The effect of insulin on the washout of [45Ca]calcium from adipocytes and soleus muscle of the rat

The role of Ca2+ and calmodulin in insulin signalling in mammalian skeletal muscle

The role of Ca2+ in mediating effects of insulin on skeletal muscle has been widely debated. It is believed that in skeletal muscle Ca2+ has a permissive role, necessary but not of prime importance in mediating the stimulatory actions of insulin. In this review, we present evidence that insulin causes a localized increase in the concentration of Ca2+. Specifically, insulin induces a rise in near-membrane Ca2+ but not the bulk Ca2+ in the myoplasm. The rise in near-membrane Ca2+ is because of an influx through channels that can be blocked by L-type Ca2+ channel inhibitors. Calcium appears to exert some of its subsequent effects via calmodulin-dependent processes as calmodulin inhibitors block the translocation of glucose transporters and other enzymes as well as the insulin-stimulated increase in glucose transport.

Modulatory effect of insulin on release of calcium from human fibroblasts by angiotensin II

BACKGROUND

Angiotensin II stimulates synthesis and deposition of collagen and might contribute to the vascular and cardiac dysfunction associated with arterial hypertension. Insulin attenuates angiotensin II-induced responses of intracellular Ca2+ concentration ([Ca2+]) in many cell types but this effect is less in insulin-resistant states. The mechanisms of the interaction between insulin and angiotensin II are still not known.

OBJECTIVE

To characterize the effects of angiotensin II on intracellular [Ca2+] and the effects of insulin on the angiotensin II-induced response of intracellular [Ca2+] in human skin fibroblasts.

METHODS

Spectrofluorophotometric measurements of intracellular [Ca2+] in monolayers of cultured human skin fibroblasts from 15 normotensive patients were performed using Fura-2 at 510 nm emission with excitation wavelengths of 340 and 380 nm.

RESULTS

Basal intracellular [Ca2+] in quiescent (24 h serum-deprived) human fibroblasts was 75 +/- 3 nmol/l (n = 20). Administration of angiotensin II elevated intracellular [Ca2+] dose-dependently with a concentration for half-maximal effect of 20 nmol/l. Administration of 100 nmol/l angiotensin II stimulated a rapid and transient increase in intracellular [Ca2+] (from 75 +/- 3 to 130 +/- 2 nmol/l, n = 20). Removal of extracellular calcium did not change peak intracellular [Ca2+], but it did reduce the time to recovery of [Ca2+] (from 64 +/- 4 to 48 +/- 2 s, n = 10, P < 0.01), suggesting that an angiotensin II-induced transmembrane calcium influx had occurred. This hypothesis was confirmed by quenching studies with manganese. The angiotensin II-induced changes in intracellular [Ca2+] were completely blocked by administration of 100 nmol/l of the angiotensin II type 1 receptor inhibitor losartan but not by administration of 100 nmol/l of the angiotensin II type 2 receptor blocker CGP42112A. Acute (20 min) exposure to 100 nmol/l insulin did not alter basal intracellular [Ca2+] in quiescent fibroblasts, but significantly blunted angiotensin II-stimulated peak of [Ca2+] (to 101 +/- 3 nmol/l, P < 0.01, n = 18) and delayed recovery of [Ca2+] (to 99 +/- 5 s, P < 0.01). The inhibitory effect of insulin was observed both with and without extracellular Ca2+.

CONCLUSIONS

Our results demonstrate that administration of angiotensin II increases intracellular [Ca2+] in human skin fibroblasts by release of Ca2+ from intracellular Ca2+ stores and by influx of Ca2+ and that administration of insulin attenuates the response of [Ca2+] to angiotensin II but prolongs the time to recovery of [Ca2+].

Effects of insulin on calcium metabolism and platelet aggregation

The influence of insulin on platelets in vitro has not been exhaustively investigated. To clarify whether insulin affects Ca2+ metabolism in platelets directly or through alteration of other systems regulating intracellular Ca2+ homeostasis, we examined the effect of insulin both alone and in combination with prostaglandin E1 on platelet aggregation and Ca2+ metabolism. Incubation of rat platelets with insulin reduced thrombin-induced Ca2+ influx but did not change thrombin-evoked release of Ca2+ from internal stores or the size of internal Ca2+ stores. The interactive effects of insulin with prostaglandin E1 were only additive, and insulin did not augment the effects of prostaglandin E1 on platelet Ca2+ metabolism. In contrast, insulin did not inhibit thrombin-induced platelet aggregation but did augment inhibition of platelet aggregation by prostaglandin E1. Our results suggest that insulin inhibits platelet function by both prostaglandin E1-dependent and -independent mechanisms.

Insulin attenuates agonist-mediated calcium mobilization in cultured rat vascular smooth muscle cells

Insulin has been shown to attenuate pressor-induced vascular contraction, but the mechanism for this vasodilatory action is unknown. This study examines the effect of insulin on angiotensin II (ANG II)-induced increments in cytosolic calcium in cultured rat vascular smooth muscle cells (VSMC). 20-min incubations with insulin (10 microU/ml to 100 mU/ml) did not alter basal intracellular calcium concentration ([Ca2+]i), but inhibited the response to 100 nM ANG II in a dose-dependent manner (ANG II alone, 721 +/- 54 vs. ANG II + 100 mU/ml insulin, 315 +/- 35 nM, P < 0.01). A similar effect of insulin on ANG II action was observed in calcium poor buffer. Moreover, insulin did not effect calcium influx. ANG II receptor density and affinity were not affected by 24-h incubation with insulin. To further clarify the mechanisms of these observations, we measured ANG II-induced production of inositol 1,4,5-triphosphate (IP3), and IP3-releasable 45Ca. Insulin treatment did not alter ANG II-stimulated IP3 production. However, IP3-stimulated release of 45Ca in digitonin permeabilized cells was significantly reduced after 5-min incubations with 100 mU/ml insulin. Thapsigargin induced release of calcium stores was also blocked by insulin. Thus, insulin attenuates ANG II-stimulated [Ca2+]i primarily by altering IP3-releasable calcium stores. Insulin effects on ANG II-induced [Ca2+]i were mimicked by preincubation of VSMC with either sodium nitroprusside or 8-bromo-cGMP. As elevations in cGMP in vascular tissue lower [Ca2+]i, it is possible that insulin affects IP3 release of calcium by a cGMP-dependent mechanism that would contribute to its vasodilatory effects.

Impaired calcium metabolism associated with hypertension in Zucker obese rats

Recent data from our laboratory indicate that reduced membrane Ca-adenosine triphosphatase (ATPase) activity in non-insulin-dependent diabetics may be responsible for increases in intracellular calcium and, consequently, for elevated vascular resistance. Since obesity is frequently associated with hypertension, even before the development of overt diabetes, we evaluated blood pressure and erythrocyte cation levels and membrane Na/K-ATPase and Ca-ATPase in Zucker obese rats and their lean controls (n = 10 per group). Intra-arterial blood pressure, determined via a femoral cannula, demonstrated elevated systolic and diastolic pressure in the obese rats (P less than .05). There were no significant differences in Na/K-ATPase between groups, but there was a decrease in Ca-ATPase (P less than .01) in the obese rats and an increase in tissue and cellular calcium content (P less than .05). These data demonstrate a specific impairment in membrane Ca-ATPase activity in obese rats they may have caused the observed increase in cellular calcium and, consequently, increased blood pressure. These phenomena may result from impaired insulin activation of membrane Ca-ATPase in these insulin-resistant animals.

A patient with hyperkalemia and metabolic acidosis

Uptake of potassium by extrarenal tissues, primarily muscle and liver, represents a major defense mechanism in the maintenance of normokalemia following an acute elevation in the serum potassium concentration. Insulin, epinephrine, and aldosterone all play major roles in maintaining the normal distribution of potassium between the intracellular and extracellular environment. In addition to hormonal regulation, changes in blood pH and tonicity also exert a strong influence on extrarenal potassium metabolism. Last, the serum potassium concentration per se directly influences its own cellular uptake and this transport mechanism appears to be inhibited by uremia.

Polymyxin B inhibits stimulation of glucose transport in muscle by hypoxia or contractions

Glucose transport can be stimulated via two separate pathways in muscle. One is activated by insulin, the other by contractile activity and hypoxia. Polymyxin B, a cationic antibiotic that displaces Ca2+ from anionic phospholipids, is reported to selectively inhibit the stimulation of glucose transport by insulin in muscle. A purpose of the present study was to determine whether the inhibition by polymyxin B is actually restricted to insulin. We found that polymyxin B (250 micrograms/ml) significantly inhibited the stimulation of glucose transport in rat skeletal muscles not only by insulin and vanadate but also by hypoxia, electrical stimulation, and K+. Polymyxin B also decreased the tension developed in response to electrical stimulation or K+. Although polymyxin B inhibited the increase in sugar transport activity induced by insulin and hypoxia, it had no inhibitory effect on sugar transport after it had been stimulated by these agents. These results show that the inhibitory effect of polymyxin B on the stimulation of glucose transport is not specific for insulin action. They suggest that polymyxin B inhibits a step that is common to the two pathways for stimulating glucose transport in skeletal muscle.

Proposal for a pathway to mediate the metabolic effects of insulin

This review seeks to assemble recent discoveries about insulin receptor/kinase, guanine nucleotide-binding proteins, phosphatidyl inositol metabolism, and protein phosphatases to provide a mechanistic pathway by which insulin would alter carbohydrate and fat metabolism. It proposes a hypothetical chain of events that leads from the insulin receptor to protein phosphatase-1. The sequence starts with insulin binding to its receptor, activating the intrinsic receptor/kinase activity. The insulin receptor phosphorylates a guanine nucleotide-binding protein, which activates a particular phospholipase C. This in turn stimulates the production of two lipid-derived messengers: inositol-phospho-glucosamine and diacylglycerol. These messengers trigger the effects of insulin. The diacylglycerol produced by insulin is thought to be analogous to the diacylglycerol produced by alpha-adrenergic stimulation, which activates protein kinase C. Activation of this kinase could account for increases in phosphorylation of certain proteins. The inositol-phospho-glucosamine is the cytosolic messenger for insulin. One of the enzymes activated by insulin is protein phosphatase type-1. It is known that the phosphatase decreases phosphorylation of certain target enzymes. In response to insulin, activation of protein phosphatase type-1 occurs with a stable conformational change that may involve rearrangement of disulfide bonds. Rearrangement is either directly in response to the cytosolic messenger or is catalyzed by an isomerase activated by the insulin messenger. Ultimately, protein phosphatase type-1 and/or the disulfide isomerase may together mediate the pleiotropic effects of insulin on carbohydrate and fat metabolism.

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.

Control of placental glucose transfer

There is little evidence to suggest that the membrane transfer mechanism of the placenta for glucose becomes saturated until maternal blood glucose concentrations are quite high. Also, recent evidence suggests that the membrane transport system for glucose in the placenta is not stimulated by maternal or fetal insulin. Furthermore, there is no solid evidence that hormonal or non-hormonal factors function in vivo to limit membrane transport of glucose in the placenta. Therefore, the limited data which are available suggest that there are no specific mechanisms which acutely regulate placental membrane transport of glucose, and that this membrane transport mechanism operates to maximize maternal-to-fetal glucose transfer. The rate of maternal-to-fetal glucose transfer is a function of the transplacental concentration gradient. This gradient appears to be under the control of fetal insulin and placental lactogen. The available data suggest that both hormones act to increase this concentration gradient: insulin by decreasing fetal blood glucose, and placental lactogen by both decreasing fetal and increasing maternal blood glucose concentrations. Furthermore, high rates of glucose uptake by fetal erythrocytes tend to promote maintenance of this concentration gradient. Therefore, these influences of the maternal-fetal concentration gradient promote transplacental glucose flux to the fetus. As illustrated by the fetal complications associated with maternal hyperglycaemia, the cellular and organismic physiology of the fetus and placenta appears to maximize, rather than optimize, glucose availability to the fetus. It may be, however, that during normal pregnancy, maximal availability is optimal for fetal development.

Stimulation of Na+-Ca2+ exchange in heart sarcolemma by insulin

Insulin was found to stimulate Na+-dependent Ca2+ uptake in dog heart sarcolemma in a concentration dependent manner (0.001 to 1 milliunits/ml). Maximal stimulation (160 to 170%) was seen at 0.1 to 1 milliunits/ml of insulin. Unlike Na+-dependent Ca2+ uptake, ATP-dependent Ca2+ uptake was unaltered by 1 microunit/ml of insulin. However, high concentrations of insulin (0.01 to 1 milliunits/ml) significantly increased the ATP-dependent Ca2+ uptake activity of heart sarcolemma; maximal increase (60%) was observed at 1 milliunit/ml of insulin. The Na+ K+-ATPase activity did not change upon incubating sarcolemma with insulin. The membrane preparation exhibited specific insulin binding characteristics. The Scatchard plot analysis of the data indicated two binding sites for insulin; the association constants for the high and low affinity sites were 2 X 10(9) M-1 and 4.4 X 10(8) M-1, respectively. These results support the view regarding the presence of insulin receptors in the heart cell membrane and indicate a dramatic effect of insulin on the sarcolemmal Ca2+ transport systems.

Energetics of active sodium-potassium transport following stimulation with insulin, adrenaline or salbutamol in rat soleus muscle

The total metabolic energy expenditure associated with active Na-K-transport over the first 20 min of stimulation with insulin, adrenaline or salbutamol (delta HmNa-K) was determined from direct calorimetric and tracer ion flux measurements in isolated muscles at rest. The reversible work performed by the Na-K-pump during the same interval of time (WrevNa-K- was calculated as the product of the ouabain-suppressible Na-K transfers and the mean free energy increase imparted to the two ions as they are transported against their electrochemical gradients across the plasma membrane. Comparison of membrane potential and intracellular Na and K concentrations before and after the stimulation indicated that part of WrevNa-K had contributed to increase in ion electrochemical gradients in the preparation (i.e. had not been lost as heat) during the 20 min period. Accordingly, the maximum value of delta HmNa-K was taken as the sum of the ouabain-suppressible heat production and WrevNa-K. Following stimulation with insulin, adrenaline or salbutamol this maximum corresponded to 10, 10 and 12% respectively, of basal metabolism. Under the same three conditions, the minimum "energetic efficiency" of the active Na-K-transport process, defined as the ratio between WrevNa-K and maximum delta HmNa-K, was 35, 41 and 38%, respectively.

Calcium control of glycogen synthase activities in mouse diaphragms, rat adipocytes and rat hepatocytes

The following article provides evidence that cellular calcium controls the activity of glycogen synthase in all three major glycogen storage tissues; muscle, fat, and liver. Depletion of cellular calcium resulted in a moderate increase of glycogen synthase %I activities in intact mouse diaphragms, in isolated rat adipocytes, and in rat hepatocytes. The increase in %I activity of glycogen synthase was more pronounced when the uridine di-phosphoglucose concentration in the glycogen synthase assay was lowered from 4.4 mM to 0.2 mM. Calcium depletion resulted in an approximately two-fold decrease in the Ka values for glucose-6-phosphate in all three tissues. The activities of glycogen synthase also correlated well with the content of cell-associated calcium in rat hepatocytes. The glucose-6-phosphate independent activities of glycogen synthase in extracts of calcium-replete and calcium-depleted tissue approached the same value following the exposure to crude phosphoprotein phosphatase. The activities of glycogen phosphorylase decreased in calcium-depleted tissues and cells. Insulin stimulated the activity of glycogen synthase in muscle and fat in the absence of added sugar and in the absence of extracellular calcium. It is concluded that glycogen synthase is under the control of calcium in the three main glycogen storage tissues. The actions of calcium are probably mediated through the actions of calcium-sensitive protein kinase(s).

Hepatic insulin effects independent of changes in Ca++ metabolism

The possibility of Ca++ acting as second messenger for insulin in rat liver was investigated using the net stimulation of 14C-glucose incorporation into glycogen by isolated hepatocytes as an index of insulin action. An insulin effect could be partially sustained in the virtual absence of Ca++ and Mg++ and a maximal insulin effect could be observed in the presence of either Ca++ or Mg++, suggesting that extracellular Ca++ is not required for insulin action. Inhibiting the activity of calmodulin, an intracellular mediator of Ca++ action, with trifluoperazine had little effect on insulin action. The efflux of 45Ca from prelabeled hepatocytes was not altered by the presence of insulin arguing against insulin-induced changes in Ca++ fluxes. Collectively, these results do not support the role of Ca++ as second messenger for insulin action in liver.

The effects of calcium on protein turnover in skeletal muscles of the rat

Several experimental procedures were used to increase the intracellular concentration of Ca2+ and determine its effects on protein turnover in isolated extensor digitorum longus and soleus muscle. These methods included the use of ionophore A23187, caffeine, dibucaine, thymol and procaine, all agents known to induce the release of calcium by acting either on the sarcolemma and/or on the sarcoplasmic reticulum. Another approach involved varying the external concentration of Ca2+ in the media in which the muscles were incubated. The changes in muscle Ca2+ concentrations after exposure to the various calcium-releasing agents were in keeping with accepted modes of action of these agents on muscle membranes. The findings suggest that increasing the sarcoplasmic concentration of Ca2+ inhibits protein synthesis and enhances protein breakdown. These catabolic effects of Ca2+ are compared with the changes induced in muscle protein turnover after exposure to insulin or cyclic nucleotides, and in myopathic muscle and situations of work overload. Attention is also drawn to some of the difficulties involved in definitively implicating Ca2+ as a factor involved in the normal regulation of protein turnover.

Insulin-induced enhancement of uptake of noradrenaline in atrial strips

1 Addition of insulin to the organ bath increased the force of contraction of guinea-pig left atrial strips driven electrically at 1 Hz. 2 The positive inotropic response to insulin remained unaltered in atria depleted of catecholamine or when beta-adrenoceptors were blocked by addition of propranolol to the organ bath. 3 The response os isolated atria to noradrenaline was significantly reduced in the presence of insulin. 4 Insulin affected neither the calcium accumulating abilities of the heart sarcolemma, mitochondria or microsomes, nor the cyclic adenosine 3',5'-monophosphate (cyclic-AMP)-protein kinase-induced stimulation of microsomal calcium uptake. 5 Addition of insulin to the organ bath enhanced significantly the ability of the cardiac tissue to take up [3H]-noradrenaline as well as [3H]-metaraminol. The activities of monoamine oxidase and catechol-O-methyl transferase were not changed after addition of insulin to homogenates of the heart. 6 The ability of insulin to facilitate uptake of noradrenaline would be expected to cause a decrease in the amount of the amine reaching the receptors, thus leading to a diminished response to this amine. This may explain, at least in part, insulin-induced subsensitivity to noradrenaline. 7 This view is supported by the observation that after blockade of amine uptake by destruction of nerve terminals, insulin failed to reduce the positive inotropic response to noradrenaline.

A (Ca2+, Mg2+)-ATPase activity in plasma membrane fragments isolated from squid nerves

A (Ca2+, Mg2+)-ATPase activity and a (Ca2+, Mg2+)-dependent phosphorylation from ATP have been found in plasma membrane fragments from squid optical nerves under conditions where contamination by intracellular organelles is unlikely. The properties of this (Ca2+, Mg2+)-ATPase activity are almost identical to those of the ATP-dependent uncoupled Ca2+ efflux observed in dialyzed squid giant axons. This gives further support to the notion that the mechanism responsible for maintaining the low levels of ionized Ca concentration in nerves at rest is not a Na+-Ca2+ exchange system but an ATP-driven uncoupled Ca2+ pump.

Insulin inhibition of alpha-adrenergic actions in liver

The effects of insulin on alpha-agonist (phenylephrine)- and [Arg8]vasopressin-induced Ca2+ and glucose release and mitochondrial Ca2+ fluxes in isolated perfused rat livers were examined. Insulin (6 nM) inhibited the ability of phenylephrine (1 and 0.5 microM) to elicit Ca2+ and glucose release, whereas it was without effect on vasopressin (10 and 2.5 nM) actions. Correspondingly, insulin inhibited the action of phenylephrine to induce a stable increase in mitochondrial Ca2+ uptake, but it did not affect the alteration caused by vasopressin. Phenylephrine and vasopressin caused transient increases in hepatocyte respiration. Insulin inhibited the effect of phenylephrine on this parameter, but not that of vasopressin. Insulin added alone did not alter any of the above parameters. It is concluded from these data that insulin does not alter cellular Ca2+ fluxes and respiration themselves, but selectively inhibits alpha-adrenergic stimulation of these processes. It is proposed that insulin acts either to inhibit binding of alpha-agonists to their specific plasma-membrane receptors or to alter generation and/or degradation of the putative alpha-adrenergic 'second messenger'. If this latter possibility is the case, then the alpha-adrenergic 'second messenger' must be different from the 'second messenger' of vasopressin.

The effect of insulin on the electrophoretic mobility of rat hepatocytes

The addition of insulin (10 microU) to a suspension of isolated hepatocytes in Krebs-Ringer bicarbonate solution, causes an increase in the negative electrophoretic mobility of the cells from - 1.68 micrometer sec-1 V-1 cm to 2.26 micrometer sec-1 V-1 cm. This observation supports the findings by other workers that the binding of insulin to its receptor leads to a marked change in the membrane.

The relationship between the transport of glucose and cations across cell membranes in isolated tissues. X. Effect of glucose transport stimuli on the efflux of isotopically labelled calcium and 3-O-methylglucose from soleus muscles and epididymal fat pads of the rat

(1) The relationship between Ca2+ and sugar transport has been studied by comparing the washout of 45Ca and 3-O-[14C]methylglucose from preloaded isolated rat soleus muscles and whole epididymal fat pads. (2) In soleus muscle, nine different agents with well established stimulating effects on glucose transport were all found to produce a marked increase in 3-O-[14C]methylglucose washout, which in each instance was preceded by or coincided with a rise in the washout of 45Ca. (3) Trypsin, 2,4-dinitrophenol, p-chloromercuriphenylsulfonic acid, H2O2 and hyperosmolarity all produced dose-dependent stimulation of the washout of 45Ca and 3-O-[3H]methylglucose. Regression analysis showed a highly significant correlation between the increases in the two parameters (P < 0.001). (4) Depolarization and Na+ influx induced by veratrine were found to be associated with a marked rise in 45Ca release followed by stimulation of 3-O-[14C]methylglucose washout. (5) In epididymal fat pads, six different agents known to stimulate glucose transport were found to produce a highly significant (P < 0.001) increase in the washout of 45Ca and 3-O-[14C]methylglucose. (6) It is concluded that in the major targets for insulin action, activation of the glucose transport system can be elicited by a rise in cytoplasmic Ca2+ concentration brought about by mobilization of Ca2+ from endogenous cellular pools.

Role of calcium influx in the regulation of sugar transport in resting left atrial muscle

The role of external Ca2+ in the regulation of sugar transport in isolated resting atria of rats and guinea pigs was studied by measuring the tissue/medium distribution of 14C-labelled 3-methylglucose and of 45Ca. Omission of Ca2+ from the medium strongly antagonized the stimulation of sugar transport by insulin, hyperosmolarity (100 mM mannitol) or Na+-pump inhibition (K+-free medium). Basal sugar transport was not affected by Ca2+ omission but was increased when 0.5 mM EGTA was also added. The Ca2+ antagonist drug D-600 decreased 45Ca influx and also inhibited sugar-transport stimulation by the above 3 treatments in the rat, while in the guinea pig it antagonized only insulin-stimulated transport. The stimulation of sugar transport by a high adrenaline concentration and the inhibition by a low concentration were both antagonized by Ca2+ omission or D-600. The results illustrate the important role of Ca2+ influx in the control of sugar transport by hormonal and other modulators and are consistent with the hypothesis that cytoplasmic Ca2+ regulates glucose transport in muscle.

Insulin and glucose 6-phosphate stimulation of Ca2+ uptake by skinned muscle fibers

Calcium uptake by skinned muscle fibers is stimulated by physiological concentrations of insulin. These fibers, which lack a functional plasma membrane, are permeable to macromolecules but retain extensive portions of their sarcolemma in the form of transverse tubules intercalated between the myofibrils. They have an active sarcoplasmic reticulum that removes 45Ca2+ from solution at concentrations below the threshold that initiates contraction (less than 1 microM). The Ca2+ uptake activity is stimulated by insulin, presumably in response to its binding to those receptors located in the transverse tubules. Addition of glucose 6-phosphate, whose intracellular concentration increases in response to insulin, also stimulates Ca2+ uptake, a unique property of this preparation. These data indicate that insulin and glucose 6-phosphate act in concert to stimulate the sarcoplasmic reticulum. The resulting decrease in myoplasmic Ca2+ and the increase in glucose 6-phosphate would serve to mediate some of the anabolic effects of the hormone.

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Direct addition of insulin inhibits a high affinity Ca2+-ATPase in isolated adipocyte plasma membranes

The mechanism by which insulin regulates cellular metabolism remains unknown although indirect evidence suggests that alterations in intracellular calcium are important. More specifically, it has been proposed that insulin triggers an increase in intracellular calcium which is responsible for the subsequent modification of metabolic activities. The cell maintains a large electrochemical gradient for ionised calcium between the cytoplasm (less than 10(-6) M, as determined for muscle and nerve) and the extracellular environment (less than 10(-3) M). The plasma membrane may, therefore, be important in the regulation of calcium homeostasis, as a slight alteration in the processes maintaining this gradient could result in marked changes in cytoplasmic calcium. One such process is the active extrusion of calcium from the cell by a high affinity calcium-stimulated ATPase (Ca2+-ATPase). Such a mechanism has been well established in red cells and is postulated in nerve, liver and muscle. We have identified a high affinity Ca2+-ATPase in a plasma membrane-enriched subcellular fraction isolated from rat adipocytes which may provide the enzymatic basis for a calcium extrusion pump. We demonstrate here that the Ca2+-ATPase is specifically inhibited by the direct addition of physiological concentrations of insulin to the direct addition of physiological concentrations of insulin to the isolated plasma membranes. This effect suggests that direct regulation of calcium homeostasis may represent an important event in the mechanism of action of insulin.

The effect of hyperosmolarity and insulin on resting tension and calcium fluxes in rat soleus muscle

1. The effect of hyperosmolarity on resting tension and on the fluxes of Na and Ca has been characterized in isolated soleus muscles of the rat. 2. When the osmolarity of the incubation medium was increased by the addition of non-permeant solutes (100-400 m-osmole), the tension showed a rapid dos-dependent rise which could be maintained for up to 60 min. 3. Tension development was unaffected by tubocurarine (2 X 10(-5) M), considerably diminished by the omission of Na or Ca from the incubation medium, and inhibited by tetracaine (10(-4) M). 4. The addition of mannitol or sucrose (200 mM) induced a prompt stimulation of the influx of 22Na and 45Ca. Both in the absence and the presence of extracellular Ca hyperosmolarity stimulated the washout of 45Ca from preloaded muscles. Tetracaine (5 X 10(-4 M) suppressed the effects of hyperosmolarity on both the influx and the efflux of 45Ca, but only gave a modest reduction in the stimulation of 22Na influx. 5. Insulin (5-100 mu./ml.) induced a considerable further rise in the resting tension of muscles exposed to mannitol or sucrose (200 mM). This effect was seen in a glucose-free medium and could be abolished by the addition of insulin antibody. 6. It is concluded that hyperosmolarity leads to a rise in the concentration of free Ca2+ ions in the sarcoplasm, partly due to a mobilization of Ca from intracellular pools, but to a considerable extent supplemented from extracelluar sources. Under these conditions, insulin further augments the Ca2+ ion level in the cytoplasm.

Calcium as modulator of the hormonal-receptors-biological-response coupling system. Effects of Ca2+ ions on the insulin activated 2-deoxyglucose transport in rat fat cells

Activation of 2-deoxyglucose transport in isolated rat fat cells by insulin is dependent upon the presence of Ca2+ in the external medium. When calcium concentration is kept below 100 micron, insulin acts like a partial agonist, giving only half of the maximal activation obtained normally with a millimolar concentration of this ion. Oxytocin, whose insulin-like action on adipocytes activates glucose oxidation by these cells, was found to be unable to affect the rate of 2-deoxyglucose transport. This, together with previous observations, suggests that calcium ions play a role in the mechanism of insulin action possibly by binding selectively to membrane sites involved in the transmission of the hormonal message to the glucose carrier. Oxytocin seems to trigger only intracellular glucose metabolism and it appears that there is an absolute requirement for calcium ions in the activation of a still unknown membrane signal.

Permissive effect of ATP on insulin-stimulated sugar transport by rat soleus muscle

The stimulatory effect of insulin (0.1 U/ml) on D-xylose uptake was progressively lost when rat soleus muscles were preincubated at 37 degrees C under anaerobic conditions for longer than 30 min; after 90 min these muscles were completely insensitive to insulin. This effect was associated with the loss of muscle ATP. When the breakdown of ATP was retarded either by lowering the preincubation temperature or by preincubation with 5 mM glucose, the effect of insulin in anaerobic muscle was correspondingly prolonged. Under certain conditions, externally added ATP promoted an effect of insulin in otherwise insulin-unresponsive muscles. This effect was small in magnitude and was complicated by the degradation of the added ATP in the incubation medium and by the fact that ATP also tended to inhibit insulin-stimulated xylose uptake. These results indicate that there is a relationship between insulin-stimulated sugar transport and muscle ATP levels. This supports the proposal that there may be some ATP-dependent reaction(s) involved in the mechanism whereby insulin promotes the process of muscle sugar transport.

The mechanism of action of polypeptide hormones with special reference to insulin's action on glucose transport

In the last decade substantial progress has been made in elucidation of the mechanisms of action of polypeptide hormones. For instance, it has become clear that these hormones bind to their target cells and that at least one component of the binding is saturable. This phenomenon, which is usually called receptor binding, is believed to be the first and necessary event in the chain of processes which lead to effects of the hormone on their target cells. Receptor binding is just one aspect of the mechanism of action of polypeptide hormones. The properties of binding differ from hormone to hormone as do the events after binding. The purpose of this communication is to describe some aspects of the mechanism of insulin action on transmembrane glucose transport, and to disucss general aspects of polypeptide hormone action in relation to the specific findings with insulin. Comprehensive reviews on polypeptide hormone receptors have been published recently (Freychet, 1976; Kahn, 1976).