Tissue selectivity of calcium channel blockers

Calcium channel blockers are also termed calcium antagonists or calcium entry blockers. The use of calcium antagonists for the management of hypertension is well established. Their control of vascular tone is related to their interaction with the alpha 1 subunit of L-type calcium channels. This interaction is not simple since prolonged depolarisation promotes the inactivated state of the channels resulting in a change of affinity which is different for various molecules so far considered. The isoforms of alpha 1 subunits and the duration of the stimulus required to activate heart or vessels are important parameters to be considered with the nature of the molecule. Those parameters influence the vascular selectivity which is quantified as the ratio of the concentrations required to reduce by 50% the contraction of heart and of vessels. This selectivity is an important component in the therapeutic action. Another component of this action is the prevention of structural changes noted in heart and arteries. As well as lowering blood pressure, calcium channel blockers have also been found to exert blood pressure independent effects. For instance, they reduce cardiac and vascular hypertrophy and avoid renal damage. In the stroke-prone rat, such protective effects are accompanied by reduction of the salt-dependent overexpression of the gene of endothelin-1 and of fetal genes associated with cardiac hypertrophy. This paper summarizes available information about those components and discuss their significance.     

The action of calcium channel blockers on recombinant L-type calcium channel alpha1-subunits

1. CHO cells expressing the alpha(1C-a) subunit (cardiac isoform) and the alpha(1C-b) subunit (vascular isoform) of the voltage-dependent L-type Ca2+ channel were used to investigate whether tissue selectivity of Ca2+ channel blockers could be related to different affinities for alpha1C isoforms. 2. Inward current evoked by the transfected alpha1 subunit was recorded by the patch-clamp technique in the whole-cell configuration. 3. Neutral dihydropyridines (nifedipine, nisoldipine, (+)-PN200-110) were more potent inhibitors of alpha(1C-)b-subunit than of alpha(1C-a)-subunit. This difference was more marked at a holding potential of -100 mV than at -50 mV. SDZ 207-180 (an ionized dihydropyridine) exhibited the same potency on the two isoforms. 4. Pinaverium (ionized non-dihydropyridine derivative) was 2 and 4 fold more potent on alpha(1C-a) than on alpha(1C-b) subunit at Vh of -100 mV and -50 mV, respectively. Effects of verapamil were identical on the two isoforms at both voltages. 5. [3H]-(+)-PN 200-110 binding experiments showed that neutral dihydropyridines had a higher affinity for the alpha(1C-b) than for the alpha(1C-a) subunit. SDZ 207-180 had the same affinity for the two isoforms and pinaverium had a higher affinity for the alpha(1C-a) subunit than for the alpha(1C-b) subunit. 6. These results indicate marked differences among Ca2+ channel blockers in their selectivity for the alpha(1C-a) and alpha(1C-b) subunits of the Ca2+ channel.     

The effect of calcium channel blockers and calcium on methotrexate accumulation in rat hepatocytes

The effects of diltiazem (DIL), verapamil (VRP) and Ca2+ on the accumulation of methotrexate (MTX) were investigated in isolated rat hepatocytes. At the physiological 2 mM Ca2+, the calcium-channel blockers DIL (100 microM) and VRP (50 microM) significantly reduced the hepatocellular accumulation of MTX. By increasing the Ca2+ concentration to 7 mM control MTX levels (at 2 mM Ca2+) were restored with VRP, and resulted in MTX levels above the controls for DIL. Ca2+ at 7 mM significantly enhanced MTX accumulation in the hepatocyte suspensions after 60 min. The concentration time curves for MTX indicated that for the first 10 min influx was the dominating process. Dixon plot analysis of this uptake phase revealed Ki values of 140 microM for DIL and 75 microM for VRP. The data suggested that DIL was a non-competitive, and VRP a competitive inhibitor of MTX influx. Hence, the inhibitory effect on MTX accumulation mediated by DIL and VRP could be due to different mechanisms.     

The relationship between anticonvulsant and anti-anxiety effects of calcium channel blockers

The effect of calcium channel blockers on the development of convulsive and anxiogenic actions of corasole and caffeine was studied on mice. Verapamil, nifedipine, cinnarizine, and fendilin attenuated corasole-induced convulsions were attenuated by convulsions. Nifedipine also weakened while fendilin increased the course of convulsions induced by caffeine. All calcium antagonists did not change the effects of corasole and caffeine in the dark-light chamber test and the test in an elevated plus maze (anxiogens shortened the time that the mice stayed in the open sections of the maze). It is concluded that there are no relations between the anticonvulsive and anxiolytic effects of calcium antagonists.     

A computerized intervention to decrease the use of calcium channel blockers in hypertension

OBJECTIVE: To determine whether a computer-assisted reminder would alter prescribing habits for the treatment of hypertension in accordance with current clinical guidelines in a general internal medicine clinic. DESIGN: A randomized trial. SETTING: The General Internal Medicine Clinic of the Veterans Affairs Puget Sound Health Care System, Seattle Division. PATIENTS/PARTICIPANTS: Clinic providers were randomized to a control group (n = 35) or intervention group (n = 36). We targeted the providers of patients being treated for hypertension with calcium channel blockers, a class of drug not recommended for initial therapy. INTERVENTION: An automated computer query identified eligible patients and their providers. A guideline reminder was placed in the charts of patients of intervention providers; the charts of patients of control providers received no reminder. MEASUREMENTS AND MAIN RESULTS: During the 5-month study period, 346 patients were seen by the 36 primary care providers (staff physicians, nurse practitioners, residents, and fellows) in the intervention group, and 373 patients were seen by the 35 providers in the control group. Intervention providers changed 39 patients (11.3%) to other medications during the study period, compared with 1 patient (<1.0%) of control providers (p < .0001). For patients whose therapy was unchanged, providers noted angina in 23.1%, indications other than those for hypertension in 9.5%, intolerable adverse effects with first-line therapy in 13.9%, and inadequate control with first-line therapy in 13.9%. Of those patients without provider-indicated contraindications, 23.6% were switched from calcium channel blockers to first-line agents during the intervention period. CONCLUSIONS: The use of a computerized, clinic-based intervention increased compliance with guidelines in the treatment of primary hypertension in general, and decreased the use of calcium channel blockers for the treatment of hypertension in particular.     

Synthesis and structure-activity relationship of substituted 1,2,3,4-tetrahydroisoquinolines as N-type calcium channel blockers

Voltage Activated Calcium Channel (VACC) blockers have been demonstrated to have utility in the treatment of pain and stroke. A series of aminomethyl substituted isoquinolinol derivatives with potent functional activity for N-type VACC's have been identified. Their synthesis and preliminary pharmacology are discussed herein.     

Effect of chloride channel blockers on the cardiac CFTR chloride and L-type calcium currents

OBJECTIVES: The aim of this study was to determine the effects of Cl- channel blockers on the cardiac cystic fibrosis transmembrane conductance regulator (CFTR) Cl- current (ICl) and the protein kinase A-regulated L-type calcium current (PKA-ICa). METHODS: Whole-cell ICl and ICa were recorded from isolated guinea pig ventricular myocytes using the patch clamp technique during stimulation of PKA by forskolin (1 or 2 microM). RESULTS: The inhibitory effects of clofibric acid, p-chlorophenoxy propionic acid, gemfibrozil, diphenylamine-2-carboxylate (DPC), anthracene-9-carboxylate, 4,4'dinitrostilbene-2,2'-disulfonic acid and indanyloxyacetic acid 94 were examined on the two currents. Clofibric acid (1 mM), p-chlorophenoxy propionic acid (1 mM) and gemfibrozil (250 microM) produced an approximate 50% decrease in ICl, but had no effect on the PKA-ICa. Surprisingly, application of DPC (500 microM and 1 mM) and anthracene-9-carboxylate (500 microM) strongly reduced both currents. However, inhibition of the Ca2+ and Cl- channels by DPC could be differentiated in two important ways. First, increasing the pH of the external solution from 7.4 to 10.0 prevented the block of ICl by DPC, but did not attenuate the reduction in the PKA-ICa. Second, DPC inhibited the PKA-ICa in mouse atrial myocytes which lacked ICl. Neither 4,4'dinitrostilbene-2,2'-disulfonic acid (100 microM) nor indanyloxyacetic acid 94 (50 microM) caused any change in either of the guinea pig ventricular currents. CONCLUSIONS: Drugs such as DPC and anthracene-9-carboxylate which block the cardiac CFTR Cl- channel also inhibit the regulation of the L-type ICa. During beta-adrenergic stimulation, changes produced by these drugs on the cardiac action potential duration will be attributable to inhibition of both the Cl- and Ca2+ currents. Analogues of clofibric acid may serve as selective blockers of the CFTR Cl- channel that can be used to determine the physiological function of ICl in cardiac excitation.     

Effects of calcium channel entry blockers on cocaine and amphetamine-induced motor activities and toxicities

The effects of calcium channel entry blockers on cocaine and amphetamine-induced behavioral responses were investigated. Cocaine and amphetamine produced dose-dependent increases in locomotor activity and stereotyped behavior with a maximum response at 40 and 1.2 mg/kg, respectively. The 1,4-dihydropyridine nimodipine and the benzothiazepine diltiazem were more effective in inhibiting cocaine (20 mg/kg)-induced responses than amphetamine (0.6 mg/kg)-induced responses. At doses of cocaine and amphetamine that caused seizures and death, nimodipine, nitrendipine and diltiazem did not offer any protection; rather, they potentiated the toxicities produced by these psychomotor stimulants.     

N-type calcium channel blockers from a marine bacterium, Cytophaga sp. SANK 71996

N-(3-Acyloxyacyl)glycines were isolated as N-type calcium channel blockers from a marine bacterium Cytophaga sp. SANK 71996. The identification and fermentation of the producing strain and structure characterization of N-(3-acyloxyacyl)glycines by spectral analyses and chemical syntheses are described together with their antagonistic activities.     

Interaction of calcium channel blockers with different agonists in aorta from normal and diseased rats

The interactions of calcium channel blockers (CCBs) with noradrenaline (NA), phenylephrine (PE), dopamine (DA) and KCl have been investigated in rat isolated aortic strip. In preparations from control and hypertensive (DOCA-saline) rats chronically treated with verapamil, nifedipine and diltiazem, there was partial inhibition of contractions to NA, PE and DA. However, with nimodipine, there was potentiation of responses. This could be related to the occurrence of different isoforms of L-type calcium channels. In preparations obtained from hyperthyroid rats the concentration-response curves of NA, PE and KCl were shifted to the right with depressed maximal response which could be secondary to the primary effect exerted on the heart. In preparations from L-thyroxine + nimodipine/nifedipine treated rats the concentration-response curves of NA, PE and KCl were shifted to the right and the maxima was depressed suggesting that this may be due to decreased alpha receptor density (NA and PE) and down-regulation of voltage operated channels (KCl).     

Selenium, calcium channel blockers, and cancer risk--the Yin and Yang of apoptosis?

It is increasingly clear that apoptosis plays a crucial role in the promotional phase of cancer development. Initiated pre-neoplastic clones in rat liver experience a high rate of apoptosis, and this rate has an important impact on the survival and growth of these clones. Suppression of apoptosis appears to be a universal property of cancer promoters, suggesting conversely that agents which inhibit cancer induction during the promotional phase increase the rate of apoptosis in initiated cells. Modulation of apoptosis is a likely explanation for recent striking evidence that use of calcium channel blockers substantially increases, whereas supplemental selenium substantially decreases, human cancer incidence. Non-genotoxic measures which are likely to upregulate apoptosis in pre-neoplastic/neoplastic cells--and thus may be useful in prevention and/or therapy--include selenium, retinoids/carotenoids, green tea polyphenols, caloric restriction, downregulation of IGF-I activity, high-dose tamoxifen and other protein kinase C antagonists, withdrawal or blockade of trophic hormones, isoflavones, limonene, vitamin D and cholecalciferol analogs, dietary fiber/sodium butyrate, hyperthermia, benzaldehyde derivatives, and creatine.     

In vitro testing of calcium channel blockers and cytotoxic chemotherapy in B-cell low-grade non-Hodgkin's lymphoma

The flux of calcium forms an important intracellular messenger system. The bcl-2 oncoprotein is thought to make cells resistant to a variety of insults, including cytotoxic drugs, by the suppression of apoptosis, which appears to involve the repartitioning of intracellular calcium. Three drugs that affect calcium pathways and may influence this repartitioning, i.e. dantrolene, azumolene (a water-soluble dantrolene analogue) and nimodipine, were studied in cell culture, using both a transformed follicle centre lymphoma cell line and primary culture of lymphoma cells in vitro in a manner that resulted in a growth pattern closely resembling that of the malignancy in vivo. Dantrolene and azumolene were potent inducers of cell death in both systems reducing the viable cell count by 70-90% in comparison with normal controls. Nimodipine, in comparison, appeared to have no significant effect. These results obtained in an in vitro setting suggest that further evaluation of dantrolene and azumolene for the treatment of low-grade non-Hodgkin's lymphoma is warranted.