Hormone-receptor coupling and the molecular mechanism of insulin action in the adipocyte: a paradigm for Ca2+ homeostasis in the initiation of the insulin-induced metabolic cascade

Effects of calcium on brazilin-induced glucose transport in isolated rat epididymal adipocytes

Brazilin increased [3H]2-deoxyglucose uptake in isolated rat epididymal adipocytes. The fact that calcium may be required for the stimulatory effects of insulin on glucose transport suggests that brazilin might also require calcium for its glucose transport-stimulating action. Changes in the concentration of extracellular calcium had no significant effect on brazilin-induced glucose transport. Nifedipine and verapamil decreased brazilin-induced glucose transport, and quin2-AM abolished the effect of brazilin on glucose transport. A23187, however, showed no effect on brazilin action. 45Ca2+ uptake into adipocytes was not influenced by brazilin treatment, and trifluoperazine significantly inhibited the effect of brazilin on glucose transport. These data suggest that calmodulin and the maintenance of the intracellular calcium concentration, rather than an increase in it, may be essential for the stimulatory action of brazilin on glucose transport.

Decreased activity of (Ca2+ + Mg2+)-adenosine triphosphatase (ATPase) and a hormone-specific defect in insulin regulation of ATPase in kidney basolateral membranes from obese fa/fa rats

The plasma membrane enzyme (Ca2+ + Mg2+)-adenosine triphosphatase (ATPase) is hormonally regulated and may participate in Ca2+ signaling by removing excess Ca2+ from the cell. Therefore, observations of a hormone-specific loss of insulin stimulation of ATPase in kidney membranes from non-insulin-dependent diabetic (NIDDM) rats may reflect their insulin-resistant state. Consequently, to evaluate whether additional insulin-resistant conditions are associated with impaired function of ATPase and with loss of regulation of the enzyme by insulin, studies were extended to investigate (Ca2+ + Mg2+)-ATPase activities and hormonal regulation of the enzyme in kidney basolateral membranes from obese and lean Zucker rats. (Ca2+ + Mg2+)-ATPase activity was lower in membranes from obese rats compared with lean rats. Maximal velocity (Vmax) of the enzyme activity was 29.2 +/- 2.6 nmol Pi/mg/min in obese rats versus 57.2 +/- 6.5 in lean rats (P < .05). However, the affinity of the enzyme for Ca2+ was similar in obese and lean rats (Km Ca2+, 0.23 +/- 0.025 v 0.23 +/- 0.032 mumol/L Ca2+). Also, the Km for ATP of the enzyme was similar in membranes from obese and lean rats. Insulin, parathyroid hormone (PTH), and cyclic adenosine monophosphate (cAMP) stimulated the ATPase activity in membranes from lean rats in a dose-dependent manner (15% to 28%). Also, the protein kinase C (PKC) stimulator 12-O-tetradecanoyl phorbol-13-acetate (TPA) increased the ATPase activity in membranes from lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone-specific defect in insulin regulation of (Ca2+ + Mg2+)-adenosine triphosphatase activity in kidney membranes from streptozocin non-insulin-dependent diabetic rats

The plasma membrane enzyme (Ca2+ + Mg2+)-adenosine triphosphatase [(Ca2+ + Mg2+)-ATPase] is hormonally regulated, and may participate in Ca2+ signaling by removing excess Ca2+ from the cell. Insulin increases ATPase activity in kidney cortical basolateral membranes (BLM) from normal rats, but fails to do so in membranes from insulin-resistant non-insulin-dependent diabetic (NIDDM) rats. To investigate mechanisms of insulin regulation of ATPase and to evaluate whether the loss of this regulation in diabetes is hormone-specific and depends on blood glucose levels, (Ca2+ + Mg2+)-ATPase function and its hormonal regulation were studied in kidney BLM from rats with mild and severe NIDDM. Km values for ATP and Ca2+ affinity of the ATPase were similar in diabetic and control rats, but the maximal velocity (Vmax) of the enzyme was higher in diabetic groups. Insulin, the protein kinase C (PKC) stimulator 12-0-tetradecanoylphorbol 13-acetate (TPA), parathyroid hormone (PTH), and cyclic adenosine monophosphate (cAMP) all increased the ATPase activity in BLM from controls by increasing the enzyme's affinity for Ca2+. A protein kinase A (PKA) inhibitor (H8 in low concentrations) abolished cAMP and PTH effects, but not those of insulin, whereas the PKC inhibitors (sphingosine and high concentrations of H8) did abolish the effects of insulin. Stimulations of ATPase activity by insulin and by PTH and cAMP were additive. Insulin and TPA lost their stimulatory effects on ATPase in BLM from rats with either mild or severe NIDDM, but PTH and cAMP maintained their stimulatory effects in these membranes. The data show [1] (Ca2+ + Mg2+)-ATPase activity is increased in NIDDM, and a hormone-specific loss of insulin stimulation of ATPase occurs; (2) these defects are not dependent on the level of glycemia; and (3) the stimulatory effects of insulin on the ATPase may be mediated in part via PKC. We suggest that the hormone-specific defect in insulin regulation of ATPase seen in the NIDDM rats may contribute to their insulin resistance.

Hepatic insulin resistance in KKA(y) mice and its amelioration by pioglitazone do not involve alterations in phospholipase C activity

It has been proposed that an abnormality in the regulation of cytosolic-free Ca2+ may be the cause of some forms of insulin resistance. In support of this proposition, it was reported that phospholipase C-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by liver plasma membranes from obese patients with non-insulin-dependent diabetes mellitus (NIDDM) was abnormally augmented (Thakker et al., J. Biol. Chem. 264, 7169-7175). The objective of this investigation was to determine if a novel antidiabetic agent, pioglitazone, ameliorated hepatic insulin resistance in KKA(y) mice and to identify any alterations in PIP2-phospholipase C activity of liver plasma membranes that may accompany changes in insulin sensitivity. Treatment of KKA(y) mice for 4 days with pioglitazone (20 mg/kg per day) decreased blood glucose and insulin and improved a variety of indices of hepatic insulin resistance, but did not alter the rate of PIP2 hydrolysis by liver plasma membranes. Acute treatment of isolated liver plasma membranes with pioglitazone (1-100 microM) also failed to alter PIP2-phospholipase C activity. Furthermore, the specific activity, Ca(2+)-requirement, pH-dependence and sensitivity to guanosine 5'-thiotriphosphate of the PIP2-phospholipase C in KKA(y) liver membranes were indistinguishable from those of C57BL/6J (normal) mice. Among C57BL/6J and KKA(y) mice fed either a control or pioglitazone-supplemented diet, there was no correlation between PIP2-phospholipase C activity in isolated liver membranes and either glucose or insulin concentrations in the circulation. These data indicate that an alteration in PIP2-phospholipase C activity of liver plasma membranes is neither a cause nor an obligatory consequence of insulin resistance in KKAy mice or its amelioration by pioglitazone. Alterations of liver membrane phospholipase C activity in NIDDM, therefore, may reflect diabetic pathology other than the insulin resistance associated with this disease.

High affinity insulin binding and insulin receptor-effector coupling: modulation by Ca2+

Insulin binding and insulin stimulated amino acid and glucose uptake were determined in cultured HTC hepatoma cells in the presence of Ca2+ and ruthenium red (RR) in order to further characterise the putative calcium binding site on the receptor. These ions increased insulin receptor high affinity binding and the sensitivity of these responses to insulin. The insulin concentration required to half-maximally stimulate amino acid uptake decreased significantly from 26.9 +/- 5.8 ng/ml to 6.0 +/- 1.3 ng/ml in the presence of 10 mM Ca2+ and to 1.3 +/- 0.5 ng/ml in the presence of RR. The effect of Ca2+ and RR was more pronounced on insulin stimulated glucose uptake. These agents also increased receptor-effector coupling, reducing the percentage of occupied receptors required for maximal insulin stimulation of amino acid uptake from 10.8% in control cells to 3.4 and 1.4% in the presence of Ca2+ and RR respectively. The receptor occupancy required to produce maximal insulin responses on glucose uptake decreased from 20% (control) to 3.8% (Ca2+ and RR). We hypothesize that since Ca2+ and RR have similar effects, that occupation of Ca2+ binding sites on the receptor produces a conformational change in the insulin receptor which increases insulin receptor affinity, insulin sensitivity and acts on an early post-receptor event responsible for coupling binding to insulin action.

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.

Endotoxicosis modulates cytosolic free calcium and basal and ACTH-stimulated lipolysis in rat adipocytes

Lipolytic rates and intracellular Ca2+ concentration ([Ca2+]i) were determined under basal conditions and upon stimulation with adrenocorticotropic hormone (ACTH), norepinephrine (NE) and insulin (I), in adipocytes isolated from control and acutely endotoxin (ET)-treated rats (1 mg/100 g body weight, LD50 at 6 h). [Ca2+]i measurements were done using the fluorescent Ca2(+)-indicator Fura-2. NE and ACTH, but not I, produced a marked increase of [Ca2+]i in cells of both control and ET-treated rats. ET treatment elicited a significant increase in [Ca2+]i of resting cells, and enhanced the ACTH effect on this parameter. The changes in lipolytic activity correlated well with changes of [Ca2+]i induced by ACTH. The results indicate that ET-induced alterations in intracellular calcium homeostasis of adipocytes may contribute to the mediation of effects on fat mobilization during endotoxemia.

Effects of food restriction and insulin treatment on (Ca2+ + Mg2+)-ATPase response to insulin in kidney basolateral membranes of noninsulin-dependent diabetic rats

Insulin increases (Ca2+ + Mg2+)-ATPase activity in cell membranes of normal rats but fails to do so in membranes of non-insulin-dependent diabetic (NIDD) rats. The loss of regulatory effect of the hormone on the enzyme might contribute to the insulin resistance observed in the NIDD animals. To further test this hypothesis, the effects of insulin treatment and acute food restriction on the ability of insulin to regulate the ATPase activity in kidney basolateral membranes (BLM) of NIDD rats were studied. Although insulin levels in NIDD and control rats were similar, plasma glucose was higher in the NIDD rats (18.3 +/- 1.5 v 19.3 +/- 1.7 microU/mL and 236 +/- 32 v 145 +/- 3 mg/dL, respectively). Insulin treatment (2 U/100 g), which increased plasma insulin in the NIDD rats (47.8 +/- 11.5 microU/mL; P less than .05), did not decrease their glucose (221 +/- 25 mg/dL). Higher insulin dose (4 U/100 g) decreased glucose level in the NIDD rats (73 +/- 3 mg/dL; P less than .001) but increased their plasma insulin 10-fold (202.5 +/- 52.5 microU/mL). Acute food restriction decreased glucose levels in the NIDD rats to levels seen in controls (135 +/- 3 mg/dL), while their insulin decreased by half (8.5 +/- 1.0 microU/mL; P less than .05). Basal (Ca2+ + Mg2+)-ATPase activity in BLM of all diabetic rats was higher than in controls (P less than .05). None of the treatments reversed this defect.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of cellular calcium metabolism in abnormal glucose metabolism and diabetic hypertension

The prevalence of hypertension in patients with non-insulin-dependent diabetes mellitus (NIDDM) is considerably higher than in the non-diabetic population. Insulin resistance may contribute to this increased prevalence. Abnormal cellular calcium (Ca2+) homeostasis may link insulin resistance and high blood pressure in patients with NIDDM. Observations of abnormal cellular Ca2+ homeostasis in animal models of NIDDM and obesity as well as in diabetic patients are consistent with this hypothesis. Abnormalities in cellular Ca2+ homeostasis are also found in hypertensive animals and humans. Alterations in cell membrane phospholipid content and distribution may be the primary cause of abnormal plasma membrane Ca2+ fluxes in patients with NIDDM and hypertension.

Flow cytometric analysis of mature adipocytes

Flow cytometry is an excellent method for studying the physiological function in adipocytes because their response to hormones, especially insulin, varies with cell maturity and therefore size. Adipocytes present a unique technical challenge. A freshly prepared adipocyte suspension contains cells and fat droplets ranging from 10 to greater than 120 microns in diameter. Stored fat occupies 90-98% of the cell volume, making it difficult to distinguish cells from fat droplets. Other difficulties include buoyancy, large size, fragility, and tendency to aggregate and clog the sample tube and nozzle. These obstacles were overcome by 1) maintaining the sample, sample line, sheath fluid, reservoir, and nozzle assembly at 37 degrees C; 2) using a 200 microns diameter orifice; 3) using a short, 300 microns inside diameter Teflon sample delivery line; 4) injecting the sample at constant flow rate into the sheath fluid at low pressure; and 5) using the pH-sensitive vital stain, biscarboxyethylcarboxyfluorescein (BCECF) to distinguish cells from fat droplets. Stained cells are brightly fluorescent when excited at 488 nm. Because fat droplets do not fluoresce, they can be distinguished from fat cells by gating on the BCECF emission. The cytosolic pH of intact, viable, mature adipocytes was derived from the ratio of the fluorescent emission intensities at 520 and 620 nm and was estimated to be 7.2. Unlike BCECF, several other useful fluorescent probes of cell function, e.g., the intracellular calcium indicator, indo-1, label both fat cells and fat droplets.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal cell calcium concentrations in cultured bone cells obtained from femurs of obese and noninsulin-dependent diabetic rats

Cytoplasmic free calcium concentration [Ca2+]i was quantified in cultured bone cells with osteoblastic characteristics. The cells were obtained from femurs of obese (fa/fa) Wistar-Kyoto rats, from nonobese, noninsulin-dependent diabetic (NIDD) Sprague Dawley rats, and from their appropriate controls. [Ca2+]i was also determined in bone cells obtained from in vivo insulin-treated NIDD rats. Obese (Wistar Kyoto) rats had increased body weight (313 +/- 13 vs. 249 +/- 4 g; P less than 0.01), decreased femur weights (0.68 +/- 0.05 vs. 0.89 +/- 0.05 g; P less than 0.05), similar glucose levels (148 +/- 5 vs. 139 +/- 3 mg/dl), and higher plasma insulin levels (6.0 +/- 0.5 vs. 0.7 +/- 0.1 ng/ml; P less than 0.01) when compared with their nonobese [(fa/+); (+/+)] littermates. Nonobese, NIDD rats, compared with their appropriate controls (nondiabetic Sprague Dawley rats) had higher plasma glucose levels (235 +/- 32 vs. 145 +/- 3 mg/dl; P less than 0.01) but their plasma insulins, body weights, and femur weights were similar to controls (0.7 +/- 0.1 vs. 0.6 +/- 0.1 ng/ml; 302 +/- 4 vs. 318 +/- 14 g; 0.97 +/- 0.4 vs. 0.98 +/- 0.04 g, respectively). Long-term (4 weeks) daily insulin treatment (2 u/100 g) of the NIDD rats increased their plasma insulin (1.9 ng/ml; P less than 0.05) and body weight (369 +/- 13 g; P less than 0.05) but did not change their plasma glucose levels (225 +/- 5 mg/dl), or femur weights (0.98 +/- 0.4 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Possible role of cytosolic free calcium concentrations in mediating insulin resistance of obesity and hyperinsulinemia

Insulin- and glyburide-stimulated changes in cytosolic free calcium concentrations [( Ca2+]i) were studied in gluteal adipocytes obtained from six obese women (139 +/- 3% ideal body wt) and six healthy, normal weight age- and sex-matched controls. Biopsies were performed after an overnight fast and twice (at 3 and 6 h) during an insulin infusion (40 mU/m2 per min) (euglycemic clamp). In adipocytes obtained from normal subjects before insulin infusion, insulin (10 ng/ml) increased [Ca2+]i from 146 +/- 26 nM to 391 +/- 66 nM. Similar increases were evoked by 2 microM glyburide (329 +/- 41 nM). After 3 h of insulin infusion, basal [Ca2+]i rose to 234 +/- 21 nM, but the responses to insulin and glyburide were completely abolished. In vitro insulin-stimulated 2-deoxyglucose uptake was reduced by insulin and glucose infusion (25% stimulation before infusion, 5.4% at 3 h, and 0.85% at 6 h of infusion). In obese patients, basal adipocyte [Ca2+]i was increased (203 +/- 14 nM, P less than 0.05 vs. normals). The [Ca2+]i response demonstrated resistance to insulin (230 +/- 23 nM) and glyburide (249 +/- 19 nM) stimulation. Continuous insulin infusion increased basal [Ca2+]i (244 +/- 24 nM) and there was no response to either insulin or glyburide at 3 and 6 h of study. Rat adipocytes were preincubated with 1-10 mM glucose and 10 ng/ml insulin for 24 h. Measurements of 2-deoxyglucose uptake demonstrated insulin resistance in these cells. Under these experimental conditions, increased levels of [Ca2+]i that were no longer responsive to insulin were demonstrated. Verapamil in the preincubation medium prevented the development of insulin resistance.

Intracellular calcium, insulin secretion, and action

Changes in cytosolic free calcium concentration [( Ca2+]i) constitute an important element of signal transduction in various cells. These changes either reflect alterations in calcium (Ca2+) fluxes or result from mobilization of intracellular Ca2+ stores. In pancreatic islet cells, an increase in [Ca2+]i is critical for secretagogue-induced insulin release. Thus, glucose evokes a rapid increase in [Ca2+]i, primarily by stimulating Ca2+ influx. Under physiologic conditions, glucose may also promote mobilization of intracellular Ca2+ stores by virtue of stimulating membrane phospholipid hydrolysis and formation of inositol triphosphate, a potent stimulus for Ca2+ mobilization. This action of glucose requires the presence of extracellular Ca2+. The magnitude of change in [Ca2+]i may not parallel the level of insulin release, suggesting that the role of [Ca2+]i in the process of insulin release must be considered in concert with other cellular mechanisms. The role of [Ca2+]i in promoting insulin action is a subject of continuous controversy. Recent observations that chelation of intracellular Ca2+ with quin-2 diminishes insulin action (and that of insulin mimetics) support the role of Ca2+ in mediating the insulin-generated signal. Insulin has also been demonstrated to increase [Ca2+]i in adipocytes in close association with its effect on 2-deoxyglucose uptake. Finally, in both pancreatic islet cells and adipocytes, high concentrations of either extracellular or intracellular Ca2+ inhibit cellular responsiveness. The optimal concentrations of cytosolic Ca2+ appear to be within the 140 to 350 nM range. When Ca2+ concentrations are too low or too high, the ability of pancreatic islets and insulin target cells to respond appropriately to physiologic stimuli is significantly diminished. Impaired cellular Ca2+ homeostasis (either primary or secondary to other cellular lesions) may represent a crucial and identical link in the pathogenesis of impaired insulin secretion and in the pathogenesis of impaired insulin action.

The role of adenosine in insulin action coupling in rat adipocytes

The involvement of adenosine in the coupling of insulin binding to action was investigated in rat adipocytes. Reduction of endogenous adenosine levels by treatment with adenosine deaminase (ADA) had no significant effect on either basal or maximally stimulated glucose transport, but reduced the insulin sensitivity of transport stimulation. Adenosine deaminase treatment also shifted the EC50 of H2O2 stimulation of transport from 0.13 mM to 0.30 mM, and the EC50 for insulin stimulation of protein synthesis from 0.40 +/- 0.06 ng/ml to 1.30 +/- 0.25 ng/ml. Adenosine appears to be acting through the pharmacological Ri adenosine receptor subtype. The mode of action of adenosine does not seem to involve inhibition of adenylate cyclase. Adenosine also influences the kinetics of insulin action. ADA treatment slows the onset of transport stimulation by a maximal insulin concentration (10 ng/ml). Increasing the hormone level to 100 ng/ml overcomes this slowing without increasing transport further. The deactivation of glucose transport following removal of insulin is accelerated by ADA treatment. Thus, adenosine is involved both in maintaining a high efficiency of an early step in the insulin signaling process and in maintaining optimal activity of the insulin-stimulated glucose transport system.

Hypertension and diabetes

Diabetes mellitus and hypertension are both common diseases, especially with an increasingly aged population. Hypertension accelerates the development of diabetic retinopathy, nephropathy, and peripheral vascular disease in the diabetic patient. Diabetes represents a type of premature aging and hypertension in the diabetic patient is characterized by many of the same pathophysiologic properties seen in the elderly hypertensive patient.

Chelation of intracellular calcium blocks insulin action in the adipocyte

The hypothesis that intracellular Ca2+ is an essential component of the intracellular mechanism of insulin action in the adipocyte was evaluated. Cells were loaded with the Ca2+ chelator quin-2, by preincubating them with quin-2 AM, the tetrakis(acetoxymethyl) ester of quin-2. Quin-2 loading inhibited insulin-stimulated glucose transport (IC50, 26 microM quin-2 AM) without affecting basal activity. The ability of insulin to stimulate glucose uptake in quin-2-loaded cells could be partially restored by preincubating cells with buffer supplemented with 1.2 mM CaCl2 and the Ca2+ ionophore A23187. These conditions had no effect on basal activity and omission of CaCl2 from the buffer prevented the restoration of insulin-stimulated glucose uptake by A23187. Quin-2 loading also inhibited insulin-stimulated glucose oxidation (IC50, 11 microM quin-2 AM) and the ability of insulin to inhibit cAMP-stimulated lipolysis (IC50, 78 microM quin-2 AM), without affecting their basal activities. Incubation of cells with 100 microM quin-2 or quin-2 AM had no effect on intracellular ATP concentration or the specific binding of 125I-labeled insulin to adipocytes. These findings suggest that intracellular Ca2+ is an essential component in the coupling of the insulin-activated receptor complex to cellular physiological/metabolic machinery. Furthermore, differing quin-2 AM dose-response profiles suggest the presence of dual Ca2+-dependent pathways in the adipocyte. One involves insulin stimulation of glucose transport and oxidation, whereas the other involves the antilipolytic action of insulin.

Plasma calcium and phosphate levels in an adult noninsulin-dependent diabetic population

Duplicate or triplicate measurements of fasting plasma glucose, calcium (Ca), phosphate (Pi), and glycosylated hemoglobins were performed on a group of non-insulin dependent diabetic patients and controls at 3-6 month intervals. In the diabetic group (48 males and 44 females), 18 were on diet only, 21 on diet and oral hypoglycemic treatment, and 51 on diet and insulin. These were a total of 217 measurements for each parameter. Results were compared to 416 measurements obtained from sex and age-matched controls. Plasma Ca levels were higher in the diabetic group (2.48 +/- 0.004 vs 2.38 +/- 0.006 mmol/liter) P less than 0.001; plasma Pi levels were similar to those of controls. The difference in plasma Ca was not influenced by age, sex, or mode of treatment. No correlation was found in the three treatment groups between plasma Ca and duration of diabetes nor with patients' weights. The results are consistent with the view that an alteration in calcium homeostasis accompanies the diabetic state.

(Na+ +K+)-ATPase activity in kidney basolateral membranes of non insulin dependent diabetic rats

Insulin resistant, Type II diabetes mellitus (NIDD) in a rat animal model results in profound changes in basal and insulin-stimulated membrane (Ca2+ +Mg2+)-ATPase activity in kidney basolateral membrane (BLM) preparations. We find that NIDD in these animals does not result in similar changes in membrane (Na+ +K+)-ATPase activity. Basal enzyme activity was the same in diabetic and control animals. Insulin treatment of diabetic animals in vivo resulted in hyperinsulinemia and increased BLM (Na+ +K+)-ATPase, while food restriction for 18 hr resulted in lowered enzyme activity. There was no direct effect of insulin on (Na+ +K+)-ATPase activity in isolated membranes from any of the animal groups. Thus, physiologic perturbations which alter insulin sensitivity and glucose homeostasis are accompanied by altered levels of (Na+ +K+)-ATPase activity. Lower levels of this membrane enzyme activity appear to be associated with optimal insulin action.

myo-Inositol 1,4,5-trisphosphate mobilizes Ca2+ from isolated adipocyte endoplasmic reticulum but not from plasma membranes

The effects of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ uptake and release from isolated adipocyte endoplasmic reticulum and plasma membrane vesicles were investigated. Effects of IP3 were initially characterized using an endoplasmic reticulum preparation with cytosol present (S1-ER). Maximal and half-maximal effects of IP3 on Ca2+ release from S1-ER vesicles occurred at 20 microM- and 7 microM-IP3, respectively, in the presence of vanadate which prevents the re-uptake of released Ca2+ via the endoplasmic reticulum Ca2+ pump. At saturating IP3 concentrations, Ca2+ release in the presence of vanadate was 20% of the exchangeable Ca2+ pool. IP3-induced release of Ca2+ from S1-ER was dependent on extravesicular free Ca2+ concentration with maximal release occurring at 0.13 microM free Ca2+. At 20 microM-IP3 there was no effect on the initial rate of Ca2+ uptake by S1-ER. IP3 promoted Ca2+ release from isolated endoplasmic reticulum vesicles (cytosol not present) to a similar level as compared with S1-ER. Addition of cytosol to isolated endoplasmic reticulum vesicles did not affect IP3-induced Ca2+ release. The endoplasmic reticulum preparation was further fractionated into heavy and light vesicles by differential centrifugation. Interestingly, the heavy fraction, but not the light fraction, released Ca2+ when challenged with IP3. IP3 (20 microM) did not promote Ca2+ release from plasma membrane vesicles and had no effect on the (Ca2+ + Mg2+)-ATPase activity or on the initial rate of ATP-dependent Ca2+ uptake by these vesicles. These results support the concept that IP3 acts exclusively at the endoplasmic reticulum to promote Ca2+ release.

Regulation of 3-O-methyl-D-glucose uptake in isolated bovine adrenal chromaffin cells

We showed earlier that insulin stimulated sugar transport in adrenal chromaffin cells (Bigornia, L. and Bihler, I. Biochim. Biophys. Acta 885, 335-344). Transport regulation and its Ca2+ -dependence was further investigated in isolated bovine adrenal chromaffin cells, serving as a model of a homogeneous neuronal cell population. Uptake of the nonmetabolizable glucose analogue, 3-O-methyl-D-glucose was stimulated by hyperosmolar medium, and this effect was abolished in the absence of external Ca2+, or depressed in the presence of La3+ or the slow Ca2+ channel blocker methoxyverapamil. Basal transport was also stimulated by factors (acetylcholine, carbamylcholine, low-Na+ medium), which cause Ca2+ -dependent catecholamine release, and these effects were abolished in Ca2+ -free medium. In addition insulin, acetylcholine, hyperosmolar and low-Na+ medium significantly increased 45Ca uptake. Thus, glucose transport in adrenal chromaffin cells was stimulated by insulin and hyperosmolarity in a Ca2+ -dependent manner, as in muscle. Sensitivity to secretory stimuli, a regulatory feature perhaps characteristic of this cell type, was also demonstrated. In contrast to muscle, sugar transport was not affected by Na+ -pump inhibition, metabolic inhibitors or the Na+ ionophore monensin, suggesting that Ca2+ influx by Na+/Ca2+ exchange does not play a significant role in the activation of sugar transport in chromaffin cells.

A calmodulin dependent Ca2+-activated K+ channel in the adipocyte plasma membrane

Increased membrane permeability (conductance) that is specific for K+ and directly activated by Ca2+ ions, has been identified in isolated adipocyte plasma membranes using the K+ analogue, 86Rb+. Activation of these K+ conductance pathways (channels) by free Ca2+ was concentration dependent with a half-maximal effect occurring at 32 +/- 4 nM free Ca2+ (n = 7). Addition of calmodulin further enhanced the Ca2+ activating effect on 86Rb+ uptake (K+ channel activity). Ca2+-dependent 86Rb+ uptake was inhibited by tetraethylammonium ion and low pH. It is concluded that the adipocyte plasma membrane possesses K+ channels that are activated by Ca2+ and amplified by calmodulin.

Regulation of endogenously catalyzed ADP-ribosylation in adipocyte plasma membranes by Ca2+ and calmodulin

The effects of Ca2+ and calmodulin on endogenously catalyzed ADP-ribosylation were investigated in adipocyte plasma membranes. Four specific proteins of 70, 65, 61 and 52 kDa were labeled with [32P]ADP-ribose and ADP-ribosylation of the proteins was highly dependent upon the conditions employed. ADP-ribosylation of the 70 kDa protein was observed only in membranes supplemented with Ca2+. Maximal incorporation of [32P] into the protein was achieved with free Ca2+ concentrations of 90 microM. Calcium-stimulated ADP-ribosylation of the 70 kDa protein was inhibited by calmodulin. Half-maximal inhibition was observed in membranes incubated with 1.2 microM calmodulin. The effect of calmodulin was characterized by an inhibition of the incorporation of [32P]ADP-ribose as opposed to a stimulation of its removal. ADP-ribosylation of the 61 kDa protein was not altered by added Ca2+ and/or calmodulin whereas ADP-ribosylation of the 65 kDa protein was partially (50%) inhibited by free Ca2+ concentrations between 10(-6) - 10(-5) M. These results provide evidence that the adipocyte plasma membrane contains ADP-ribosyltransferase activities and demonstrate that ADP-ribosylation of a 70 kDa protein is regulated by Ca2+ and calmodulin.

The insulin receptor contains a calmodulin-binding domain

Substantial evidence suggests that calcium has a pivotal role in regulating the initial events through which insulin alters plasma membrane metabolism. Because binding of insulin to its receptor represents the initial site of insulin action in the plasma membrane, studies were undertaken to determine whether the insulin receptor is a calmodulin-binding protein. Preparations enriched in the insulin receptor and calmodulin-binding proteins were isolated from detergent-solubilized rat adipocyte membranes by chromatography with wheat germ agglutinin agarose and calmodulin-conjugated Sepharose, respectively. Substantial purification of a manganese-dependent, insulin-sensitive phosphoprotein of 95K identified as the beta subunit of the insulin receptor was accomplished. Binding and photocovalent cross-linking of iodine-125-labeled calmodulin to these affinity-purified preparations and to isolated plasma membranes, followed by immunoadsorption with insulin receptor antibodies bound to protein A Sepharose, resulted in significant purification of a binding complex of 110K to 140K. These results indicate that the adipocyte insulin receptor or a polypeptide closely associated with the receptor is a calmodulin-binding protein.

Vanadyl and vanadate inhibit Ca2+ transport systems of the adipocyte plasma membrane and endoplasmic reticulum

Vanadate and vanadyl have many insulin-mimetic effects on cellular metabolism and also have been shown to alter cellular Ca2+ fluxes. In this report, vanadate and vanadyl, like insulin, are shown to inhibit the plasma membrane (Ca2+ + Mg2+)-ATPase/Ca2+ transport system as well as Ca2+ transport by endoplasmic reticulum from rat adipocytes. Ca2+ transport by the endoplasmic reticulum was inhibited half-maximally (I50) by vanadate and vanadyl at concentrations of 30 and 33 microM, respectively. Inhibition of the plasma membrane Ca2+ transport by vanadate and vanadyl was less sensitive, with I50 values of 144 and 92 microM, respectively. These I50 values for plasma membrane Ca2+ transport were similar when measured under conditions of calmodulin-stimulated and non-calmodulin-stimulated Ca2+ transport. The predominant effect of both ions on the kinetic parameters of Ca2+ transport was a substantial decrease in the Vmax by 43-46% for both transport systems. An increase in intracellular Ca2+ following the inhibition of the (Ca2+ + Mg2+)-ATPase/Ca2+ pump in the plasma membrane and endoplasmic reticulum by these vanadium ions may result, at least in part, in the observed insulin-mimetic alterations in cellular metabolism.

Mechanism of action of insulin on acetylcholine-evoked amylase secretion in the mouse pancreas

The effects of insulin and acetylcholine (ACh) on amylase secretion, transmembrane movement of 45Ca2+ and K+, membrane potential and cyclic nucleotide levels in the isolated mouse pancreas were investigated. Insulin alone had no effect on either amylase secretion or 45Ca2+ fractional efflux but it markedly potentiated the ACh-evoked amylase secretion and significantly reduced the ACh-induced 45Ca2+ fractional efflux. These effects were dose related. Insulin evoked a small membrane hyperpolarization and an increase in K+ efflux. The islet hormone had virtually no effect on ACh-induced membrane depolarization but it markedly enhanced the ACh-elicited K+ efflux. Both insulin and ACh had marked time-dependent effects on the metabolism of adenosine 3',5'-cyclic monophosphate (cyclic AMP). Insulin increased and ACh decreased cyclic AMP concentration when applied separately. However, when added together, insulin and ACh caused a rapid and sustained elevation of cyclic AMP levels. Superfusion of mouse pancreatic fragments with an exogenous lipid-soluble derivative of cyclic AMP (dibutyryl adenosine 3',5'-cyclic monophosphate) caused dose-dependent increases in amylase secretion. Dibutyryl cyclic AMP also markedly enhanced, in a dose-dependent manner, the ACh-evoked amylase secretion. It is concluded that insulin may exert its potentiating action on ACh-evoked amylase output in the mouse pancreatic acinar cells by elevating both cytoplasmic Ca2+ and cyclic AMP levels.