Sphingolipids--the enigmatic lipid class: biochemistry, physiology, and pathophysiology

The "sphingosin" backbone of sphingolipids was so named by J. L. W. Thudichum in 1884 for its enigmatic ("Sphinx-like") properties. Although still an elusive class of lipids, research on the involvement of sphingolipids in the signal transduction pathways that mediate cell growth, differentiation, multiple cell functions, and cell death has been rapidly expanding our understanding of these compounds. In addition to the newly discovered role of ceramide as an intracellular second messenger for tumor necrosis factor-alpha, IL-1beta, and other cytokines, sphingosine, sphingosine-1-phosphate, and other sphingolipid metabolites have recently been demonstrated to modulate cellular calcium homeostasis and cell proliferation. Perturbation of sphingolipid metabolism using synthetic and naturally occurring inhibitors of key enzymes of the biosynthetic pathways is aiding the characterization of these processes; for examples, inhibition of cerebroside synthase has indicated a role for ceramide in cellular stress responses including heat shock, and inhibition of ceramide synthase (by fumonisins) has revealed the role of disruption of sphingolipid metabolism in several animal diseases. Fumonisins are currently the focus of a FDA long-term tumor study. This review summarizes recent research on (i) the role of sphingolipids as important components of the diet, (ii) the role of sphingoid base metabolites and the ceramide cycle in expression of genes regulating cell growth, differentiation, and apoptosis, (iii) the use of cerebroside synthase inhibitors as tools for understanding the role of sphingolipids as mediators of cell cycle progression, renal disease, and stress responses, and (iv) the involvement of disrupted sphingolipid metabolism in animal disease and cellular deregulation associated with exposure to inhibitors of ceramide synthase and serine palmitoyltransferase, key enzymes in de novo sphingolipid biosynthesis. These findings illustrate how an understanding of the function of sphingolipids can help solve questions in toxicology and this is undoubtedly only the beginning of this story.     

Electrophysiological studies on oxindole, a neurodepressant tryptophan metabolite

1. The aim of the present work was to investigate the electrophysiological effects of oxindole, a tryptophan metabolite present in rat blood and brain, and recently proposed as a contributing factor in the pathogenesis of hepatic encephalopathy. 2. Using rat hippocampal slices in vitro and extra- or intracellular recordings, we evaluated oxindole effects on the neurotransmission of the CA1 region following orthodromic stimulation of the Schaffer collaterals. 3. Oxindole (0.3-3 mM) decreased the amplitude of population spikes extracellularly recorded at the somatic level and of the fEPSPs recorded at the dendritic level. In intracellular recordings, oxindole (0.1-3 mM) did not affect the resting membrane potential or the neuronal input resistance, but reduced the probability of firing action potentials upon either synaptic or direct activation of the pyramidal cells. 4. Oxindole (0.3-3 mM) increased the threshold and the latency of firing action potentials elicited by depolarizing steps without changing the duration or the peak amplitude of the spikes. It also significantly increased the spike frequency adaptation induced by long lasting (400 ms) depolarizing stimuli. 5. In separate experiments, performed by measuring AMPA or NMDA-induced responses in cortical slices, oxindole (1-3 mM) did not modify glutamate receptor agonist responses. 6. Our results show that concentrations of oxindole which may be reached in pathological conditions, significantly decrease neuronal excitability by modifying the threshold of action potential generation.     

In vivo EPR: an effective new tool for studying pathophysiology, physiology and pharmacology

The development of spectrometers working at lower frequencies with improved resonators now permits the routine use of non-invasive EPR spectroscopy in vivo. The capabilities of EPR spectra to reflect environmental conditions, combined with the use of paramagnetic materials as selective non-toxic labels, has led to increasingly widespread and productive applications of the technique to complex problems involving physiology, pharmacology and pathophysiology. Some of the especially promising applications in which EPR techniques uniquely appear to provide valuable information are illustrated, including the measurement of oxygen and oxygen gradients, monitoring of the metabolism of xenobiotics, monitoring pharmacokinetics of drugs, measurement of perfusion, measurement of pH, recognition and labeling of receptors, and characterization of drug releasing systems.     

The insulin-like growth factor-I/binding protein axis: physiology, pathophysiology and therapeutic manipulation

Insulin-like growth factor-I (IGF-I) is a single-chain polypeptide which has multiple metabolic actions and effects on the differentiation and proliferation of a wide variety of cell types. IGF-I has endocrine, paracrine and autocrine actions and is bound in the circulation to a complex system of binding proteins which alter its bioavailability and activity. Thus its physiology is complex and is altered in a number of pathological states. This review will discuss these states and the actual and proposed therapeutic applications of recombinant human IGF-I (rhIGF-I).     

Endogenous somatostatin-28 modulates postprandial insulin secretion. Immunoneutralization studies in baboons

Somatostatin-28 (S-28), secreted into the circulation from enterocytes after food, and S-14, released mainly from gastric and pancreatic D cells and enteric neurons, inhibit peripheral cellular functions. We hypothesized that S-28 is a humoral regulator of pancreatic B cell function during nutrient absorption. Consistent with this postulate, we observed in baboons a two to threefold increase in portal and peripheral levels of S-28 after meals, with minimal changes in S-14. We attempted to demonstrate a hormonal effect of these peptides by measuring their concentrations before and after infusing a somatostatin-specific monoclonal antibody (mAb) into baboons and comparing glucose, insulin, and glucagon-like peptide-1 levels before and for 4 h after intragastric nutrients during a control study and on 2 d after mAb administration (days 1 and 2). Basal growth hormone (GH) and glucagon levels and parameters of insulin and glucose kinetics were also measured. During immunoneutralization, we found that (a) postprandial insulin levels were elevated on days 1 and 2; (b) GH levels rose immediately and were sustained for 28 h, while glucagon fell; (c) basal insulin levels were unchanged on day 1 but were increased two to threefold on day 2, coincident with decreased insulin sensitivity; and (d) plasma glucose concentrations were similar to control values. We attribute the eventual rise in fasting levels of insulin to its enhanced secretion in compensation for the heightened insulin resistance from increased GH action. Based on the elevated postmeal insulin levels after mAb administration, we conclude that S-28 participates in the enteroinsular axis as a decretin to regulate postprandial insulin secretion.     

The giant protein titin. Emerging roles in physiology and pathophysiology

Activation of the nociceptin receptor stably expressed in Chinese hamster ovary cells induced a transient mitogen-activated protein kinase (MAPK) activation, via pertussis toxin-sensitive G-proteins. The nociceptin receptor-mediated MAPK activation was partially blocked by down-regulation or inhibition of protein kinase C, and suppressed by pretreatment with a phosphatidylcholine-specific phospholipase C inhibitor, D609. Furthermore, a tyrosine protein kinase inhibitor, genistein, and phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002, affected the nociceptin-induced MAPK activity. The nociceptin-induced MAPK activation may lead to activation of phospholipase A2 and induce changes in gene expression.     

The potential relevance of growth hormone to female reproductive physiology and pathophysiology

OBJECTIVE: To assess possible interfacing between the somatotrophic and reproductive axes. DESIGN: Literature review. MAIN OUTCOME MEASURES: Ovarian growth hormone reception and action. RESULTS: The available literature strongly supports a permissive role for the somatotrophic axis in the reproductive process. CONCLUSIONS: Although a role for growth hormone in reproductive biology appears highly likely, its relevance to the process of puberty and to the normal workings of the menstrual cycle, as well as its possible application in reproductive pathology must await further investigation.     

Defective insulin secretion and enhanced insulin action in KATP channel-deficient mice

ATP-sensitive K+ (KATP) channels regulate many cellular functions by linking cell metabolism to membrane potential. We have generated KATP channel-deficient mice by genetic disruption of Kir6.2, which forms the K+ ion-selective pore of the channel. The homozygous mice (Kir6.2(-/-)) lack KATP channel activity. Although the resting membrane potential and basal intracellular calcium concentrations ([Ca2+]i) of pancreatic beta cells in Kir6.2(-/-) are significantly higher than those in control mice (Kir6.2(+/+)), neither glucose at high concentrations nor the sulfonylurea tolbutamide elicits a rise in [Ca2+]i, and no significant insulin secretion in response to either glucose or tolbutamide is found in Kir6.2(-/-), as assessed by perifusion and batch incubation of pancreatic islets. Despite the defect in glucose-induced insulin secretion, Kir6.2(-/-) show only mild impairment in glucose tolerance. The glucose-lowering effect of insulin, as assessed by an insulin tolerance test, is increased significantly in Kir6.2(-/-), which could protect Kir6.2(-/-) from developing hyperglycemia. Our data indicate that the KATP channel in pancreatic beta cells is a key regulator of both glucose- and sulfonylurea-induced insulin secretion and suggest also that the KATP channel in skeletal muscle might be involved in insulin action.     

Troglitazone improves defects in insulin action, insulin secretion, ovarian steroidogenesis, and fibrinolysis in women with polycystic ovary syndrome

Women with polycystic ovary syndrome (PCOS) are characterized by defects in insulin action, insulin secretion, ovarian steroidogenesis, and fibrinolysis. We administered the insulin-sensitizing agent troglitazone to 13 obese women with PCOS and impaired glucose tolerance to determine whether attenuation of hyperinsulinemia ameliorates these defects. All subjects had oligomenorrhea, hirsutism, polycystic ovaries, and hyperandrogenemia. Before and after treatment with troglitazone (400 mg daily for 12 weeks), all had 1) a GnRH agonist (leuprolide) test, 2) a 75-g oral glucose tolerance test, 3) a frequently sampled iv glucose tolerance test to determine the insulin sensitivity index and the acute insulin response to glucose, 4) an oscillatory glucose infusion to assess the ability of the beta-cell to entrain to glucose as quantitated by the normalized spectral power for the insulin secretion rate, and 5) measures of fibrinolytic capacity [plasminogen activator inhibitor type 1 (PAI-1) and tissue plasminogen activator]. There was no change in body mass index (39.9 +/- 1.4 vs. 40.2 +/- 1.4 kg/m2) or body fat distribution after treatment. Both the fasting (91 +/- 3 vs. 103 +/- 3 mg/dL; P < 0.001) and 2 h (146 +/- 8 vs. 171 +/- 6 mg/dL; P < 0.02) plasma glucose concentrations during the oral glucose tolerance test declined significantly. There was a concordant reduction in glycosylated hemoglobin to 5.7 +/- 0.1 from a pretreatment level of 6.1 +/- 0.1% (P < 0.03). Insulin sensitivity increased from 0.58 +/- 0.14 to 0.95 +/- 0.26 10(-5) min-1/pmol.L (P < 0.01) after treatment as did the disposition index (745 +/- 135 vs. 381 +/- 96; P < 0.05). The ability of the beta-cell to appropriately detect and respond to an oscillatory glucose infusion improved significantly after troglitazone treatment; the normalized spectral power for the insulin secretion rate increased to 5.9 +/- 1.1 from 4.3 +/- 0.8 (P < 0.05). Basal levels of total testosterone (109.3 +/- 15.2 vs. 79.4 +/- 9.8 ng/dL; P < 0.05) and free testosterone (33.3 +/- 4.0 vs. 21.2 +/- 2.6 pg/mL; P < 0.01) declined significantly after troglitazone treatment. Leuprolide-stimulated levels of 17-hydroxyprogesterone, androstenedione, and total testosterone were significantly lower posttreatment compared to pretreatment. The reduction in androgen levels occurred independently of any changes in gonadotropin levels. A decreased functional activity of PAI-1 in blood (from 12.7 +/- 2.8 to 6.3 +/- 1.4 AU/mL P < 0.05) was associated with a decreased concentration of PAI-1 protein (from 64.9 +/- 9.1 to 44.8 +/- 6.1 ng/mL; P < 0.05). No change in the functional activity of tissue plasminogen activator (from 5.3 +/- 0.4 to 5.1 +/- 0.5 IU/mL) was observed despite a decrease in its concentration (from 9.6 +/- 0.9 to 8.2 +/- 0.7 ng/mL; P < 0.05). The marked reduction in PAI-1 could be expected to improve the fibrinolytic response to thrombosis in these subjects. We conclude that administration of troglitazone to women with PCOS and impaired glucose tolerance ameliorates the metabolic and hormonal derangements characteristic of the syndrome. Troglitazone holds potential as a useful primary or adjunctive treatment for women with PCOS.     

Oscillatory insulin secretion after pancreas transplant

In vivo studies of beta-cell secretory function have demonstrated the existence of rapid insulin oscillations of small amplitude recurring every 8-15 min in normal subjects. This study evaluated the effects of pancreas transplant on rapid insulin oscillations. Samples for glucose, insulin, and C-peptide were drawn during constant glucose infusion at 2-min intervals for 90 min from six successful Px patients with type I diabetes mellitus, from six normal nondiabetic control subjects, and from three Kx subjects. A computerized algorithm (ULTRA) was used for pulse detection. In the Px group, the average insulin pulse period was significantly shorter than in both the control and Kx groups (Px 8.1 +/- 0.5, control 12.5 +/- 0.7, Kx 12.4 +/- 0.5 min, P < 0.0005). By contrast, the C-peptide pulse periods (Px 16.8 +/- 2.3, control 14.7 +/- 1.2, Kx 15.3 +/- 1.5 min) were similar in the three groups. Spectral analysis confirmed that the frequency of the insulin pulses was increased in the Px group. The absolute amplitude of the insulin pulses was greater in the Px group (P < 0.001) while the amplitude of the C-peptide pulses did not differ between the groups. Cross-correlation analysis demonstrated maximal correlation coefficients at a lag of 0 min between insulin and C-peptide (control r = 0.33, P < 0.0001; Kx r = 0.17, P = 0.06) and between insulin and glucose (control r = 0.21, P < 0.001; Kx r = 0.20, P < 0.02) in the control and Kx groups, respectively, whereas no significant correlations were observed at any lag in the Px group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of eicosanoids in biosynthesis and secretion of insulin

A new technique of unipolar laparoscopic coagulation is particularly useful for the treatment of pelvic or abdominal bleeding during laparoscopic procedures. Blood present at the coagulation site is blown clear with a stream of argon gas prior to tissue coagulation. Of particular importance is the virtual absence of smoke, and because the procedure involves a nontouch technique, eschar is not removed inadvertently. The speed of tissue coagulation is faster than that of conventional bipolar and unipolar techniques.     

Reversal of somatostatin inhibition of insulin and glucagon secretion

These studies were designed to elucidate the mechanism of inhibitory action of somatostatin (SRIF) on glucagon (IRG) and insulin (IRI) secretion. Studies were carried out in the unrecirculated isolated rat pancreas perfusion with arginine 19.2 mM and glucose 5.5 mM as stimulus primarily for IRG but also IRI secretion. The effects of excess Ca++ (15.2 mEq./L.) and excess K+ (12.8 mEq./L.) on IRG, IRI, and the SRIF-inhibited pancreas were studied. Ca++ excess in five perfusions strikingly stimulated IRG secretion (+92 per cent) but only stabilized IRI secretion compared with control perfusions. K+ excess (in seven perfusions) markedly inhibited IRG secretion (-39 per cent) while stimulating IRI secretion (+16 per cent). Restoration of normal concentrations of K+ resulted in a rebound of IRG to levels 120 per cent that of controls. SRIF, at concentrations from 0.1-20 ng./ml., produced inhibition of both IRG and IRI. In 11 perfusions, with SRIF at 10 ng./ml., IRG decreased more than IRI (-75.2 per cent IRG and -46.9 per cent IRI). In five perfusions, addition of Ca++ (15.2 mEq./L.) 10 minutes after SRIF was started resulted in a reversal of IRG inhibition to 69.4 per cent and IRI to 73.2 per cent of the arginine controls. The reversal by Ca++ of SRIF effect on IRG was greater at higher concentrations of Ca++, suggesting some form of competition. In four perfusions, excess K+ reversed SRIF-induced IRI inhibition to 79.6 per cent that of controls but had no effect on IRG inhibition. Studies in vitro with isolated islets revealed that SRIF (2 mug./ml.) inhibited 45Ca uptake of islets as did epinephrine (10(-5) M). It was concluded that SRIF-induced inhibition of hormone release appears related to an action on Ca++ uptake.     

Temporal profiles and clinical significance of pulsatile insulin secretion

In this article, recent experiments are reviewed which have addressed the role of oscillatory insulin secretion in the pathophysiology of glucose intolerance and diabetes. The ultradian oscillations of insulin secretion appear to be an integral part of the feedback loop between glucose and insulin secretion and as a result are abnormal in states of glucose intolerance. Treatment of impaired glucose tolerance with troglitazone, a thiazolidinedione that improves insulin sensitivity, leads to an improvement in the ability of the beta-cell to sense and respond to a glucose stimulus restoring the ability of glucose to entrain the ultradian oscillations. The rapid oscillations of insulin secretion appear to be an inherent feature of the cellular mechanisms of insulin secretion since they persist in the isolated perfused pancreas and in perifused islets. These oscillations are paralleled by changes in intracellular Ca2+ and are also abnormal in states of glucose intolerance and diabetes. Available evidence indicates that these alterations are due to decreased expression of voltage-dependent Ca2+ channels on the beta-cell membrane.