Mechanism of insulin action on resting membrane potential of frog skeletal muscle

Disruption of Sur2-containing K(ATP) channels enhances insulin-stimulated glucose uptake in skeletal muscle

ATP-sensitive potassium channels (K(ATP)) are involved in a diverse array of physiologic functions including protection of tissue against ischemic insult, regulation of vascular tone, and modulation of insulin secretion. To improve our understanding of the role of K(ATP) in these processes, we used a gene-targeting strategy to generate mice with a disruption in the muscle-specific K(ATP) regulatory subunit, SUR2. Insertional mutagenesis of the Sur2 locus generated homozygous null (Sur2(-/-)) mice and heterozygote (Sur2(+/-)) mice that are viable and phenotypically similar to their wild-type littermates to 6 weeks of age despite, respectively, half or no SUR2 mRNA expression or channel activity in skeletal muscle or heart. Sur2(-/-) animals had lower fasting and fed serum glucose, exhibited improved glucose tolerance during a glucose tolerance test, and demonstrated a more rapid and severe hypoglycemia after administration of insulin. Enhanced glucose use was also observed during in vivo hyperinsulinemic euglycemic clamp studies during which Sur2(-/-) mice required a greater glucose infusion rate to maintain a target blood glucose level. Enhanced insulin action was intrinsic to the skeletal muscle, as in vitro insulin-stimulated glucose transport was 1.5-fold greater in Sur2(-/-) muscle than in wild type. Thus, membrane excitability and K(ATP) activity, to our knowledge, seem to be new components of the insulin-stimulated glucose uptake mechanism, suggesting possible future therapeutic approaches for individuals suffering from diabetes mellitus.

Insulin increases the turnover rate of Na+-K+-ATPase in human fibroblasts

Insulin stimulates K+ transport by the Na+-K+-ATPase in human fibroblasts. In other cell systems, this action represents an automatic response to increased intracellular [Na+] or results from translocation of transporters from an intracellular site to the plasma membrane. Here we evaluate whether these mechanisms are operative in human fibroblasts. Human fibroblasts expressed the alpha(1) but not the alpha(2) and alpha(3) isoforms of Na+-K+-ATPase . Insulin increased the influx of Rb+, used to trace K+ entry, but did not modify the total intracellular content of K+, Rb+, and Na+ over a 3-h incubation period. Ouabain increased intracellular Na+ more rapidly in cells incubated with insulin, but this increase followed insulin stimulation of Rb+ transport. Bumetanide did not prevent the increased Na+ influx or stimulation of Na+-K+-ATPase. Stimulation of the Na+-K+-ATPase by insulin did not produce any measurable change in membrane potential. Insulin did not affect the affinity of the pump toward internal Na+ or the number of membrane-bound Na+-K+-ATPases, as assessed by ouabain binding. By contrast, insulin slightly increased the affinity of Na+-K+-ATPase toward ouabain. Phorbol esters did not mimic insulin action on Na+-K+-ATPase and inhibited, rather than stimulated, Rb+ transport. These results indicate that insulin increases the turnover rate of Na+-K+-ATPases of human fibroblasts without affecting their number on the plasma membrane or modifying their dependence on intracellular [Na+].

Association of insulin resistance with salt sensitivity and nocturnal fall of blood pressure

Insulin resistance was demonstrated in hypertensive patients and in salt-sensitive subjects. It was recently reported that the salt-sensitive state was related to a reduced fall in blood pressure during the night in essential hypertension. In the present study, the relationship among insulin sensitivity, blood pressure response to salt intake, and nocturnal fall in blood pressure was examined in 20 subjects with nondiabetic and nonobese essential hypertension during a low-salt and a high-salt diet. The subjects were maintained on a low-salt diet (50 mmol/d) and a high-salt diet (255 mmol/d) for 1 week each, in random order. On the sixth day of each diet, blood pressure was measured every hour for 24 hours with an automatic device. Insulin sensitivity was measured according to the steady-state plasma glucose (SSPG) method on the seventh day of each diet. Salt-induced increase in blood pressure, which we defined as the change in 24-hour mean arterial pressure between the low and the high dietary salt intakes, was significantly correlated with SSPG (r=0.60, P<0.01) during the high-salt period. There was a significant negative correlation (r=-0.61, P<0.01) between SSPG and a nocturnal fall in mean arterial pressure during the high-salt period. Salt-induced increase in blood pressure was inversely correlated with a nocturnal fall in mean arterial pressure (r=-0.52, P<0.02) with the high-salt diet. These results suggest that insulin resistance, salt sensitivity, and failed nocturnal fall in blood pressure are associated with each other in subjects with essential hypertension.

Improvement of insulin resistance in essential hypertension by long-acting Ca antagonist benidipine

To investigate whether the long-acting Ca channel blocker, benidipine improves insulin resistance in patients with essential hypertension, insulin sensitivity was measured using the steady state plasma glucose (SSPG) method in 11 or 14 nonobese and nondiabetic hypertensive subjects before and after treatment with benidipine or placebo, respectively, and 11 healthy control subjects. SSPG level was significantly higher in two hypertensive groups, indicating reduced insulin sensitivity than in controls. SSPG level significantly decreased after benidipine treatment, with a decrease of blood pressure. SSPG level and blood pressure did not change in the placebo group. As for oral glucose tolerance test, the area under the curve of insulin diminished significantly after benidipine treatment. SSPG level significantly correlated with intra-platelet Ca2+ concentrations in 9 hypertensive subjects. The long-acting Ca channel blocker benidipine has partially improved insulin resistance in essential hypertension, contributing to the prevention of atherosclerosis associated with insulin resistance.

Modulation of ATP-sensitive K+ channel by insulin in rat skeletal muscle fibers

In the present study we evaluated the modulation of the sarcolemmal ATP sensitive K+ channel by insulin. The "in vivo" administration of insulin to the rats led to an hyperpolarization of the skeletal muscle fibers. This effect is antagonized by "in vitro" incubation of the muscle with glybenclamide, an ATP sensitive K+ channel blocker. Patch clamp experiments revealed that insulin enhanced the mean current of the ATP sensitive K+ channel by a factor of 1.4. This effect is mediated by an increase of the channel open probability, while no change occurred in the single channel conductance nor in the channel density. In the treated rats, the sensitivity of the channel to ATP and glybenclamide is abnormally reduced. Our results are consistent with an activation of the ATP sensitive K+ channel by insulin. This contributes to the hyperpolarization of the skeletal muscle fibers.

Phase-advanced daily rhythms of melatonin, body temperature, and locomotor activity in food-restricted rats fed during daytime

This study was performed to investigate possible effects of a timed caloric restriction on the light-dark (LD) synchronization of four biological rhythms pair-studied in the same animals. In Experiment 1, food-restricted rats kept under a photoperiod of 12 h light:12 h dark received 50% of previous ad libitum food 2 h after the onset of light. Their daily rhythm of pineal melatonin and rhythms of plasma melatonin and corticosterone were examined and compared to those of ad libitum control rats after 1 or 2 months of food restriction. A significant phase advance (about 2 h) was found for the pineal melatonin rhythm and for the daily onset of plasma melatonin. Timing of nocturnal peak of circulating corticosterone was unchanged, and a diurnal peak anticipated food presentation by about 2 h. In Experiment 2, effects of a timed caloric restriction under 12L:12D were studied on the expression of daily rhythms of body temperature and locomotor activity. To discriminate between the effects of timed meal feeding and those of the added caloric restriction, these rhythms were analyzed in food-restricted rats, as in Experiment 1, and were compared to those in sham-restricted rats, concomitantly fed twice more than food-restricted rats (i.e., a timed meal feeding without caloric restriction). Acrophase of the nocturnal peak of body temperature rhythm reached the greatest phase advance (7 h) in food-restricted rats, in which it was close to LD transition. The nocturnal component of locomotor activity rhythm also was markedly phase advanced (6 h) by caloric restriction, as indicated by wheel-running and general activity occurring form early afternoon to midnight. A smaller 4-h phase advance of the nocturnal peak of body temperature also was observed in sham-restricted rats, although the onset of locomotor activity rhythm apparently was unaffected by meal feeding and the end of activity rhythm was phase advanced by 2 h. These results indicate that timed caloric restriction is a potent phase-shifting agent that interacts with the LD cycle zeitgeber. This nonphotic stimulus phase advances melatonin, corticosterone, body temperature, and activity rhythms to different extents and thus suggests a change in the internal synchronization of the circadian system.

Insulin resistance rather than hyperinsulinemia more closely associated with essential hypertension

In order to clarify which of the two, insulin resistance or hyperinsulinemia, are more contributable to non obese and non diabetic hypertension, insulin sensitivity test was performed. By multiple regression analysis, mean, systolic, and diastolic blood pressure were inversely correlated with glucose clearance. During insulin sensitivity test, plasma catecholamines levels and FENa were not changed by insulin infusion. In the present study, it is demonstrated that insulin has no hypertensive effect under the mild hyperinsulinemia (45-55 microU/ml). We conclude that insulin resistance rather than hyperinsulinemia may be more closely associated with non obese and non diabetic hypertension.

Isoproterenol- and insulin-induced hyperpolarization in rat skeletal muscle

Using conventional microelectrode techniques, we investigated the combined effects of isoproterenol (Iso) and insulin (Ins) on the resting membrane potential (RMP) of isolated rat skeletal muscles. In soleus muscle, Iso (1 microM) and Ins (4 units/L) separately induced a hyperpolarization of 9.2 mV and 4.8 mV, respectively. Combined administration of Iso and Ins induced a hyperpolarization of 12.4 mV, larger than either one separately. A similar observation was made in Na(+)-loaded rewarming experiments. 8-Br-cAMP (1 mM and 3 mM) and forskolin (10 microM, an adenylate cyclase activator) induced a hyperpolarization of 5.3 mV, 8 mV, and 6.0 mV, respectively. This hyperpolarizing action was blocked by ouabain, indicating that the Na-K pump was involved in the hyperpolarization. 8-Br-cGMP (3 mM) had no effect on RMP; however, it blocked or reversed the hyperpolarization caused by 8-Br-cAMP (1 mM). In addition, 8-Br-cGMP partially inhibited the hyperpolarizing effect of Iso (1 microM) by 40% and completely prevented the effect of Ins. The phorbol ester, PMA, (1 microM, a PKC activator) induced a ouabain-inhibitable hyperpolarization. These results suggest that cAMP and PKC are involved in the Iso- and Ins-induced hyperpolarization and that Iso and Ins influence the RMP presumably through regulation of the electrogenic Na-K pump via different mechanisms.

Disparate effects of insulin on isolated rabbit afferent and efferent arterioles

Despite evidence that insulin per se may be an important regulator of glomerular hemodynamics, little is known about its direct action on the glomerular afferent arterioles (Af-Art) and efferent arterioles (Ef-Art), the crucial vascular segments that control glomerular hemodynamics. In the present study, we examined the direct effect of physiological concentrations of insulin on isolated microperfused rabbit Af- and Ef-Arts. After cannulation, vessels were equilibrated in insulin-free medium for 30 min. To determine whether insulin causes vasodilation or constriction, increasing doses (5, 20, and 200 microU/ml) were added to the bath and lumen of arterioles that were either preconstricted to 50% of control diameter with norepinephrine or left nonpreconstricted. Insulin caused no vasoconstriction in either Af- or Ef-Arts, but it reversed norepinephrine-induced constriction in Ef-Arts but not Af-Arts (suggesting a vasodilator action selective to the Ef-Art): at 200 microU/ml, insulin increased Ef-Art luminal diameter by 75.8 +/- 7.0% from the preconstricted level (n = 6; P < 0.008). The vasorelaxant effect of insulin on Ef-Arts was not affected by blockade of either endothelium-derived relaxing factor/nitric oxide or prostaglandin synthesis. Despite the lack of effect of insulin on Af-Art when added after the equilibration period, when Af-Arts were equilibrated in the presence of either 20 or 200 microU/ml insulin, their basal diameter was significantly reduced (11.7 +/- 0.9 microns; P < 0.025, n = 6, and 12.0 +/- 0.9 microns; P < 0.025, n = 7, respectively) compared with nontreated Af-Arts (16.2 +/- 1.3 microns; n = 7). In conclusion, this study demonstrates that at physiological concentrations, insulin dilates NE-constricted Ef-Arts, while insulin pretreatment enhances Af-Art tone. The disparate actions of insulin on the Af- vs the Ef-Art may contribute to its beneficial effect on glomerular hypertension.

Respiratory muscles in endocrinopathies

The aim of this review was to demonstrate that RM function is altered in various endocrinopathies and that RM weakness is a common finding. RM function has been well-studied in diseases such as thyroid dysfunction, and steroid induced RM myopathies. Less well documented reports on RM function were found in parathyroid dysfunctions, disorders of mineralocorticoids and pituitary disturbances. Controversial reports were found in diabetes mellitus. No report was found connecting RM function with androgens, pheochromocytoma or adrenaline deficiency in humans. These diseases could potentially cause RM impairment leading to severe respiratory failure (pump failure) putting life in great danger. Therefore, it is obvious that further studies are needed to investigate the performance of RMs in endocrinopathies. Such studies are extremely urgent in Cushing's and Addison's disease, acromegaly, disorders of the adrenal medulla, and in diabetes insipidus.

Epidemiologic and clinical aspects of insulin resistance and hyperinsulinemia

Epidemiologic studies have shown that insulin is a risk factor for coronary heart disease (CHD). Clinical studies have also demonstrated positive correlations between insulin and blood pressure, triglycerides, total cholesterol, fibrinogen, and plasminogen activator inhibitor. Moreover, there is an inverse correlation between insulin and high-density lipoprotein (HDL). These studies have provided evidence in support of the biologic plausibility of epidemiologic observations, but they have not clearly established insulin's role in the pathogenesis of human cardiovascular diseases (CVD) such as hypertension. In fact, there is considerable evidence that insulin resistance (abnormal nonoxidative glucose disposal), not hyperinsulinemia, is the primary insulin-related abnormality in human hypertension, and that hyperinsulinemia occurs as a response to insulin resistance. Skeletal muscle appears to be the primary site of insulin resistance in essential hypertension, although other organs, such as the kidneys and liver--key sites for cell and water homeostasis and lipoprotein regulation, respectively--may respond normally to insulin. Adipocytes also appear to be a site of insulin resistance. Thus, the putative interrelationship between hyperinsulinemia and insulin resistance, on the one hand, and with blood pressure and lipoproteins, on the other, is a complex one and may involve organ-specific insulin resistance. Altered cation transport is one of several mechanisms by which insulin resistance might raise blood pressure. The Na+, K(+)-ATPase and Ca(2+)-ATPase pumps are insulin sensitive. Thus, when insulin resistance is present, the activity of these pumps in the smooth muscle of the arterial wall might be reduced. This would lead to an intracellular accumulation of sodium and calcium, thereby sensitizing the vascular wall to pressor substances. Moreover, secondary hyperinsulinemia will occur, and insulin has been shown to stimulate sympathetic nervous system activity and to increase renal tubular absorption of sodium. Insulin is also a growth factor and therefore might have a trophic effect on the vessel wall, one that could initiate and/or sustain hypertension as well as atherosclerosis. Abnormal lipoprotein metabolism is yet another possible explanation for the accelerated atherosclerosis that has been observed in persons with abnormal carbohydrate tolerance and insulin resistance. Hyperinsulinemia and insulin resistance both play a role in the expression of elevated very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) levels as well as in the depression of HDL levels. Coronary risk reduction has been disappointing when blood pressure has been lowered with treatment regimens based on thiazide diuretics and/or beta blockers. Thiazides and some beta blockers may further impair tissue insulin sensitivity and often cause blood lipoprotein abnormalities.(ABSTRACT TRUNCATED AT 400 WORDS)

The sensitivity of insulin-stimulated and basal Na efflux to ouabain in frog skeletal muscle cells

1. The sensitivity of Na efflux to ouabain in frog skeletal muscle cells was studied in the presence and absence of insulin. 2. Insulin increased the ouabain-sensitive Na efflux, about two-fold, without any significant effect on the ouabain-insensitive Na efflux; i.e. all components of the Na efflux increased by insulin can be blocked by ouabain. 3. There was no significant difference between the time course of the inhibitory action of ouabain on Na efflux in the presence and absence of insulin: i.e. the binding affinity of the insulin-stimulated Na/K pump to ouabain is same as that of the control.

Protective effect of insulin on suppressed electrical activity caused by ouabain in isolated frog retina

The reduction in amplitude of the a-wave of the frog ERGs in response to light at seven different intensities recorded from isolated frog retinas (Rana catesbeiana) caused by ouabain (10(-5) M) was partially prevented dose-dependently by the insulin pre-treatment (between 6.6 x 10(-8) and 6.6 x 10(-5) M). Inversely, however, the already suppressed a-waves by ouabain could not be recovered by the application of insulin. The preventive effect of insulin pretreatment was also significantly dependent upon the stimulus intensity, but not upon the presence of glucose in the external media. The effect of insulin itself on the amplitude of the a-wave was below significance level over the range of its concentrations employed in the present experiments. It was suggested that this effect of insulin on the a-wave can be explained by assuming that insulin enhanced the Na-K pump activity under its suppressed conditions in visual cell.

Effects of increasing extracellular pH on protein synthesis and protein degradation in the perfused working rat heart

Increasing the extracellular pH over the range pH 7.4-8.9 stimulated protein synthesis by about 60% in the rat heart preparation anterogradely perfused in vitro. Protein degradation was inhibited by this pH increase. The magnitudes of the effects at pH 8.9 on protein synthesis and degradation were similar to those of high concentrations of insulin. Cardiac outputs were increased, as were cardiac phosphocreatine contents, indicating that the alterations in extracellular pH did not adversely affect the physiological viability of the preparation. ATP contents were unaltered. The creatine kinase equilibrium was used to assess the magnitude of the change in intracellular pH induced by these treatments. The increase in intracellular pH was about 0.2 for a 1-unit increase in extracellular pH. Thus small changes in intracellular pH have dramatic effects on cardiac protein turnover.

Food-deprivation-induced phase shifts in Sminthopsis macroura froggatti

Past research has shown that there is a circadian oscillator in laboratory rats that is entrained by restricted feeding schedules. However, in laboratory rats at least, the light-dark (LD) cycle is the dominant zeitgeber in the entrainment of wheel-running activity rhythms. Given that dasyurid marsupials are predominantly carnivorous, the episodic intake of food in the wild and the high nutritive content of that food suggest that food may be an important zeitgeber in these species. Twelve Sminthopsis macroura froggatti were presented with a daily meal at 0900 hr under an LD 12:12 cycle with lights-on at 0600 hr for 37 days. Activity in anticipation of the meal was observed in most animals. Following this, all animals were exposed to periods of 12-18 days ad lib. food interspersed with 3-day periods of deprivation--a technique used previously to demonstrate persistent meal-associated rhythms. The meal-associated activity rhythms previously observed in rats during the 3-day deprivation period were not seen, but the 3-day deprivation period produced large phase-shifts in the activity rhythms of several S.m. froggatti. It is concluded that meal feeding does not dominate the LD cycle in entraining dasyurid marsupials, but that the frequent observation of phase shifts suggests a different and, perhaps, stronger role for food intake in biological rhythmicity than has been observed previously in laboratory rats.

31P NMR study of insulin effects on the isolated perfused rabbit urinary bladder

Insulin stimulates hexose transport, intermediary metabolism, and cell growth and development. These effects are well-documented in skeletal but not smooth muscle. 31P NMR spectroscopy was performed on rabbit urinary bladders (n = 4) to characterize insulin's actions on smooth muscle. The bladder and its vasculature were surgically isolated from the animal and perfused with a PSS/red blood cell perfusate. After a control steady state was achieved (approx 1-2 h), insulin (0.100 mU/ml) was added to the perfusate. Relative levels of intracellular phosphorylated compounds, pH, and free Mg2+ were measured and compared to control values. Also, extracellular pH and fractional volume were assessed using phenylphosphonate, a 31P NMR extracellular pH and volume indicator. Insulin induced significant increases in PCr (16 +/- 9%) at the expense of Pi, intracellular pH (delta pH 0.24 +/- 0.07), and fractional extracellular volume (49 +/- 1%). Intracellular free Mg2+ and extracellular pH did not change. These results indicate that in situ smooth muscle is sensitive to physiological levels of insulin. In fact, insulin improves the energy state of smooth muscle cells and the overall tissue perfusion.

Effects of internal Na and external K concentrations on Na/K coupling of Na,K-pump in frog skeletal muscle

To clarify the dependency of the Na/K coupling of the Na,K-pump on internal Na and external K concentrations in skeletal muscle, the ouabain-induced change in membrane potential, the ouabain-induced change in Na efflux and the membrane resistance were measured at various internal Na and external K concentrations in bullfrog sartorius muscle. Upon raising the internal Na concentration from 6 mmol/kg muscle water to 20 mmol/kg muscle water, the magnitude of the ouabain-induced change in membrane potential increased about eightfold and the magnitude of the ouabain-induced change in Na efflux increased about fivefold while the membrane resistance was not significantly changed. As the external K concentration increased from 1 to 10 mM, the magnitude of the ouabain-induced change in membrane potential decreased (1/5.5 fold), while the magnitude of the ouabain-induced change in Na efflux increased (about 1.5-fold). The membrane resistance decreased upon raising the external K concentration from 1 to 10 mM (1/2-fold). These observations imply that the values of the Na/K coupling of the Na,K-pump increases upon raising the internal Na concentration and decreases upon raising the external K concentration.

Hypertension, non-insulin-dependent diabetes, and intracellular sodium metabolism

The present study was designed to investigate whether non-insulin-dependent diabetic hypertensive patients exhibit abnormalities in intracellular sodium metabolism similar to those described for essential hypertensive patients. Both normotensive and hypertensive non-insulin-dependent diabetic patients had similar average values of both Na+-Li+ countertransport and Na+-K+ cotransport compared with nondiabetic controls. Within the group of diabetic patients, hypertensive patients did not exhibit any abnormalities in either of the sodium transport pathways studied. The possible implications of these findings are addressed.

Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.

The effect of the internal Na concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscles

Insulin induced a hyperpolarization of the membrane by stimulating the Na,K-pump in frog skeletal muscles. The Na,K-pump activity was dependent on the internal Na concentration. As the internal Na concentration was raised from 5 mmol/kg muscle water to 18 mmol/kg muscle water, the magnitude of the insulin-induced increase in the ouabain-sensitive Na efflux (an index of the Na,K-pump activity) rose by 5-fold and the magnitude of the insulin-induced hyperpolarization rose by 8.5-fold. On the other hand, the specific membrane resistance was not significantly changed by a rise in the internal Na concentration. The Na/K coupling of the Na,K-pump was calculated at low, normal or high internal Na concentration by using the values of the insulin-induced changes in the ouabain-sensitive Na efflux and the membrane potential. As a result of the calculation, it was suggested that in frog skeletal muscles the Na/K coupling would increase with a rise of the internal Na concentration.

Insulin-induced changes in membrane potential and 3-O-methylglucose uptake at various external K concentrations in frog skeletal muscle

Insulin induced a hyperpolarization of the membrane and stimulated the 3-O-methylglucose (3-O-MG) uptake in frog skeletal muscle. In the present study, the relationship between the insulin-induced changes in the membrane potential and the 3-O-MG uptake was investigated. The stimulatory action of insulin on the 3-O-MG uptake was mediated by two different mechanisms. One of them was dependent on the change in the membrane potential and the other was independent of the change in the membrane potential. Both values of the insulin-induced changes in the membrane potential and the 3-O-MG uptake were diminished by increasing the external K concentration. One of the causes for the diminution of the 3-O-MG uptake with a rise of the external K concentration would be the decrease in the magnitude of the insulin-induced hyperpolarization.

Effects of feeding cycles on circadian rhythms in squirrel monkeys

Squirrel monkeys (Saimiri sciureus) were housed singly in cages equipped with a tree for climbing to measure locomotor activity, and with a movable food cup that could be arrested automatically. The animals were kept in continuous dim illumination (LL), twice interrupted by several weeks of entrainment by a light-dark (LD) 12:12 cycle. Apart from three control sections in which the food cups were unlocked continuously (ad libitum feeding), food was accessible for 3 hr per day only, with interfeeding intervals varying from 23 to 26 hr (periodic restricted feeding, or RF). During LD entrainment, the imposition of an RF schedule resulted in anticipatory behaviors, represented by increased tugs at the food cup and a pause in locomotor activity preceding the feeding time. In LL, the animals showed free-running circadian rhythms of locomotor and "feeding" activity that nearly always persisted when ad libitum feeding was replaced by RF. The period (tau) of the free-running rhythm was slightly modulated in relation to the varying interfeeding intervals (T), but entrainment was never achieved except in one test with an animal whose tau was very close to T. It is concluded that periodic availability of food represents an extremely weak zeitgeber, if any, for the circadian pacemaker of squirrel monkeys.

Effects of external K concentration on the electrogenicity of the insulin-stimulated Na,K-pump in frog skeletal muscle

Insulin hyperpolarized the membrane of frog skeletal muscle by stimulating the electrogenic Na,K-pump. At external K concentrations of 1,2,5 and 10 mM, both the insulin-induced hyperpolarization and the insulin-stimulated ouabain-sensitive Na efflux (an index of Na,K-pump activity) were observed. By increasing the external K concentration, the insulin-stimulated Na efflux increased, but the magnitude of the insulin-induced hyperpolarization decreased; i.e., although the activity of the insulin-stimulated Na,K-pump increased, on the contrary, the magnitude of the hyperpolarization decreased. To clarify the causes of this phenomenon, the specific membrane resistance was measured and found to decrease upon increasing the external K concentration. One of the reasons for the decrease in magnitude of the hyperpolarization is the decrease in the specific membrane resistance. However, the decrease in magnitude of the hyperpolarization with a rise of the external K concentration, which increased the insulin-stimulated Na,K-pump activity, cannot be explained only by the decrease in the specific membrane resistance. It is suggested that the decrease in magnitude of the hyperpolarization is mainly caused by a decrease in the electrogenicity of the insulin-stimulated Na,K-pump upon an increase in the external K concentration. The conclusion of the present study is that the electrogenicity of the insulin-stimulated Na,K-pump in muscles is variable and decreases with increasing the external K concentration.

Relationship of muscle growth in vitro to sodium pump activity and transmembrane potential

Serum stimulates embryonic avian skeletal muscle growth in vitro and the growth-related processes of amino acid transport and protein synthesis. Serum also stimulates myotube Na pump activity (measured as ouabain-sensitive rubidium-86 uptake) for at least 2 h after serum addition. Serum-stimulated growth depends on this Na pump activity since ouabain added at the same time as serum totally inhibits the growth responses. The relationship of myotube growth, Na pump activity, and transmembrane potential was studied to determine whether serum-stimulated Na pump activation and growth are coupled by long-term membrane hyperpolarization. When myotube amino acid transport and protein synthesis are prestimulated by serum, ouabain was found to have little inhibitory effect, indicating that the already stimulated growth-related processes are not tightly coupled to continued Na pump activity. Serum-stimulated protein synthesis is tightly coupled to Na pump activity, but only during the first 5-10 min after serum addition. When myotube transmembrane potentials were measured using the lipophilic cation tetraphenylphosphonium, serum at concentrations that stimulate myotube growth and Na pump activity was found to have little effect on the cell's transmembrane potential. Furthermore, partial depolarization of the myotubes with 12- to 55-mM extracellular potassium does not prevent serum stimulation of myotube growth. Monensin was found to hyperpolarize the myotubes, but causes myotube atrophy. These results indicate that although Na pump activity is associated with initiation of serum-stimulated myotube growth, continued Na pump activity is not essential, and there is little relationship between myotube growth and the myotube's transmembrane potential.

Effects of streptozotocin diabetes and fasting on intracellular sodium and adenosine triphosphate in rat soleus muscle

The hypothesis that part of the insulin transduction system consists of a co-ordinated stimulation of the Na pump and of Na-H exchange by the hormone (Moore, 1981) requires that insulin plays a physiological role in the regulation of intracellular Na+. Moreover, this model predicts that in hypoinsulinaemic states, such as diabetes and fasting, intracellular pH and intracellular ATP levels would be depressed. The present study tests the hypothesis that in hypoinsulinaemic states intracellular Na+ is increased and intracellular ATP is decreased by measuring these parameters in soleus muscles removed from both diabetic and fasted rats. When rats were made diabetic by injection of streptozotocin (SZ) plasma insulin significantly decreased by 24 hr and plasma glucose and triglyceride levels increased. Intracellular Na+ was significantly elevated by 48 hr after injection of SZ. The elevation ranged from 18 to 48% and persisted for the duration of the experimental observation (up to 28 days). Intracellular ATP decreased significantly by the seventh day after SZ injection and remained depressed by about 24% for the duration (35 days) of the observation. In one series, a significant negative correlation was seen between plasma insulin levels and intracellular Na+ of both SZ-diabetic animals and their controls. Intracellular Na+ also significantly increased when hypoinsulinaemia was induced by fasting. Again, intracellular ATP did not decrease until after the elevation of intracellular Na+. After 72 hr of fasting, intracellular ATP was still decreased in spite of normal plasma glucose levels. Insulin therapy of SZ diabetic rats restored intracellular ATP and plasma glucose to normal, but did not restore intracellular Na+ to normal levels. The results confirm two predictions of the 'insulin transduction system model (Moore, 1981). Most strongly supported is that part of the model which indicates that the Na pump is regulated by physiological levels of insulin. This is especially convincing since hypoinsulinaemia produced by a non-pharmacological procedure, fasting, was associated with an increase in intracellular Na+.

Free-running activity rhythms in the rat: entrainment by melatonin

The pineal gland hormone melatonin may play a role in synchronization of rat circadian rhythms. Free-running activity rhythms of the rat were entrained by a daily melatonin injection, with entrainment occurring when the onset of activity coincided with the time of daily injections. When injections were stopped, activity rhythms became free-running again. Thus in pharmacological experiments, the time of day of melatonin administration is crucial.

Transport of electrolytes in muscle

The muscle fiber stands alongside the red blood cell and the giant axon as one of the three classical cell types that have had major application in investigating ion transport processes in cell membranes. Of these three cell types, the muscle fiber was the first to provide definite evidence for a sodium pump. The ability of the sodium pump to produce an electrical potential difference across the cell membrane was also first demonstrated in muscle fibers. This important property of the sodium pump is now known to have physiological significance in many other types of cells. In this review, electrolyte transport investigations in skeletal muscle are traced from their inception to the current state of the field. Applications of major research techniques are discussed and key results are summarized. An overview of electrolyte transport in muscle, this article emphasizes relationships between the muscle fiber membrane potential and ionic transport processes.

The comparison of vanadyl (IV) and insulin-induced hyperpolarization of the mammalian muscle cell

Extracellularly applied vanadyl (IV) hyperpolarized the membrane potential of mouse diaphragm muscle from about -74.0 mV up to -81.7 mV. The hyperpolarizing effect of 10(-4) mol.I-1 vanadyl (IV) is comparable with hyperpolarization induced by 100 mU.ml-1 insulin. Both compounds increased the intracellular K+ concentration, the hyperpolarizing effect of vanadyl (IV) and insulin is blocked by ouabain and is unaffected by removal of K+ from the external medium. Triggering of the release of intracellular K+ associated with cellular proteins is proposed as the mechanism of vanadyl (IV) and insulin-induced hyperpolarization.

Rapid hyperpolarization of rat skeletal muscle induced by insulin

It has been proposed that the increase produced by insulin in electrical potential differences across membranes of target cells may be a mechanism by which the cell surface insulin-receptor complex causes at least some of the metabolic effects of insulin. If insulin-induced hyperpolarization is a transducer of common effector responses it must precede those responses. The problem has not been addressed previously, so that rapid responses to insulin have not been sought. Two methods were used. In one method, the bathing solution was changed rapidly so as to include insulin in supramaximal concentrations, and a series of measurements of membrane potentials. Er, were made. Insulin hyperpolarized by 9.4 mV within 1 min. In the other method, nanoliter amounts of highly concentrated insulin solution were ejected from a micropipette onto the surface of an impaled muscle fiber. In 21 out of 32 insulin injections, hyperpolarization occurred with 1 s; in 11 control injections there was no change. This is the most rapid response to insulin yet reported, and is consistent with the hypothesis that insulin-induced hyperpolarization may transduce effector responses.

Stimulation of Na:H exchange by insulin

In frog skeletal muscle, the increase of intracellular pH (pHi) induced by insulin is correlated with an increase in intracellular Na+ when the sodium pump is inhibited by ouabain. Reversing the Na+ free energy gradient by substituting either Mg2+ or choline for extracellular Na+ converts the effect of insulin to a decrease in pHi, indicating that the action of insulin upon pHi is determined by the Na+ free energy gradient. Moreover, estimates of the Na+ free energy gradient indicate that both the direction and magnitude satisfy the hypothesis that this is the source of energy for the observed changes in pHi. Both the increase in intracellular pH induced by insulin and the associated increase in intracellular Na+ produced by this hormone in the presence of ouabain are blocked by amiloride. This drug also blocks the decrease in pHi by insulin when Mg2+ is substituted for Na+ in the Ringer. In Ringer containing Na+, the increase in pHi by insulin occurs when both metabolic and atmospheric sources of CO2 are eliminated by using a 100% N2 atmosphere. Thus, the mechanism stimulated by insulin is not a Na+-CO3(2-) cotransport system, but is either an Na:H exchange or a Na+-OH- cotransport system which can be inhibited by amiloride. The suggestion is advanced that the Na:H exchange mechanism is part of the membrane transduction system for insulin.

Mechanism, prevention and therapy of sodium-dependent hypertension

This review considers the mechanism, prevention and therapy of sodium-dependent, low-renin, presumably volume-expanded, hypertension. Certain evidence suggests that in susceptible persons the basic problem is a genetic or acquired deficiency in the ability of the kidney to excrete sodium and hence water. This places them at a disadvantage in a society such as ours in which the salt intake is uniformly high, to a large extent because of the salt content in commercially processed foods. Other evidence suggests that the blood pressure level rises in part because the volume expansion evokes the release of an unknown, slowly-acting, pressor agent which operates by stimulating the contractility of cardiovascular muscle through suppression of the cellular sodium-potassium pump, much in the manner of the cardiac glycosides. Several investigators and the Select Committee on GRAS Substances suggest that the incidence of salt-dependent hypertension could be significantly decreased in a society such as ours if salt intake were reduced from the present level of approximately 10 g/day to 12 g/day. An obvious starting point is a reduction of the salt content in processed foods. The Food and Nutrition Board of the National Research Council suggest that a judicious combination of dietary sodium restriction and the use of an appropriate diuretic is the most rational approach to the treatment and management of diseases characterized by retention of sodium.