Pituitary adenylate cyclase-activating polypeptide-27 causes a biphasic chronotropic effect and atrial fibrillation in autonomically decentralized, anesthetized dogs

We investigated the effects of a neuropeptide, pituitary adenylate cyclase-activating polypeptide- (PACAP) 27, on the sinoatrial nodal pacemaker activity and the mechanisms for the cardiac effects of PACAP-27 in the autonomically decentralized heart of the anesthetized dog. PACAP-27 (0.01-0.3 nmol) injected into the sinus node artery increased followed by decreased sinus rate. PACAP-27 (0.1 and 0.3 nmol) caused atrial fibrillation spontaneously. After atropine, PACAP-27 never decreased but only increased sinus rate as did vasoactive intestinal peptide. However, propranolol did not affect the negative and positive chronotropic effects. Tetrodotoxin but not hexamethonium abolished the negative chronotropic response to PACAP-27 in atropine nontreated dogs, and tetrodotoxin also inhibited the positive chronotropic response by 34% in atropine-treated dogs. In atropine- and propranolol-treated dogs, positive chronotropic responses to PACAP-27 were inhibited by PACAP-(6-27), a PACAP receptor antagonist but not by vasoactive intestinal peptide (10-28), a vasoactive intestinal peptide receptor antagonist. These results indicate that PACAP-27 causes the negative chronotropic effect through the postganglionic parasympathetic nerve activation and it produces the positive chronotropic effect mediated by PACAP receptors with an activation of non-adrenergic, nonvasoactive intestinal peptide-ergic nerves at least in part in the dog heart. Atropine and tetrodotoxin abolished atrial fibrillation induced by PACAP-27 but other blockers did not. These results suggest that neurally released acetylcholine induced by PACAP-27 participates in the induction of atrial fibrillation.     

Impairment of autonomically mediated heart rate control in patients with cardiac dysfunction

Patients with cardiac disorders have defective parasympathetic control of heart rate. To evaluate the possibility of similar changes in sympathetic control of heart rate, we compared reflex chronotropic responses to 80 degree upright tilt and nitroglycerin-induced hypotension in 31 cardiac patients and 7 normal individuals before and after partial parasympathetic blockade with atropine. Tilting revealed an attenuation of the normal heart rate increase in patients; the magnitude of this defect was greatest in patients with more severe symptoms (class III) and evidence of left ventricular dysfunction (the heart rate increase averaged 25 plus or minus 3 beats/min in normal subjects, 12 plus or minus 2 beats/min in class I-II patients, and 7 plus or minus 1 beats/min in class III patients). Class III symptoms due to mechanical causes (mitral stenosis), however, were not associated with this defect. A marked reduction in heart rate rise with hypotension was seen only in those class III patients without mitral stenosis (0.4 plus or minus 0.1 beats min-minus 1 mm Hg-minus 1 vs. 3.0 plus or minus 0.5 beats min-minus 1 mm Hg-minus 1 in normal subjects). This abnormality also persisted after atropine administration, thus confirming a defect in the sympathetic as well as the parasympathetic component of baroreceptor-mediated reflex heart rate control in patients with cardiac dysfunction. Infusions of isoproterenol produced equivalent rises in heart rate in patients and normal individuals, excluding a reduction in beta-receptor responsiveness as a cause of impaired sympathetic influence. Norepinephrine depletion, however, is a well-recognized concomitant of cardiac failure. It is possible that the reduction in sympathetically mediated heart rate responses results in part from depletion of the sympathetic neurotransmitter.     

Regional differences in cardiac effects of pituitary adenylate cyclase-activating polypeptide-27 in the isolated dog heart

Recent observations indicate that several neuropeptides may be involved in the regulation of cardiac function, but the effects of these peptides on the atrium are not always the same as those on the ventricle. To compare the effect of pituitary adenylate cyclase-activating polypeptide (PACAP)-27 on the atrium with that on the ventricle, we investigated the effects of PACAP-27 on the sinus rate and atrial and ventricular contractility in isolated, blood-perfused dog heart preparations. PACAP-27 (0.01-0.3 nmol) caused transient positive followed by negative chronotropic and inotropic responses in a dose-dependent manner in the isolated right atrium, whereas it caused only a dose-dependent positive inotropic response in the left ventricle. After atropine treatment, PACAP-27 caused only positive cardiac responses in isolated atria. The order of the increase in response to PACAP-27 was atrial contractile force > sinus rate > or = ventricular contractile force. Tetrodotoxin blocked the negative chronotropic and inotropic responses to PACAP-27 in isolated atria. Propranolol did not affect the positive response. PACAP-(6-27), a type I PACAP receptor antagonist, attenuated the positive responses similarly in the atropine-treated right atrium and the left ventricle. Thus, we demonstrated that (1) PACAP-27 caused negative cardiac effects in the atrium and sinoatrial node by activation of intracardiac parasympathetic nerves, but had no negative effect on the ventricle; (2) PACAP-27 had positive effects in the atrium, sinoatrial node and ventricle mediated by type I PACAP receptors, but PACAP-27 was more effective in the atrium and sinoatrial node than in the ventricle of the dog heart.     

Potentiating action of MKC-242, a selective 5-HT1A receptor agonist, on the photic entrainment of the circadian activity rhythm in hamsters

1. Purinergic and cholinergic components of parasympathetic neurotransmission and contractile responses to exogenous alpha,beta-methylene ATP, acetylcholine, substance K, substance P, calcitonin gene-related peptide, vasoactive intestinal polypeptide and capsaicin have been investigated in the urinary bladder of hibernating hamsters (4 weeks), cold exposed (4 weeks) and age-matched controls. 2. Electrical field stimulation (EFS) evoked increased frequency-dependent contractions in the detrusor strips from hibernating hamsters compared with those obtained from cold-exposed and age-matched animals. Tetrodotoxin (10(-6) M) completely blocked the frequency-dependent contractions in all groups. 3. The purinergic component of the parasympathetic neurotransmission was not affected in hibernating and cold-exposed animals while the cholinergic component was increased with respect to age-matched animals. The neurogenic response to EFS, still present after incubation with atropine (10(-6) M) and suramin (10(-4) M), was attenuated by indomethacin (10(-6) M) and blocked by tetrodotoxin (10(-6) M). 4. Exogenous administration of alpha,beta-methylene ATP elicited a significantly reduced contraction in strips from hibernating and cold-exposed hamsters relative to age-matched animals. The contractile response to exogenous acetylcholine was greater in the detrusors from hibernating hamsters than in cold-exposed and age-matched animals. Substance K elicited reduced contractions in preparations from hibernating animals compared with cold-exposed and control animals. Calcitonin gene-related peptide, vasoactive intestinal polypeptide, substance P and capsaicin did not elicit any relaxant or contractile response either at resting tone or in carbachol (5 x 10(-7) M)-precontracted tissues. 5. In summary, our findings indicate that 4 weeks of hibernation can significantly increase neurogenic responses in the hamster urinary bladder. This appears to be due to an increase in postjunctional responses to acetylcholine. In contrast, there was a decrease of the postjunctional responses to the parasympathetic cotransmitter ATP and also to the sensory-motor neurotransmitter substance K.     

Effect of endurance exercise training on heart rate variability at rest in healthy young and older men

Heart rate variability (HRV) (SD of the RR interval), an index of parasympathetic tone, was measured at rest and during exercise in 13 healthy older men (age 60 to 82 years) and 11 healthy young men (age 24 to 32 years) before and after 6 months of aerobic exercise training. Before exercise training, the older subjects had a 47% lower HRV at rest compared with the young subjects (31 +/- 5 ms vs 58 +/- 4 ms, p = 0.0002). During peak exercise, the older subjects had less parasympathetic withdrawal than the young subjects (-45% vs -84%, p = 0.0001). Six months of intensive aerobic exercise training increased maximum oxygen consumption by 21% in the older group and 17% in the young group (analysis of variance: overall training effect, p = 0.0001; training effect in young vs old, p = NS). Training decreased the heart rate at rest in both the older (-9 beats/min) and the young groups (-5 beats/min, before vs after, p = 0.0001). Exercise training increased HRV at rest (p = 0.009) by 68% in the older subjects (31 +/- 5 ms to 52 +/- 8 ms) and by 17% in the young subjects (58 +/- 4 ms to 68 +/- 6 ms). Exercise training increases parasympathetic tone at rest in both the healthy older and young men, which may contribute to the reduction in mortality associated with regular exercise.     

Chronic spinal muscular atrophy of facioscapulohumeral type

Adult male rats were progressively trained 5 days/weeks on a motor-driven treadmill. The training period lasted 12 weeks and consisted of 60 min/day of wind-sprints and endurance work. No significant difference in resting heart rates was observed between the control and exercise groups during week 1 (394 +/- 7 vs. 388 +/- 5). However, at week 12 the exercise group had a lower resting heart rate (359 +/- 6 vs. 331 +/- 4). Heart rates observed following saline, propranolol, atropine, and propranolol plus atropine injections were lower in the exercise group in all cases. The difference in heart rates between the control and exercise groups was 19 beats/min following propranolol plus atropine which was less than the 28 beats/min difference observed under control conditions. With atropine and then with propranolol the differences were 33 and 27 beats/min. These heart rate differences were observed without the presence of cardiac hypertrophy as assesssed from ventricle weights. Our data indicate that the bradycardia resulting from exercise training is due primarily to changes other than neural influences on the heart.     

Local actions of acetylcholine on vasomotor regulation in rat incisor pulp

Local actions of acetylcholine on vasomotor regulation in rat incisor pulp were investigated in anaesthetized animals. Pulpal blood flow was measured with laser Doppler flowmetry. lontophoresis (40 or 60 muA for 20 s) with acetylcholine or carbachol on an exposed dentin surface caused a biphasic response: an initial vasoconstriction was followed by an atropinesensitive vasodilation. The vasoconstrictor response was enhanced by 171% in the presence of atropine, whereas in control experiments, using isotonic saline as a medium for the direct current, similar vasoconstriction was unaffected by atropine. Carbachol-induced vasodilation was significantly reduced by 30% after intravenous injection of the nitric oxide synthesis blocker N omega-nitro-L-arginine methyl ester (10 mg kg-1), and the remaining vasodilation was abolished by atropine. The present results support the concept of a cholinergic modulation of sympathetic vasoconstrictor function and of a partial contribution of nitric oxide in carbachol-induced vasodilation in rat incisor pulp.     

Motor and sensory evoked potential studies in brainstem strokes

BACKGROUND: Anterior chamber miotic solutions are widely used during anterior chamber surgery. We examined the effects of solutions containing miotic agents such as carbachol and/or acetylcholine on corneal endothelial pumping activity. METHODS: We monitored, in vitro, the transendothelial electrical potential difference of isolated rabbit corneal endothelial preparations. As controls, we used solutions without miotics. RESULTS: We found that a solution containing 55 mM acetylcholine and minimal amounts of salts (Miochol E) maintains transendothelial electrical potential difference some 30% above control levels for up to 4 h. Two other solutions, one including balanced salts and 0.55 mM carbachol (Miostat), the other a mixture of 0.19 mM carbachol and 55 mM acetylcholine plus minimal salts, are adequate to maintain the potential difference at control levels. Lastly, a solution with acetylcholine but without any salts (Miochol) greatly decreases the potential difference, to 30% of the control level, in 100 min. CONCLUSION: Our results indicate that: (1) 55 mM (1%) acetylcholine stimulates the endothelial electrical potential difference; (2) addition of 0.19 mM (0.003%) carbachol negates the stimulatory effect of acetylcholine; and (3) absence of electrolytes severely depresses the endothelial electrical activity.     

Regulation of beta-adrenoceptors in the guinea-pig sinoatrial node

This study examined the changes of beta-adrenoceptors in the guinea-pig sinoatrial nodal region following 7 day (-)-isoprenaline (400 micrograms/kg/h s.c.) infusion and the relationship between beta-adrenoceptor desensitization and receptor down-regulation. Changes in beta 1- and beta 2-adrenoceptor density were measured using quantitative autoradiography and function in organ bath studies. (-)-Isoprenaline treatment produced a marked decrease in total (from 57.5 to 33.9 fmol/mg protein), beta 1- (from 49.4 to 32.8 fmol/mg protein), and beta 2-adrenoceptor density (from 8.1 to 1.05 fmol/mg protein) in the sinoatrial node. In adjacent right atrium, treatment produced no change in total (39.5 and 36.7 fmol/mg protein) or beta 1-adrenoceptors (35.9 and 36.4 fmol/mg protein) but did decrease beta 2-adrenoceptors (from 3.7 to 0.3 fmol/mg protein). Chronotropic effects were measured in spontaneously beating right atrium. Procaterol, a selective beta 2-adrenoceptor agonist, caused a biphasic chronotropic response in control right atria, the first part of which was abolished in the tissue from treated animals. The maximum increase in right atrial rate to RO363, a beta 1-adrenoceptor selective partial agonist, was reduced from 114 bpm in control to 43 bpm in treated animals. In electrically driven right atrium with the sinoatrial node removed procaterol failed to produce a positive inotropic response via beta 2-adrenoceptors, but the maximum response to RO363 was reduced from 0.75 g in the control tissue to 0.12 g in the treated tissue. This study showed that changes in beta 2-adrenoceptor density following 7 day (-)-isoprenaline infusion are compatible with reduced functional responsiveness in the SA node. The reduction of beta 1-adrenoceptor number in the SA node was also compatible with the reduced chronotropic response in this tissue. However the lack of effect on beta 1-adrenoceptor density in the right atrium was not consistent with the decrease in beta 1-adrenoceptor mediated inotropic response in this tissue. This suggests that beta-adrenoceptor desensitization is not always associated with receptor down-regulation but depends also on the changes in the cell signalling system beyond the level of the receptor which differ according to the cardiac location.     

Carbachol stimulates c-fos expression and proliferation in oligodendrocyte progenitors

To determine if muscarinic receptor-activation plays a role in oligodendrocyte development, the effect of carbachol a stable acetylcholine analog, on gene expression and proliferation was investigated. Using Northern blot analysis we showed that carbachol caused a time and concentration-dependent increase in c-fos mRNA. This effect was blocked by atropine, a non-selective muscarinic antagonist. In addition, the muscarinic-stimulated c-fos increase was inhibited by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), but not by N-2-(p-bromocinnamylamino)-ethyl-5-isoquinoline-sulfonamide (H-89), a potent inhibitor of protein kinase A, suggesting the involvement of PKC in mediating the response. Down-regulation of PKC by overnight pre-treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) blocked only the phorbol ester-stimulated c-fos accumulation while no effect was observed in the carbachol-induced response. These results suggested that carbachol stimulated an H-7 sensitive PKC pathway which may be different than that activated by TPA. Further evidence for two separate mechanisms of proto-oncogene induction was provided by the additive effect of carbachol and TPA. Induction of c-fos mRNA by carbachol was dependent on both influx of extracellular Ca2+ and release from intracellular stores, as both EDTA and BAPTA blocked the response. Since activation of muscarinic receptors can affect cell division in other cellular systems, the effect of carbachol on [3H]thymidine and bromodeoxyuridine incorporation into oligodendrocyte DNA was measured. Carbachol stimulated DNA synthesis in oligodendrocyte progenitors. This effect was mediated by muscarinic receptors as [3H]thymidine incorporation was prevented or significantly reduced by the addition of atropine. In conclusion, the present findings suggest that, the neurotransmitter, acetylcholine may act as a trophic factor in developing oligodendrocytes, regulating their growth and development in the central nervous system.     

Activation of muscarinic receptors during blockade of GABA(A)-mediated inhibition induces synchronous epileptiform activity in immature rat hippocampus

We investigated the effects of the cholinergic agonist carbachol (25 microM) on the synaptic potentials recorded extracellularly and intracellularly from the CA3 area of immature hippocampal slices of the rat (postnatal days 10-20). In control conditions, carbachol reduced the amplitude of evoked synaptic responses (n=8) and did not induce any spontaneous synchronous activity (n=12); the depressant effect of carbachol was mimicked by acetylcholine (100 microM, in eserine 10 microM, n=5) and was reversed by the muscarinic antagonist atropine (1 microM, n=2). The GABA(A)-receptor antagonist bicuculline (10 microM) enhanced the amplitude and duration of the evoked synaptic responses and induced infrequent (0.016-0.045 Hz) spontaneous synchronous discharges in 23/37 of the slices. Application of carbachol in the presence of bicuculline reduced the amplitude of the evoked synaptic responses (n=21) and in addition induced synchronous discharges with rates of occurrence 0.075-0.225 Hz, in 64/68 slices. Both effects were mimicked by acetylcholine and eserine, and antagonized by atropine. The specific muscarinic antagonists pirenzepine (M1-type), tripitramine (M2-type), 4-diphenylacetoxy-N-methylpiperidine methiodide (M3-type) and tropicamide (M4-type) (all tested at 0.1-1 microM) reversibly reduced the frequency of synchronous carbachol-induced discharges. In addition, these discharges were reversibly blocked by high Ca2+ perfusion medium (7 mM CaCl2, n=4) and by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM, n=7). Synchronous epileptiform discharges were recorded from both CA1 and CA3 areas in intact slices (n=3), but only from CA3 following disruption of the CA1-CA3 synaptic connections (n=3). These experiments suggest that activation of muscarinic receptors during blockade of GABA(A)-mediated potentials, may enhance synchronous epileptiform activity in immature (postnatal days 10-20) hippocampus, through activation of local excitatory circuits and that endogenous acetylcholine may be sufficient to play this role.     

Heart muscle cells acutely infected with Trypanosoma cruzi: characterization of electrophysiology and neurotransmitter responses

Primary cell cultures of mouse ventricular myocardium were infected with Trypanosoma cruzi, to study the consequences of T. cruzi-muscle cell interaction on the rate of spontaneous contractions, on the responses to norepinephrine, and on action potential parameters. Single cells or small cell groups of infected cultures were subjected to pharmacological and electrophysiological experiments. In concentrations ranging from 1 nM to 100 microM, norepinephrine exerted positive or negative chronotropic effects mediated by alpha-adrenergic receptors. A significant number of infected cells (25%) did not respond to the agonist. Two days after infection the cultures exhibited a higher frequency of spontaneous contractions (20%), paralleled by an increase in firing rate and a decrease in the action potential duration without significant changes in maximum diastolic potential and action potential amplitude. A decrease in alpha-adrenergic receptor-mediated positive chronotropic response to norepinephrine was also observed in 2-day infected cells. Cells made to phagocyte ferritin particles showed an increase in the rate of spontaneous contractions, but no changes in the positive chronotropic responses to norepinephrine. In conclusion, these observations show that during acute infection with T. cruzi, there are alterations in automaticity and in the chronotropic responses to norepinephrine, whose mechanisms are related to the process of parasite endocytosis by the cardiac cells.     

Inborn errors of copper metabolism: kinky hair disease and hepatolenticular degeneration. Therapeutic approaches

1. The effects of secretin on inotropic and chronotropic activity were investigated in nine isolated canine atrium preparations which were suspended in a bath and perfused with arterial blood from a carotid artery of a heparinized donor dog. 2. Secretin administered into the cannulated sinus node artery in a dose range of 0-1-10 units produced a dose-related positive inotropic and a biphasic chronotropic effect. 3. The positive chronotropic and inotropic responses to secretin were not suppressed by treatment with alprenolol in doses which blocked responses to noradrenaline. 4. The negative chronotropic response to secretin was not blocked by atropine in doses which blocked the response to acetylcholine. 5. After treatment with glucagon, secretin produced dose-related negative chronotropic and a positive inotropic effects. Thus glucagon may antagonize the positive chronotropic effect of secretin. 6. From these results, it is concluded that secretin has a direct effect on atrial rate and contractility.     

Influence of autonomic tone on QT interval duration

The autonomic tone has been shown to influence the duration of the QT interval, however the independent contribution of sympathetic and parasympathetic tone is not fully elucidated. The influence of autonomic tone on QT duration was studied in 10 young healthy volunteers by evaluating the changes in QT and RR duration induced by i.v. isoproterenol infusion and by standing before and after i.v. administration of propranolol or atropine. Furthermore, the relationship between RR interval and QT duration was evaluated during nocturnal sinus arrhythmia and submaximal exercise test. Low doses of isoproterenol reduced RR (p < 0.01) but not QT interval duration, while higher doses influenced both RR (p < 0.0001) and QT (p < 0.001) duration. Propranolol did not influence standing-induced shortening of RR and QT intervals; on the contrary, atropine administration abolished standing-induced QT interval shortening, without influencing RR changes. QT duration resulted significantly related to preceding RR interval at peak exercise (r = 0.87, p < 0.001) and during nocturnal sinus arrhythmia (r = 0.73, p < 0.0005), however, the regression lines showing the correlation between QT and preceding RR interval were different. Both sympathetic and parasympathetic tone appear to contribute to heart rate-independent changes in QT duration. In the basal state parasympathetic more than sympathetic tone influences the relation QT-heart rate. Major increases of sympathetic nervous system activity may change the relation QT-heart rate. Thus, in case of abrupt autonomic changes, any proposed formula for heart rate correction of QT may result inappropriate, also in the normal range of heart rate.     

Inhibition of nitric oxide synthase slows heart rate recovery from cholinergic activation

The role of nitric oxide (NO) in the cholinergic regulation of heart rate (HR) recovery from an aspect of simulated exercise was investigated in atria isolated from guinea pig to test the hypothesis that NO may be involved in the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Inhibition of NO synthesis with NG-monomethyl-L-arginine (L-NMMA, 100 micro M) significantly slowed the time course of the reduction in HR without affecting the magnitude of the response elicited by bath-applied ACh (100 nM) or vagal nerve stimulation (2 Hz). The half-times (t1/2) of responses were 3.99 +/- 0.41 s in control vs. 7. 49 +/- 0.68 s in L-NMMA (P < 0.05). This was dependent on prior adrenergic stimulation (norepinephrine, 1 micro M). The effect of L-NMMA was reversed by L-arginine (1 mM; t1/2 4.62 +/- 0.39 s). The calcium-channel antagonist nifedipine (0.2 micro M) also slowed the kinetics of the reduction in HR caused by vagal nerve stimulation. However, the t1/2 for the reduction in HR with antagonists (2 mM Cs+ and 1 micro M ZD-7288) of the hyperpolarization-activated current were significantly faster compared with control. There was no additional effect of L-NMMA or L-NMMA+L-arginine on vagal stimulation in groups treated with nifedipine, Cs+, or ZD-7288. We conclude that NO contributes to the cholinergic antagonism of the positive cardiac chronotropic effects of adrenergic stimulation by accelerating the HR response to vagal stimulation. This may involve an interplay between two pacemaking currents (L-type calcium channel current and hyperpolarization-activated current). Whether NO modulates the vagal control of HR recovery from actual exercise remains to be determined.     

Effect of physical training on exercise-induced hyperkalemia in chronic heart failure. Relation with ventilation and catecholamines

BACKGROUND: The exercise-induced rise in arterial potassium concentration ([K+]a) may contribute to exercise hyperpnea and could play a role in exertional fatigue. This study was designed to determine whether the exercise-induced rise in [K+]a is altered in patients with chronic heart failure (CHF) and whether physical training affects K+ homeostasis. METHODS AND RESULTS: We evaluated 10 subjects with CHF (ejection fraction, 23 +/- 3.9%) and 10 subjects with normal left ventricular function (NLVF) who had undergone previous coronary artery graft surgery (ejection fraction, 63 +/- 8.6%). Subjects performed an incremental cycle ergometer exercise test before and after a physical training or detraining program. Changes in [K+]a and ventilation (VE) during exercise were closely related in both groups. Subjects with CHF did less absolute work and had reduced maximal oxygen consumption (VO2max) compared with subjects with NLVF (P < .01). Exercise-induced rises in [K+]a, VE, norepinephrine, lactate, and heart rate were greater at matched absolute work rates in subjects with CHF than in subjects with NLVF (P < .01). However, when the rise in [K+]a was plotted against percentage of VO2max to match for relative submaximal effort, there were no differences between the two groups. Physical training resulted in reduced exercise-induced hyperkalemia at matched submaximal work rates in both groups (P < .01) despite no associated change in the concentration of arterial catecholamines. At maximal exercise when trained, peak increases in [K+]a were unaltered, but peak concentrations of catecholamines were raised (P < .05). The decrease in VE at submaximal work rates after training was not significant with this incremental exercise protocol, but both groups had an increased peak VE when trained (P < .01). CONCLUSIONS: Exercise-induced rises in [K+]a, catecholamines, and VE are greater at submaximal work rates in subjects with CHF than in subjects with NLVF. Physical training reduces the exercise-induced rise in [K+]a but does not significantly decrease VE during submaximal exercise with this incremental cycle ergometry protocol. The reduction in exercise-induced hyperkalemia after training is not the result of altered concentrations of arterial catecholamines. The pathophysiological significance of the increased exercise-induced hyperkalemia in CHF and the mechanisms of improved K+ homeostasis with training have yet to be established.     

Inactivation of (Gialpha) proteins increases arrhythmogenic effects of beta-adrenergic stimulation in the heart

Chronic treatment of rats with carbachol downregulates M-cholinoceptors and inhibitory, pertussis toxin (PTX)-sensitive G proteinalpha-subunits (Gialpha) and sensitizes the heart to arrhythmogenic effects of isoprenaline (ISO), suggesting a causal relationship. To test this hypothesis by a more direct and quantitative approach, nine groups of rats were treated for 24 h with increasing doses of PTX (1.25-200 microg/kg i.v.). Inactivation of cardiac Gialpha was determined biochemically by 32P-ADP-ribosylationin vitro and functionally by measuring contractile effects of carbachol. Effects of ISO were studied in spontaneously beating right atria (RA) and isolated papillary muscles (PM; paced at 1 Hz). PTX increased heart rate in conscious animals (ECG) with a bell-shaped dose-dependency (maximal increase 120 beats/min at 7.5 microg/kg). PTX dose-dependently inactivated 25-85% of total cardiac Gialpha, which linearly correlated with a loss of the direct negative chronotropic effect of carbachol in atria, but not with a loss of its indirect negative inotropic effect in PM. The latter was resistant up to PTX 20 microg/kg (=70% inactivation). The decrease in Gialpha closely correlated with an increased efficacy of ISO to induce spontaneous contractile activity (automaticity) in PM. At 3 micromol/l ISO, all PM from PTX 200 microg/kg beat spontaneously compared to 10% in control. In contrast, pretreatment with PTX only modestly and not clearly dose-dependently increased the inotropic potency of ISO (PTX 100 microg/kg: EC50 28v 81 nmol/l in control) and did not affect the chronotropic effect of ISO. The disparity of the functional consequences of PTX treatment suggest that under physiological conditions, Gialpha serve mainly to suppress arrhythmogenic, but not or to a minor extent, positive chronotropic or inotropic effects of beta-adrenoceptor activation.     

Attenuated cardiac sympathetic responsiveness during dynamic exercise in patients with heart failure

BACKGROUND: Cardiac norepinephrine (NE) spillover is increased in patients with chronic heart failure. This elevation is partly due to augmented NE release but also to reduced capacity for cardiac NE removal processes. In patients with mild to moderate heart failure, it is not known whether the described alteration in cardiac sympathetic function also affects cardiac NE spillover during intense sympathetic activation and whether other organs respond in proportion to the heart. METHODS AND RESULTS: Twenty-two patients with heart failure and 15 age-matched healthy subjects were studied. Whole-body and regional (NE) spillovers from the heart and kidneys were assessed at baseline and during supine cycling exercise (10 minutes) with the use of steady-state infusions of tritiated NE (isotope dilution). Cardiac performance was evaluated by means of catheterization of the right side of the heart. Cardiac NE spillover was higher (P < .05) at baseline in the patient group than in healthy subjects, whereas renal and whole-body NE spillovers were similar between the study groups. During exercise, cardiac NE spillover increased 13-fold (P < .05) in healthy subjects but only 5-fold (P < .05) in the cardiac failure group, the latter reaching a lower peak value (P < .05). In contrast, there was no difference between the study groups in either renal or whole-body NE spillover responsiveness to exercise. CONCLUSIONS: Patients with mild to moderate heart failure demonstrated a selective attenuation of cardiac sympathetic responsiveness during dynamic exercise. This attenuation may convey reduced inotropic and chronotropic support to the failing heart.     

M3 muscarinic receptor-mediated enhancement of NMDA-evoked adenosine release in rat cortical slices in vitro

Acetylcholine plays an important role in cortical arousal. Adenosine is released during increased metabolism and has been suggested to be a sleep-promoting factor. To understand the interaction of acetylcholine and adenosine in regulating cortical excitability, we examined the effect of carbachol on NMDA-evoked adenosine release and identified the muscarinic receptor subtype that mediated this effect in adult rat cortical slices in vitro. Carbachol (to 300 microM) alone did not affect the basal release of adenosine. However, carbachol (100 microM) induced a 253% increase in NMDA (20 microM)-evoked adenosine release in the presence of Mg2+. In the absence of Mg2+, carbachol's potentiating effect was less (60% increase). The nonselective muscarinic antagonist atropine (1.5 microM) blocked the facilitatory effect of carbachol on NMDA-evoked adenosine release, and this was mimicked by the M3-selective antagonist 4-diphenylacetoxy-N-methylpiperidine (1 microM). Neither an M1-selective dose of pirenzepine (50 nM) nor the M2-selective antagonist methoctramine (1 microM) affected carbachol's action on NMDA-evoked adenosine release. Carbachol had no effect on adenosine release evoked by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA). These results suggest that acetylcholine does not affect basal adenosine release but enhances NMDA receptor-mediated evoked adenosine release by acting at M3 receptors in the cortex. This interaction may have a role in regulating cortical neuronal excitability on a long-term basis.     

Exercise training alters endothelium-dependent vasoreactivity of rat abdominal aorta

We tested the hypothesis that adaptations in peripheral arterial vasoreactivity are induced by exercise training. Male rats were trained to run on a treadmill at 30 m/min (15 degrees incline) for 1 h/day 5 days/wk for 10-12 wk. Efficacy was indicated by a 51% increase (P < 0.05) in citrate synthase activity in soleus muscle of exercise-trained (ET) rats compared with that of sedentary (SED) control rats. Responses to vasoactive compounds were examined in vitro using rings of abdominal aorta. Maximal isometric contractile tension evoked by KCl, norepinephrine (NE), and phenylephrine were not different between groups; sensitivity to phenylephrine was also not different between groups. However, sensitivity was lower for both KCl and NE in vessels from ET animals. Endothelium removal did not influence KCl sensitivity but did abolish the difference in NE sensitivity of vessel segments between ET and SED animals. Maximal vasodilator responses induced by acetylcholine (ACh; NE or prostaglandin F2 alpha preconstriction) were greater in vessel rings from ET rats. However, dilatory responses by sodium nitroprusside (NE or prostaglandin F2 alpha preconstriction) and forskolin (NE preconstriction) were not different between groups, indicating that the augmented ACh-induced dilatory response resulted from an adaptation of the endothelium. Blockade of nitric oxide synthase activity diminished ACh-induced vasodilation by 79 and 100% in SED and ET rats, respectively. These results indicate that training alters vasomotor function in rat abdominal aortas through adaptations of both endothelium and smooth muscle.