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

Role of Cellular Calcium Metabolism in Abnormal Glucose Metabolism and Diabetic Hypertension JOSEPH LEVY,

M.D., MICHAEL

6. ZEMEL,

PH.D.,JAMES R. SOWERS, M.D.

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 (Ca”) homeostasis may link insulin resistante and high blood pressure in patients with NIDDM. Observations of abnormal cellular Ca”+ homeostasis in animal models of NIDDM and obesity as well as in diabetic patients are consistent with this hypothesis. Abnormalities in cellular Ca’+ 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.

From the Division of Endocrlnoiogy and Hypertension, Wayne State University, School of Medicine, Detroit, Michigan. Requests for reprints should be addressed +o Dr. Joseph Levy, University Health Center, 4th Floor, POD 4H, 4201 St. Antoine, Detroit, Michigan 48201. ,

he pathophysiology of hypertension, obesity, and T non-insulin-dependent diabetes mellitus (NIDDM) is complex. Factors contributing to hypertension may include an increase in sympathetic tone [1,2], abnormal handling of sodium [3,4], abnormal hormonal milieu [1,3,5], and abnormal calcium (Ca”) homeostasis [6,7]. The pathophysiology of obesity is still obscure. Among factors contributing to development of obesity are increased food intake, hypothalamic lesions, abnormal energy utilization, defective thermogenesis, adipose tissue hyperplasia, defective adipose cell hpolysis, abnormal autonomic innervation, and genetic factors [8,9]. NIDDM is characterized by relative insulin deficiency and insulin resistante [lO,ll], but the exact mechanism(s) of these defects is unclear [12,13]. Regardless of the cause of these three conditions, a considerable body of evidence shows an increased incidence of hypertension in patients who are obese or have NIDDM [1,14-171. Krolewski et aZ [18] reported that 26 percent of the male and 34 percent of female diabetics followed at the Joslin Clinic were hypertensive, compared with 13.7 percent and 19.5 percent of the general American population of men and women, respectively. Higher prevalence rates have been reported in other large-scale surveys of diabetic patients [19-211, and a synthesis of several reports concludes that the prevalence of hypertension among diabetics is 30 to 55 percent, approximately twice that of the general population [17,20-221. The increased prevalence of hypertension in patients who have NIDDM cannot be attributed solely to obesity, which is common in these patients [211. Although an increased prevalence of hypertension among the obese is well documented [1,17], a study in a community of subjects aged 50 to 80 years demonstrated consistent association between diabetes and hypertension in both sexes at all ages; adjustment for obesity reduced the extent but not the presence of the relationship [23]. The mechanisms that render the obese and diabetic populations more susceptible to hypertension remain unclear. Hyperinsulinemia (which might reflect insulin resistance) has been suggested as the link between hypertension, diabetes, and obesity [24]. It is possible that these three conditions share a common abnormality at the cellular level, which accounts for their frequent association. One defect common to all three pathologic states is the existence of peripheral insulin resistance [13,24-291. Recent reports that hypertensive patients who were neither obese nor glucoseintolerant exhibit peripheral insulin resistance support this concept [28,29]. Levy et al [30] have suggested that abnormal cell Caa+ homeostasis might contribute to insulin resistance in NIDDM. This concept has received support from observations suggest-

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ing that abnormal cell Ca” homeostasis may contribute to the insulin resistance in obesity and other insulin-resistant states like steroid-induced insulin resistance [31-341. Accordingly, altered cellular Ca’+ homeostasis might be a common fundamental mechanism for abnormal glucose metabolism and elevated blood pressure in patients with NIDDM. This review summarizes the role of cellular Ca2+ homeostasis in glucose metabolism and hypertension in patients with NIDDM, obesity, and other insulin-resistant conditions, and addresses the possibility that abnormal handling of cellular Ca2+ might be a common link between abnormal glucose metabolism and high blood pressure. ROLE OF CELLULAR CALCIUM IN GLUCOSE HOMEOSTASIS Insulin Secretion

Appropriate insulin secretion plays a key role in maintaining normoglycemia under physiologic conditions. Modulation of insulin release from the pancreatic islets results from complex interplay between substrates, hormones, and neural stimuli. Although it is generally accepted that intracellular Ca’* ([Ca ‘Ii) plays an important role in insulin secretion [35,361, the relationship between changes in [Ca2+li and insulin secretion is complex. Glucose, the primary physiologic stimulus for insulin secretion, has been shown to have 6A-BS

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Figure 1. Effects of intravenous (A) glucose (300 mgikg body weight) and (6) tolbutamide (1 mg) on the serum levels of glucose, insulin and c-peptide in a 14.year-old girl with mild diabetes. Data are from [49].

a stimulatory effect on the influx of Ca2’ in pancreatic islets [37-391. Under some conditions it causes a transient decrease in 45Ca2’ efflux, followed by an increase in 45Ca2+ efflux [381. Indirect approaches have suggested that the initial, rapid phase of glucoseinduced insulin release de ends on the mobilization of intracellular stores of Ca B+ [40] and that the second, sustained phase of insulin release reflects an increase in [Ca”]i derived from extracellular sources via voltage-dependent Ca2+ influx [41]. Studies using purified subcellular islet fractions have identified an adenosine triphosphate (ATP)de endent Ca2’ transport pump and an associated Ca5 +-adenosine triphosphatase (ATPase) activity localized to the islet cell plasma membrane [42]. ATPdependent Ca” transport has been demonstrated to correlate with the Ca +-stimulated ATPase [43,44]. Other investigators have reported an indirect inhibitory effect of D-glucose on the Ca2+-stimulated ATPase activity [45,461. Acidic phospholipids (phosphatidic acid and phosphatidylserine) as well as diacylglycerol dilinolein have been observed to stimulate islet-cell plasma membrane Ca2+-ATPase activity [44]. These observations further emphasize the importance of an intact membrane phospholipid structure in insulin secretion. A Ca2’ transport pump and associated Ca”-stimulated ATPase with regulatory and kinetic parameters distinct from those of the plasma

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membrane Ca2+ transport pump has been found in the islet cell endoplasmic reticulum [471. The physiolo ‘c function of this Ca” pump may be to sequester CaSl+ within the endoplasmic reticulum. However, the pumps’ activities in the two membranes differ significantly. Less [Ca2’]i is needed to activate the Ca2’ transport system in the plasma membrane, and this Ca2+-ATPase reveals both a high and low affinity for Ca” [42,43]. The plasma membrane pump is also stimulated by calmodulin [421. In contrast, endoplasmic reticulum pump activity is stimulated by higher [Ca”]i, has only one affinity for Ca2+, and is not calmodulin dependent [43]. Introduction of new techniques and the use of clonal lines of insulin-secreting cells have clarified the role of Ca”+ in mediating the glucose stimulus. It is evident that, in addition to stimulating Ca2+ entry, glucose also promotes active sequestration of calcium ions in intracellular stores. In addition, glucose stimulates extrusion of Ca2+ from beta-cells. The balance between these processes determines the activity of Ca” in the cytoplasm and consequently, the rate of insulin release [39]. The observation that glucose can not only increase but also lower cytoplasmic Ca2+, suggests that exposure to glucose under certain conditions results in paradoxical inhibition of insulin release [41,48,49] (Figure 1). These findings suggest that intact cellular Ca2+ homeostasis, including membrane Ca”-ATPase, is an essential requirement for proper physiologic response of the beta-cell to insulin secretagogues. Thus, altered cellular Ca” homeostasis might contribute to impairment in insulin secretion. NIDDM is characterized by decreased maximal insulin responsiveness to the potentiating effects of glucose, suggesting a generalized impairment of beta-cell function in patients with NIDDM [501. The question of whether abnormal cellular Ca2’ can contribute to these abnormalities needs further evaluation. THE IMPORTANCE OF CALCIUM IN INSULIN ACTION Although the direct role of Ca”+ in the mechanism of insulin action is controversial [51,52], a large body of evidence corroborates its im ortance. Several insulin-sensitive enzymes are Ca L -dependent [53-551. Extracellular Ca” is needed for a number of insulin effects on cellular metabolism [56]. Several Ca”altering compounds mimic the effects of insulin [57]. Insulin affects Ca’* fluxes in intact cells and alters Ca” homeostasis in plasma membranes [51,58,59]. Insulin appears to increase [Ca2+]i concentration of adipoc es [60] and platelets 1611. Further, chelation of [Ca P‘Ii prevents stimulation of glucose transport by insulin and insulinomimetic agents in adipocytes [62]. In order to consider the possible mechanisms by which insulin regulates [Ca2+]i normal cell Ca2+ homeostasis will be reviewed. [&Ii is maintained at a 104-fold lower level than the extracellular Ca2+ concentration. Maintenance of this low [Ca”]i is achieved by the concerted effects of a variety of mechanisms operating in various subcellular fractions (Figure 2). [Ca’+li is controlled by the uptake and release of Ca2+ across the plasma membrane, the endoplasmic reticulum, and the mitochondrial membrane. Each of these membranes has transport systems that regulate uptake and release of the cation and thus modulate [Ca2’li [63]. In the plasma membrane, two mechanisms for the active transport of calcium out of the cell

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lgure 2. Major pathways tor the transport ot Ca” across cellular membranes. PM = plasma membrane; a = steady Ca2+ Influx; b = voltage-dependent Ca’ ’ influx; c = plasma membrane Ca*+-ATPase; d = plasma membrane Na’/Ca’+ exchange (direction depends on thermodynamic balance between Nat and Ca*‘; electrochemical potential gradients and can vary with tissue and condition). ER = endoplasmic reticulum; e = endoplasmlc reticulum Ca2--ATPase; f = second-messenger-activated Ca’+-efflux; IMM = inner mitochondrial membrane; g = mitochondrral Ca’kiporter; h = mitochondrial Na ‘/Ca’+ exchange; Ca.Xbinding of Ca*+ to cytosolic molecules. Data are from [63].

have been described; the sodium (Na’)/Ca” exchange system and a Ca2+-ATPase linked Ca2+ extrusion pump [64-661. Generally, this specific ATPase predominates in nonexcitable cells and the exchanger in excitable ones, which demand the rapid ejection of comparatively large amounts of Ca2’. This ATPase has a high Ca2’ affinity and low pumping capacity, and the Nac/Ca2+ exchanger has the opposite characteristics. Thus, Ca’+-ATPase may be responsible mainly for fine-tuning [Ca’+]i [66]. If Ca2+ plays an important role in insulin action, insulin should be able to modulate these Ca2+ pump activities. Indeed insulin has been found to directly regulate the activity of the plasma membrane Ca2+ATPase in kidney, heart, liver, and adipocytes [30,58,67-701. Insulin also stimulates the Na+/Ca’+ exchanger in heart sarcolemmal membranes and decreases the inward Ca2’ currents in myocardial cells [69,711. Further, insulin may also induce its effect on cellular Ca2+ homeostasis by affecting movement of other ions. For example, insulin can stimulate the Na+/hydrogen (H+) exchange in adipocytes [‘72] and can indirectly stimulate (Naf + K+)-ATPase activity [‘73] in a variety of tissues [74,751. These effects of insulin on cell Na+ homeostasis might further affect cell Ca2+ homeostasis via the Na+/Ca2’ exchanger [76]. Thus, insulin may modulate cellular Ca2+ concentration by several routes, and the net effect of insulin on cell [Ca’+]i will be, therefore, the result of these integrated actions.

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The exact mechanism by which insulin affects the membrane Ca” pumps is not clear. Plasma membrane Ca”+-ATPase is located in a favorable position for exposure to external stimuli (hormones) and is regulated by a number of hormones (parathyroid hormone, vasopressin, thyroid hormones, growth hormone, glucagon and calcitonin) in different tissues [67,70,77-801. Therefore! the importance of this ATPase in insulin action is of particular interest. Cumulative data suggest several pathways by which the hormone might affect the Ca’+-ATPase activity. Calmodulin, trypsin, free fatty acids, and cholesterol all have been found to increase plasma membrane Ca”ATPase activity [64,81-831. Glucose has been shown to decrease the ATPase activity [841. Of particular interest is the requirement of a complex phospholipid environment for this membrane-bound ATPase [85]. The role of acidic phospholipids in the activation of this enzyme is still controversial. Lowering of the enzyme’s activity by phosphatidic acid and phosphatidylserine has been reported by some investigators [86], whereas others have reported activation of the’Ca”ATPase by acidic phospholipids and fatty acids 1871. These findings are of particular importance in view of the known effects of insulin on membrane phospholipid metabolism [88-90], and might suggest a physiologic mechanism by which insulin can modulate cellular Ca” homeostasis [go]. However, insulin could also affect the ATPase activity by increasing membrane calmodulin content [91] and/or phosphorylation of calmodulin [92,93]. The enzyme’s activity can be stimulated either by increasing its affinity for Ca2+ or by increasing the number of active units. These two mechanisms for stimulating enzyme activity are regulated independently [94]. Recent observations in our laboratory suggest that insulin’s primary effect on the enzyme is to increase its affinity for Ca2+ [951. ROLE OF CELLULAR CA2+ METABOLISM IN DEVELOPMENT OF HYPERTENSION Controversy still exists about the relative roles of Na+ and Ca” in the development of essential hyper6A-10s

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Figure 3. Dependence of the high-affinity Ca*’ t Mg*+-ATPase on free Ca2+ concentration in BLM from diabetic (open circles) and control (solid circles) rat kidney cortex. Data are means F SE of five experiments from different membrane preparations. *p <0.05; **p
tension [96,971. A growing body of evidence suggests that there is an abnormality in Ca2+ homeostasis in hypertensive patients [97]. Abnormalities in [Ca2’]i as well as defects in membrane binding and transport kinetics have been identified in red blood cells, platelets, and adipocytes of patients with essential hypertension [6,7,97-1011. Platelet [Ca2+Ji, has been reported to be elevated in patients with essential hypertension. Antihypertensive therapy reduced the elevated platelet [Ca2’]i, and this reduction correlated with the decrease in blood ressure and peripheral vascular resistance [loll. [Ca%‘Ii was also elevated in red blood cells from essential hypertensive patients [97,98] and increased total Ca” was found in skeletal muscle from essential hypertensive patients [ 1021. Considered together, these findings suggest that the disturbance in cellular Ca2+ homeostasis seen in hypertension is not confined to one tissue but represents a more generalized defect. Such increases in [Ca2’]i in vascular smooth muscle cells may be of primary importance in the origin of increased peripheral vascular resistance, a characteristic pathology of the hypertensive state [91. Controversy also exists about whether the altered Ca2’ metabolism in red blood cells of hypertensive patients is a persistent marker or a sequel of essential hypertension [102]. The abnormalities in cellular Ca”+ homeostasis described in hypertension cannot be attributed directly to changes in calmodulin because calmodulin content and distribution in red blood cells from hypertensive subjects are normal [1031. However, the ability of calmodulin to activate Ca”ATPase is impaired in platelets of hypertensive patients [104]. Although decreased Ca’+-ATPase has been found in patients with hypertension [98,100,1041, an altered Na’+-Ca2+ exchanger could not be verified [1051. In contrast to the ATP-dependent Ca2+ transport, Na+-Ca’+ exchange may not play an important role in either vascular smooth muscle cells or erythrocytes [1051. The origin of the cellular Ca” shifts in essential hypertension is not clear. Genetic defects in cell membrane Ca2+ transport or transport of Ca2+

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IT, Figure 4. Rates of maximally stimulated 2-deoxy glucose uptake (at insulin concentration 25 ngi ml) at the various levels of cytosolic free Ca”. Hatched area represents the window of optimal response to isulin. Data are from [31].

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across intracellular membrane systems may contribute to alterations in cellular Ca” homeostasis. Additional evidence suggests that changes in membrane lipid composition can cause these defects [106]. It has been documented that the physiologic activity of Ca’“-ATPase and (Na+ + K+)-ATPase depend on their phospholipid milieu [85]. Thus, abnormal membrane phospholipid composition may cause changes in basal activities of ATPase and alter the Ca2+-ATPase response to calmodulin as well as to hormonal regulation. More research is needed to evaluate this possibility. ABNORMALITIES IN CELLULAR CALCIUM HOMEOSTASIS IN PATIENTS WITH DIABETES AND OBESITY Abnormalities in Plasma Membrane Ca2+-ATPaseActivity

Ca2+-ATPase activity has been shown to be high in kidney basolateral membranes (BLM) from a NIDDM animal model, at all Ca2’ concentrations studied [30] (Figure 3). In streptozotocin-induced diabetic rats, Chan and Junger [107] found increased influx of 45Ca2+ in inside-out liver membrane vesicles. A similar increase in Ca2+-ATPase activity was also found in erythrocyte membranes of the NIDDM rats [108] suggesting that changes in Ca2+-ATPase activity are generalized and not restricted to one tissue in patients with NIDDM. Further, it has recently been observed that diabetic patients exhibit changes in erythrocyte Ca2+-ATPase activity [log]. These observations suggest an abnormal cellular Ca2’ homeostasis in NIDDM patients. However, although increased Ca2+-ATPase activity was observed in non-obese, non-insulin-dependent diabetic rats, decreased enzyme activity was observed in obese, non-diabetic insulin-resistant rats [34,110,111]. Thus, abnormal cell Ca2’ homeostasis may exist in two types of patients: non-insulin-dependent diabetic patients without obesity but with insulin resistance; and obese, insulinresistant patients who are not diabetic. Abnormal cell Ca” homeostasis might characterize the insulin resistance in both conditions, but the nature of this impairment appears to differ. Indeed, when [Ca2’]i was measured in bone cells from rats with NIDDM, an increased [Ca2’li was seen in cells from the obese, whereas a decreased [Ca’+]i was seen in cells from the non-obese rats with NIDDM [112]. This hypothesis is further supported by recent observations that there is an optimal range of [Ca2’]i for mediating insulin action; [Ca2’li levels higher or lower than the optimal December

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range are associated with impaired insulin action [31] (Figure 4). Observing decreased (Naf + I(+)-ATPase

in the obese and in some diabetic tissues [113-1171 might suggest an additional route for impaired cellular Ca2+ homeostasis in these conditions [114]. impaired Effect of Insulin on Ca’+-ATPase Activity and [Ca2’]i in NIDDM and Obesity

Insulin has been found to directly stimulate Ca’+ATPase activity in kidney membranes from normal rats and dogs, indicating that this effect is not speciesspecific [30,6’7]. The effect of insulin is dose-dependent and only the biologically active insulin molecule has this effect. Desoctapeptide insulin (a biologically inactive insulin molecule) has no effect on the enzyme (Figure 5) [30]. The action of insulin on ATPase ap-

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other hormones to regulate ATPase is preserved in these conditions [341. Altered Cellular Ca2+ Regulation in Other InsulinResistant Conditions

[Ca”+]i increases as human fibroblasts age in culture 11191 and skin Ca’+., but not magnesium, increases with age [1201. Adlpocytes from 12-month-old rats have higher [Ca”]i levels than those of twomonth-old rats. Further, insulin’s ability to increase [Ca’+li is reduced in cells from the aged [121]. Abnormal Ca’+-ATPase activity has been observed in kidney basolateral membranes of steroid-treated rats [33]. These two conditions (aging and steroid treatment) are associated with impaired insulin action [122]. It is possible that abnormal cellular Ca2+ homeostasis is common in these conditions and contributes to the insulin resistance.

POSSIBLEMECHANISMFORABNORMALCELLCA’+ REGULATION AND DECREASED RESPONSE OF MEMBRANECA*+-ATPASE TO INSULININ NIDDM Changes in Membrane Phospholipids

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Figure 6. Insulin effect on the (Ca’+ t MgL’).ATPase activity in ELM from control and diabetic rats. Data are means * SE of five experiments performed in quadruplicate. A, Percentage increase in ATPase activity over the basal level (activity in the absence of isulin). B, Changes in absolute activity levels are compared with the basal value in each membrane group. *p <0.05, **p
pears to be hormone specific. Insulin-like growth factor-1 and insulin-like growth factor-It, which also have receptors in kidney BLM, do not directly affect the enzyme’s activity [67]. The effect of the insulin on the enzyme is also specific for Ca’+-ATPase, since no direct regulatory effect of the hormone on (Na+ + K+)-ATPase has been observed [73]. In diabetic rats, insulin loses its regulatory effect on the plasma membrane Ca’+-ATPase (Figure 6), but partially regains its effect in membranes from food-restricted diabetic animals, in which an improvement in insulin resistance is also observed [30,111]. These findings give additional support to the notion that regulation of cellular Ca2+ by insulin, in part through modulation of Ca2+ATPase activity, is important for insulin action. It also suggests that impaired activity of Ca2’ATPase and loss of the regulatory effect of the hormone on the pump enzyme might be important factors in the impaired insulin action seen in NIDDM patients. Additional support for this theory comes from observations that anti-insulin receptor antibodies, known to be associated with insulin resistance in vivo, abolish insulin’s stimulatory effect on the enzyme [118]. Further, insulin’s effect on the enzyme was markedly reduced in membranes from obese rats [341. This last observation is in concert with a reduced effect of insulin on [Ca2’li in adipocytes obtained from obese patients (Figure 7) [32]. Both of these defects in the ability of insulin to regulate the enzyme (in obesity and NIDDM) are hormone specific, as the ability of 6A-12s

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McMillan [123] hypothesized that abnormalities in insulin’s effect on its target tissues in diabetic patients might be related to changes in cell membrane phospholipid composition. Indeed, changes in membrane phospholipid content or distribution in diabetic patients have been reported [90,124,125]. The diabetic condition appears to be associated with only small changes in phospholipid content, affecting mainly those phospholipids that predominate in the inner half of the membrane lipid bilayer (phosphatidylethanolamine, phosphatidylserine, phosphatidyinositol, and phosphatidic acid) [116,126]. Although the absolute change in phospholipid content is relatively small, redistribution of phospholipids in the membrane bilayers might have additional important effects in regulating membrane properties [124]. As discussed previously, altered membrane phospholipid content might be of importance in regulating membrane ATPase activities. Further, insulin has been found to acutely increase phospholipids in the phosphatidate-inositide cycle in rat adipose tissue and monocytes, both in vivo and in vitro [88,127]. An insulin-induced increase in phosphatidic acid content has been noted in kidney BLM of normal rats but not in membranes from NIDDM rats [go]. As acidic phospholipids increase Ca2+ -ATPase activity [87], it is possible that the increase in phosphatidic acid content in membranes from control rats could mediate the stimulating effect of insulin on the enzyme activity [go]. Similarly, a lack of increase in phosphatidic acid in diabetic rat membranes may be important in explaining the lack of regulatory effect of the hormone on Ca2’ATPase in membranes from diabetic rats. The possibility that a similar mechanism applies to the lack of insulin effect on the ATPase in obese patients [341 needs to be evaluated. Effects of Insulin on Calmodulin Metabolism

In addition to phospholipid regulation, the membrane Ca2+-ATPase is also regulated by calmodulin. Calmodulin increases the maximal velocity of the enzyme and increases the ATPase’s affinity for Ca2” [66]. Therefore, insulin-induced changes in calmodulin function may be important in mediating the hormone’s

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effect on membrane Ca”+-ATPase. Insulin has been shown to increase calmodulin binding to the plasma membrane [91]. Insulin was also found to stimulate phosphorylation of calmodulin in isolated membranes and intact fat cells, via the insulin receptor tyrosine kinase [93,128]. The exact effect of these changes in calmodulin function on the membrane Ca’+-ATPase is not clear. Insulin-induced phosphorylation may alter binding and response of calmodulin to Ca” [931 and might alter sensitivity of cellular enzymes to calmodulin and Ca2+ [93]. Insulin-induced enhanced binding of calmodulin to the membrane might explain the stimulatory effect of the hormone on the enzyme. However, an alternative explanation would be that an increase in binding of calmodulin to other proteins in the membrane decreases its availability for the Ca’+-ATPase molecule, and consequently, results in an inhibitory effect on Ca2+-ATPase as was observed in adipocytes under certain conditions [91,129]. Changes in calmodulin function may also explain decreased basal activity of the enzyme in erythrocytes of insulin-treated diabetic patients [log]. Glycosylation of the calmodulin molecule has also been suggested to explain the reduced activity of the enzyme [log]. Another mechanism by which changes in calmodulin can affect the ATPase in diabetes and obesity is via impairment in its phosphorylation. Insulin receptor tyrosine kinase activity is decreased in patients with NIDDM [130] and increased in obesity [131]. Therefore, it is possible that insulin-induced phosphorylation of calmodulin will be impaired in diabetes and obese subjects. This might consequently result in the blunted effect of the hormone on the ATPase in these conditions. POSSIBLE ROLE OF CHANGES IN CELLULAR CA*+ HOMEOSTASIS AND MEMBRANE PHOSPHOLIPID CONTENT IN HYPERTENSION ASSOCIATED WITH DIABETES Hypertension in diabetes is characterized by increased vascular resistance and exaggerated pressor responses to vasoactive substances [17,21,132,133]. Increased [Ca”]i in vascular smooth muscle cells plays a critical role in establishing and maintaining a state of enhanced vascular resistance [134]. Further, altered cellular Caz’ metabolism, as seen in the inDecember

sulin-resistant states of NIDDM and obesity, may alter the hormonal signal of calcium-mobilizing hormones [135] and result in an increased pressor response even without an increase in the plasma level of the hormone [132,133]. Indeed, altered cellular Ca2+ homeostasis characterized by decreased Ca’+-ATPase activity and increased erythrocyte [Ca”]i, has been observed in diabetic hypertensive patients [136]. In addition, decreased 45Ca + efflux has been observed in 45Ca2+-loaded aortic strips (a preparation mainly of smooth muscle cells) from obese, insulin-resistant, hypertensive rats (Figure 8) [137]. These findings further suggest a possible role of abnormal cellular Ca” homeostasis in patients with the high blood pressure seen in insulin-resistant conditions. Reports that angiotensin increases inositol triphosphate and calcium in vascular smooth-muscle cells [138] further emphasize the role membrane phospholipid content plays in mediating pressor responses to circulating vasoactive hormones and maintenance of physiologic blood pressure. Further, these findings support the concept that altered membrane phospholipid content, as seen in patients with diabetes [90,124,1251 might also alter the effect of vasoactive hormones on their target cells, and raise blood pressure. COMMENTS Cumulative evidence suggests that abnormal cellular Ca2’ homeostasis is common in insulin-resistant patients; this abnormal cell Ca2’ metabolism can contribute to impaired insulin action and secretion, and result in glucose intolerance. Similarly, altered cellular Ca2+ homeostasis may cause increased peripheral vascular resistance and exaggerated pressor responses to vasoactive substances, resulting in hypertension. It has been suggested recently that another complication of diabetes, osteopenia, can be partially explained by a primary defect in cellular Ca” homeostasis [112]. More research is needed to evaluate whether other complications of diabetes are related to abnormalities in cellular handling of Ca2’ in this disease. The exact sequence of events leading to insulin resistance and hypertension is unknown. Figure 9 suggests a hypothetical scheme for a chain of events lead8, 1989

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Figure 8. Impaired ?a*’ effluxfrom obese rat aortae compared with lean rat aortae. Data are from [137].

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ing to impaired glucose tolerance and hypertension. Altered membrane phospholipid content may be of primary importance in this cascade. Although genetic factors may predispose patients to changes in membrane phospholipid content, dietary habits are also known to influence these phospholipids [139]. Altered membrane phospholipids can result in changes in calcium fluxes and Ca’+-ATPase activity. The possibility that a genetic predisposition for altered ATPase activity also contributes cannot be ruled out. The resultant abnormal cellular Ca’* homeostasis with diminished response of Ca 2f-ATPase to insulin could then create an insulin-resistant state. This may be manifested as either type II diabetes or obesity, depending on the pancreatic capacity to secrete insulin. Abnormal cellular Ca2+ homeostasis, as a generalized defect, can cause abnormal insulin secretion, increased vascular resistance, and altered response of vascular smooth muscle cells to Ca2+-mobilizing vasoactive hormones. The vascular changes, in turn, may cause hypertension. Finally, the hypertensive state and the insulinresistant state may have further deleterious effects on membrane phospholipid content and cellular Ca2’ 6A-14s

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Figure 9. Proposed relationship between abnormal calcium homeostasis and insulin resistance, obesity, and hypertension.

homeostasis, creating a relentless cycle [1021. Much research is needed to further elucidate the details of this pathophysiologic process.

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