Ability of insulin to increase calcium uptake by adipocyte endoplasmic reticulum

Insulin action on different smooth muscle preparations

1. The importance of insulin in diabetic gastro-intestinal complications has been postulated. The present study was designed to investigate short-term effect of insulin on different smooth muscles isolated from non-diabetic animals. 2. The contractile responses of isolated guinea-pig ileum to acetylcholine and histamine and the serotonin-induced contractions of rat stomach fundus strips were inhibited by insulin in a non-competitive manner. The contractions of rat vas deferens elicited by noradrenaline and tyramine were also inhibited in the presence of insulin. 3. Insulin caused dose-dependent relaxation on the isolated rat duodenum. The relaxing response to insulin was not affected in the presence of atropine, phentolamine, propranolol, nicotinic acid, tetrodotoxin, tetraethylammonium, ouabain and nifedipine. The relaxing effect of insulin on the isolated rat duodenum was inhibited by sodium orthovanadate, trifluoperazine, verapamil, aspirin and dexamethasone, non-competitively. 4. The above results strongly suggest that the relaxing or inhibitory effect of insulin on the smooth muscles is closely related with prostanoid metabolism. Furthermore, it is concluded that this effect of insulin on the smooth muscles may be due to activation of Ca2+-pump ATPase(s).

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.

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.

Subcellular localization of calcium in the coronet cells and tanycytes of the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson

The intracellular localization of calcium in the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson, was studied by means of ultracytochemical and X-ray microanalytical techniques. Using a variant of the glutaraldehyde/potassium pyroantimonate-osmium tetroxide method, Ca was detected in mitochondria, smooth endoplasmic reticulum and primary vesicles of coronet cells, and in mitochondria and smooth endoplasmic reticulum of tanycytes. Mitochondria and smooth endoplasmic reticulum in both cell types are considered as general Ca-stores. The primary vesicles in the ciliary globules of coronet cells are viewed as additional Ca-reservoirs. Possible roles of these Ca-stores in the regulation of transport activities of coronet cells in the homeostasis of the CSF are discussed.

Calcium ions and the control of proliferation in normal and cancer cells

Several lines of evidence suggest tha Ca2+ ions control cell proliferation: Ca2+ entry into cytoplasm acts as a general mitogen; serum and serum-replacements induce Ca2+ influx; the Ca2+ concentrations in growth media required to support the proliferation of normal cells are much higher than those required for cancer cells; serum and growth factors reduce the Ca2+ requirements of normal cells; tumour promoters alter Ca2+ fluxes via a mechanism used principally by growth factors. Minor supporting evidence includes the effects of various drugs and viruses, and the behaviour of tumour cell mitochondria and intercellular junctions. It is still not possible to decide exactly where and when inside cells the critical effect of Ca2+ on proliferation occurs, but we discuss at length the practical problems of understanding Ca2+ movements in tissue-culture cells. Carried to its logical conclusion, present evidence suggests that an overridden or bypassed Ca2+ control process may be the key, common determinant of unrestrained proliferation in cancer cells.

The effect of glucagon on the kinetics of hepatic mitochondrial calcium uptake

Previous work by this and other laboratories has shown that glucagon administration stimulates calcium uptake by subsequently isolated hepatic mitochondria. This stimulation of hepatic mitochondrial Ca2+ uptake by in vivo administration of glucagon was further characterized in the present report. Maximal stimulation of mitochondrial Ca2+ accumulation was achieved between 6-10 min after the intravenous injection of glucagon into intact rats. Under control conditions, Ca2+ uptake was inhibited by the presence of Mg2+ in the incubation medium. Glucagon treatment, however, appeared to obliterate the observed inhibition by Mg2+ of mitochondrial Ca2+ uptake. Kinetic experiments revealed the usual sigmoidicity associated with initial velocity curves for mitochondrial calcium uptake. Glucagon treatment did not alter this sigmoidal relationship. Glucagon treatment significantly increased the V max for Ca2+ uptake from 292 +/- 22 to 377 +/- 34 nmoles Ca2+/min per mg protein (n = 8) but did not affect the K 0.5, (6.5-8.6 microM). Since the major kinetic change in mitochondrial Ca2+ uptake evoked by glucagon is an increase in V max, the enhancement mechanism is likely to be an increase either in the number of active transport sites available to Ca2+ or in the rate of Ca2+ carrier movement across the mitochondrial membranes.

Characterization of the hormone-sensitive Ca2+ uptake activity of the hepatic endoplasmic reticulum

The characteristics and kinetics of calcium uptake activity were studied in isolated hepatic microsomes. The sustained accumulation of calcium was ATP- and oxalate-dependent. Glucagon increased microsomal Ca2+ uptake upon either in vivo injection, or in vitro perfusion of the hormone in the liver. In contrast, the effect of insulin depended on the route of administration. Calcium accumulation by subsequently isolated hepatic microsomes increased when insulin was injected intraperitoneally whereas it decreased when the hormone was perfused directly into the liver. These effects of glucagon and insulin were dose dependent. When insulin was added to the perfusate prior to the addition of glucagon, insulin blocked the glucagon-stimulated increase in microsomal Ca2+ uptake. Cyclic AMP mimicked the effect of glucagon on microsomal Ca2+ accumulation when the cyclic nucleotide was perfused into the liver. The effects of glucagon and insulin on the kinetics of hepatic microsomal Ca2+ uptake were investigated. In microsomes isolated from perfused rat livers treated with glucagon the V of the uptake was significantly increased over the control values (12.2 vs. 8.6 nmol Ca2+ per min per mg protein, P less than 0.02). In contrast, the addition of insulin to the perfusate significantly decreased the V of Ca2+ uptake by subsequently isolated microsomes (6.8 vs. 8.3 nmol Ca2+ per min per mg protein, P less than 0.05). However, neither hormone had an effect on the apparent Km for Ca2+ (4.1 +/- 0.5 microM) of the reaction. The effect of these hormones on the activity of Ca2+-stimulated ATPase was also studied. No significant changes in either V or Km for Ca2+ of the enzymatic reaction were detected.

Relationship between calcium ion transport and (Ca2+ + Mg2+)-atpase activity in adipocyte endoplasmic reticulum

Calcium uptake by adipocyte endoplasmic reticulum was studied in a rapidly obtained microsomal fraction. The kinetics and ionic requirements of Ca2+ transport in this preparation were characterized and compared to those of (Ca2+ + Mg2+)-ATPase activity. The time course of Ca2+ uptake in the presence of 5 mM oxalate was nonlinear, approaching a steady-state level of 10.8--11.5 nmol Ca2+/mg protein after 3--4 min of incubation. The rate of Ca2+ transport was iM oxalate. The calculated initial rate of calcium uptake was 18.5 nmol Ca2+/mg protein per min. The double reciprocal plot of ATP concentration against transport rate was nonlinear, with apparent Km values of 100 muM and 7 muM for ATP concentration ranges above and below 50 muM, respectively. The apparent Km values for Mg2+ and Ca2+ were 132 muM and 0.36--0.67 muM, respectively. The energy of activation was 23.4 kcal/mol. These kinetic properties were strikingly similar to those of the microsomal (Ca2+ + Mg2+)-ATPase. The presence of potassium was required for maximum Ca2+ transport activity. The order of effectiveness of monovalent cations in stimulating both Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity was K+ greater than Na+ = NH4+ greater than Li+. Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity were both inhibited 10--20% by 6 mM procaine and less than 10% by 10 mM sodium azide. Both processes were completely inhibited by 3 mM dibucaine or 50 muM p-chloromercuribenzene sulfonate. The results indicate that Ca2+ transport in adipocyte endoplasmic reticulum is mediated by a (Ca2+ + Mg2+)-ATPase and suggest an important role for endoplasmic reticulum in control of intracellular Ca2+ distribution.

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.