Insulin and insulin-like growth factor-1 modulation of glucose transport in arterial smooth muscle cells: implication of GLUT-4 in the vasculature

Insulin-Independent and Dependent Glucose Transporters in Brain Mural Cells in CADASIL

Typical cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is caused by mutations in the human NOTCH3 gene. Cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy is characterized by subcortical ischemic strokes due to severe arteriopathy and fibrotic thickening of small vessels. Blood regulating vascular smooth muscle cells (VSMCs) appear as the key target in CADASIL but the pathogenic mechanisms remain unclear. With the hypothesis that brain glucose metabolism is disrupted in VSMCs in CADASIL, we investigated post-mortem tissues and VSMCs derived from CADASIL patients to explore gene expression and protein immunoreactivity of glucose transporters (GLUTs), particularly GLUT4 and GLUT2 using quantitative RT-PCR and immunohistochemical techniques. In vitro cell model analysis indicated that both GLUT4 and -2 gene expression levels were down-regulated in VSMCs derived from CADASIL patients, compared to controls. In vitro studies further indicated that the down regulation of GLUT4 coincided with impaired glucose uptake in VSMCs, which could be partially rescued by insulin treatment. Our observations on reduction in GLUTs in VSMCs are consistent with previous findings of decreased cerebral blood flow and glucose uptake in CADASIL patients. That impaired ability of glucose uptake is rescued by insulin is also consistent with previously reported lower proliferation rates of VSMCs derived from CADASIL subjects. Overall, these observations are consistent with the development of severe cerebral arteriopathy in CADASIL, in which VSMCs are replaced by widespread fibrosis.

Nutrient pathways and breast cancer risk: the Long Island Breast Cancer Study Project

The relative importance of biochemical pathways has not been previously examined when considering the influence of diet on breast cancer risk. To address this issue, we used interview data from a population-based sample of 1463 breast cancer cases and 1500 controls. Dietary intake was assessed shortly after diagnosis using a 101-item food frequency questionnaire. Age- and energy-adjusted odds ratios (ORs) for individual micro- and macronutrients were estimated with logistic regression. Hierarchical modeling was used to account for biologically plausible nutrient pathways (1-carbon metabolism, oxidative stress, glycemic control, and phytoestrogens). Effect estimates from hierarchical modeling were more precise and plausible compared to those from multivariable models. The strongest relationship observed was for the glycemic control pathway, but confidence intervals (CI) were wide [OR (95% CI): 0.86 (0.62, 1.21)]. Little or no effect was observed for the 1-carbon metabolism, oxidative stress, and phytoestrogen pathways. Associations were similar when stratified by supplement use. Our approach that emphasizes biochemical pathways, rather than individual nutrients, revealed that breast cancer risk may be more strongly associated with glycemic control factors than those from other pathways considered. Our study emphasizes the importance of accounting for multiple nutrient pathways when examining associations between dietary intake and breast cancer.

Role of angiotensin II type-1 and type-2 receptors on vascular smooth muscle cell growth and glucose metabolism in diabetic rats

This study investigates the mechanisms whereby angiotensin II (Ang II) signaling contributes to cell growth and glucose metabolism in cultured vascular smooth muscle cells (VSMCs) from male Wistar fatty rats (WF) and their littermates (Wistar lean rats, WL). The levels of the medial outgrowth rate of VSMCs and Ang II type-1 receptors (AT1R) in aortae from WF were more enhanced than those in aortae from WL, but the level of Ang II type-2 receptors (AT2R) was not different. A mixture of insulin and Ang II additively increased the values of [(3)H]-thymidine incorporation in WF and WL, which was inhibited by olmesartan, an AT1 receptor blockade (ARB), but not by PD123,319, an AT2 receptor blockade. Similarly, insulin and Ang II phosphorylated extracellular-regulated protein kinase 1/2, retinoblastoma tumor suppressor protein, and cyclic AMP response element binding protein, and these levels were higher in WF than in WL. In contrast, the phosphorylation was suppressed by olmesartan but not PD123,319. Insulin-stimulated Akt phosphorylation and 2-deoxy-d-glucose uptake in WF were significantly reduced by Ang II, and the reduction was ameliorated by olmesartan but not PD123,319. Differently from the result of Akt, the phosphorylation of the insulin-stimulated insulin receptor beta-subunit was not affected by Ang II, olmesartan, or PD123,319. However, the phosphorylation of insulin-stimulated insulin-related substrate (IRS)-1 was suppressed by Ang II, and the suppression was ameliorated by olmesartan, but not PD123,319, in both WF and WL. In contrast, the phosphorylation of IRS-1 on Ser(307) was elevated by the Ang II, and the elevation was suppressed by olmesartan, but not by PD123,319, in both WF and WL. These findings demonstrated that Ang II signaling contributes to cell proliferation and inhibition of the insulin signaling pathways through AT1R, but not trough AT2R, in both non-diabetic and diabetic VSMCs.

ERK1/2 activation by angiotensin II inhibits insulin-induced glucose uptake in vascular smooth muscle cells

Clinical evidence suggests a relationship between hypertension and insulin resistance, and cross-talk between angiotensin II (Ang II) and insulin signaling pathways may take place. We now report the effect of Ang II on insulin-induced glucose uptake and its intracellular mechanisms in vascular smooth muscle cells (VSMC). We examined the translocation of glucose transporter-4 (GLUT-4) and glucose uptake in rat aortic smooth muscle cells (RASMC). Mitogen-activated protein (MAP) kinases and Akt activities, and phosphorylation of insulin receptor substrate-1 (IRS-1) at the serine and tyrosine residues were measured by immunoprecipitation and immunoblotting. As a result, Ang II inhibited insulin-induced GLUT-4 translocation from cytoplasm to the plasma membrane in RASMC. Ang II induced extracellular signal-regulated kinase (ERK) 1/2 and c-Jun N-terminal kinase (JNK) activation and IRS-1 phosphorylation at Ser307 and Ser616. Ang II-induced Ser307 and Ser616 phophorylation of IRS-1 was inhibited by a MEK inhibitor, PD98059, and a JNK inhibitor, SP600125. Ang II inhibition of insulin-stimulated IRS-1 tyrosyl phophorylation and Akt activation were reversed by PD98059 but not by SP600125. Ang II inhibited insulin-induced glucose uptake, which was also reversed by PD98059 but not by SP600125. It is shown that Ang II-induced ERK1/2 activation inhibits insulin-dependent glucose uptake through serine phophorylation of IRS-1 in RASMC.

Insulin-like growth factor binding protein-1 and interleukin-6 are markers of fetal stress during parturition at term gestation

OBJECTIVE

Maintaining an adequate blood glucose level is essential for neuron integrity. The increased energy demand imposed on the fetus by the birth process in combination with a limited glucose production capacity therefore threatens brain function. It is logical to presume that mechanisms increasing glucose mobilization as well as decreasing peripheral glucose utilization has evolved to preserve brain function, even after complicated deliveries.

DESIGN

We studied umbilical cord levels of hormones involved in acute glucose regulation as well as insulin-like growth factor-I (IGF-I), modulating factors insulin-like growth factor binding protein (IGFBP)-1 and -3 as well as interleukin-6 (IL-6) in 149 infants born after different degrees of birth stress. We measured glucose, insulin, IGF-I, IGFBP-1, IGFBP-3, glucagon, growth hormone (GH), prolactin, adrenocorticotropin (ACTH), cortisol and IL-6 in umbilical cord blood of infants born at term gestation after: A) elective Cesarean-section (n = 37), B) normal delivery (n = 87) or C) complicated delivery (n = 25). All infants were of normal birth weight for gestational age. Arterial pH and lactate as well as S-100B, a marker of neuronal damage, were used as stress variables.

RESULTS

With increasing fetal stress, we found significant and generally progressive elevations in glucose, IGFBP-1, IL-6, ACTH, cortisol, glucagon, GH, prolactin and lactate. This was accompanied by significant decreases of IGF-I, insulin and arterial pH. S-100B and IGFBP-3 levels did not differ between groups. IGFBP-1 showed a significant positive correlation to IL-6 and lactate and a significant negative correlation to both IGF-I and arterial pH.

CONCLUSIONS

Increasing stress and energy demands during birth are accompanied by increasing fetal levels of glucose-mobilizing hormones in combination with depressed levels of insulin and IGF-I, despite increasing blood glucose. Furthermore, IGFBP-1 and IL-6 increase steeply, presumably aimed at diminishing insulin-like activity of IGF-I, thereby reducing peripheral glucose utilization. We believe that IGFBP-1 and IL-6 deserve evaluation as potential intrapartum indicators of fetuses at risk for asphyxia.

GLUT4 facilitative glucose transporter specifically and differentially contributes to agonist-induced vascular reactivity in mouse aorta

OBJECTIVE

We hypothesized that GLUT4 is a predominant facilitative glucose transporter in vascular smooth muscle cells (VSMCs), and GLUT4 is necessary for agonist-induced VSMC contraction.

METHODS AND RESULTS

Glucose deprivation and indinavir, a GLUT4 antagonist, were used to assess the role of GLUT4 and non-GLUT4 transporters in vascular reactivity. In isolated endothelium-denuded mouse aorta, approximately 50% of basal glucose uptake was GLUT4-dependent. Norepinephrine-mediated contractions were dependent on both GLUT4 and non-GLUT4 transporters, serotonin (5-HT)-mediated contractions were mainly GLUT4-dependent, and prostaglandin (PG) F(2alpha)-mediated contractions were dependent on non-GLUT4 transporters, whereas indinavir had no effect in GLUT4 knockout vessels. We also observed a 46% decrease in GLUT4 expression in aortas from angiotensin II hypertensive mice. Indinavir caused a less profound attenuation of maximal 5-HT-mediated contraction in these vessels, corresponding to the lower GLUT4 levels in the hypertensive aortas. Finally, and somewhat surprisingly, chronic GLUT4 knockout was associated with increased vascular reactivity compared with that in wild-type animals, suggesting that chronic absence or reduction of GLUT4 expression in VSMCs leads to opposite effects observed with acute inhibition of GLUT4.

CONCLUSIONS

Thus, we conclude that GLUT4 is constitutively expressed in large arteries and likely participates in basal glucose uptake. In addition, GLUT4, as well as other non-GLUT4 facilitative glucose transporters, are necessary for agonist-induced contraction, but each transporter participates in VSMC contraction selectively, depending on the agonist, and changes in GLUT4 expression may account for some of the functional changes associated with vascular diseases like hypertension.

Insulin increases glycolysis without further vasodilation in porcine coronary arteries exposed to hypoxia

In acute ischaemia, glucose-insulin-potassium administration reduces mortality and beta-adrenoceptor antagonists have favourable effects on the outcome of ischaemic heart disease. The present study was designed to investigate whether insulin (1.4x10(-7) M) and the beta-adrenoceptor antagonist, propranolol (10(-5) M), increase hypoxic vasodilation in correspondence with changes in glycolysis. Porcine coronary arteries, precontracted with 10(-5) M prostaglandin F(2alpha), were mounted in a pressure myograph and a microdialysis catheter was inserted in the tunica media. Hypoxic vasodilation, interstitial lactate/pyruvate ratio and interstitial glucose were measured at low (2 mM) and high (20 mM) glucose concentrations. Hypoxia (60 min) caused vasodilation and doubled the lactate/pyruvate ratio. Treatment with insulin quadrupled the lactate/pyruvate ratio during hypoxia, but did not change hypoxic vasodilation. Propranolol blocked isoprenaline-evoked vasodilation, but hypoxic increases in lactate/pyruvate ratio and vasodilation did not change. The combination of insulin and propranolol did not cause further changes compared with each drug added alone, although the combination increased vasoconstriction during reoxygenation. Interstitial glucose fell during hypoxia at an organ bath glucose concentration of 2 mM, and rose at a glucose concentration of 20 mM. Addition of insulin and propranolol alone or in combination had no effect on interstitial glucose concentration. Accordingly, arteries were found to contain only minute amounts of the glucose transporter isoform GLUT4. Our findings suggest that insulin increases arterial glycolysis, but treatment with insulin, propranolol, or both, is not associated with enhanced coronary vasodilation during hypoxia.

Insulin stimulates glucose transport via nitric oxide/cyclic GMP pathway in human vascular smooth muscle cells

OBJECTIVE

In cultured human vascular smooth muscle cells, insulin increases cyclic GMP production by inducing nitric oxide (NO) synthesis. The aim of the present study was to determine whether in these cells the insulin-stimulated NO/cyclic GMP pathway plays a role in the regulation of glucose uptake.

METHODS AND RESULTS

Glucose transport in human vascular smooth muscle cells was measured as uptake of 2-deoxy-d-[3H]glucose, cyclic GMP synthesis was checked by radioimmunoassay, and GLUT4 recruitment into the plasma membrane was determined by immunofluorescence. Insulin-stimulated glucose transport and GLUT4 recruitment were blocked by an inhibitor of NO synthesis and mimicked by NO-releasing drugs. Insulin- and NO-elicited glucose uptake were blocked by inhibitors of soluble guanylate cyclase and cyclic GMP-dependent protein kinase; furthermore, glucose transport was stimulated by an analog of cyclic GMP.

CONCLUSIONS

Our results suggest that insulin-elicited glucose transport (and the corresponding GLUT4 recruitment into the plasma membrane) in human vascular smooth muscle cells is mediated by an increased synthesis of NO, which stimulates the production of cyclic GMP and the subsequent activation of a cyclic GMP-dependent protein kinase.

Characterization of multiple signaling pathways of insulin in the regulation of vascular endothelial growth factor expression in vascular cells and angiogenesis

The effects of insulin on vascular endothelial growth factor (VEGF) expression in cultured vascular cells and in angiogenesis were characterized. Insulin increased VEGF mRNA levels in mouse aortic smooth muscle cells from 10(-9) to 10(-7) m with an initial peak of 3.7-fold increases at 1 h and a second peak of 2.8-fold after 12 h. The first peak of VEGF expression was inhibited by LY294002, an inhibitor of phosphatidylinositol (PI) 3-kinase, and by the overexpression of dominant negative forms of p85 subunit of PI 3-kinase or Akt. Inhibitors of MEK kinase, PD98059, or overexpression of dominant negative forms of Ras was ineffective. In contrast, the chronic effect of insulin on VEGF expression was partially inhibited by both LY294002 or PD98059 as well as by the overexpression of dominant negatives of PI 3-kinase or Ras. The importance of PI 3-kinase-Akt pathway on VEGF expression was confirmed in mouse aortic smooth muscle cells isolated from insulin receptor substrate -1 knockout (IRS-1-/-) mice that showed parallel reductions of 46-49% in insulin-stimulated VEGF expression and PI 3-kinase-Akt activation. Insulin-induced activation of PI 3-kinase-Akt on hypoxia-induced VEGF expression and neovascularization was reduced by 40% in the retina of neonatal hypoxia model using IRS-1-/- mice. Thus, unlike other cells, insulin can regulate VEGF expression by both IRS-1/PI 3-kinase-Akt cascade and Ras-MAPK pathways in aortic smooth muscle cells. The in vivo results provide direct evidence that insulin can modulate hypoxia-induced angiogenesis via reduction in VEGF expression in vivo.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

IGF-I promotes a shift in metabolic flux in vascular smooth muscle cells

13C-nuclear magnetic resonance (NMR) spectroscopy was used to test our hypothesis that insulin-like growth factor I (IGF-I) stimulates glucose flux into both nonoxidative and oxidative pathways in vascular smooth muscle cells (VSMC). Rat VSMC were exposed to uniformly labeled [13C]glucose ([U-13C]glucose; 5.5 mM) and [3-13C]pyruvate (1 mM) in the presence and absence of IGF-I (100 ng/ml). IGF-I increased glucose flux through glycolysis and the tricarboxylic acid (TCA) cycle as well as total anaplerotic flux into the TCA cycle. Previous work in our laboratory identified an increase in GLUT1 content and glucose metabolism in neointimal VSMC that was sufficient to promote proliferation and inhibit apoptosis. To test whether IGF-I could potentiate the GLUT1-induced increased flux in the neointima, we utilized VSMC harboring constitutive overexpression of GLUT1. Indeed, IGF-I markedly potentiated the GLUT1-induced increase in glucose flux through glycolysis and the TCA cycle. Taken together, these findings demonstrate that upregulation of glucose transport through either IGF-I or increased GLUT1 content stimulates glucose flux through both nonoxidative and oxidative pathways in VSMC.

Decreased vascular glucose transporter expression and glucose uptake in DOCA-salt hypertension

OBJECTIVE

Because glucose uptake and metabolism can affect vascular smooth muscle cell function, we proposed that animals with hypertension might develop alterations in glucose transporter expression in vascular smooth muscle cells that were responsible for some of the vascular abnormalities characteristic of hypertension.

DESIGN AND METHOD

Male Sprague-Dawley rats (250-300 g) were left uni-nephrectomized and either implanted or not with deoxycorticosterone acetate (DOCA, 200 mg/kg) impregnated silastic. All animals were fed normal rat chow. The DOCA-implanted rats were given water supplemented to 1% NaCl and 0.2% KCl for 7, 14 or 28 days.

RESULTS

The insulin-response glucose transporter (GLUT4) polypeptide levels were depressed several-fold in aortae and carotid arteries from DOCA-salt hypertensive rats compared with sham rats. Uptake of the glucose analog, 2-deoxyglucose (2-DOG), was also reduced 53% in hypertensive compared with sham aortae. There were no changes in GLUT4 expression in other tissues in the DOCA-salt animals, nor were there significant changes in aortae from spontaneously hypertensive rat/stroke prone animals. As previously demonstrated, carotid arteries from DOCA-salt animals exhibited a significant increased contractile sensitivity to ergonovine. Inhibition of glucose metabolism with 2-DOG in sham arteries caused a marked enhancement of contractile responsiveness to ergonovine, whereas 2-DOG had no effect on the already enhanced contractility of DOCA-salt arteries, suggesting that reduction in glucose uptake and metabolism substantially increases the contractile response of DOCA-salt arteries.

CONCLUSIONS

Alterations in glucose uptake and metabolism in vascular smooth muscle cells may participate in the contractile abnormalities characteristic of certain forms of hypertension.

Human rhabdomyosarcoma cells retain insulin-regulated glucose transport activity through glucose transporter 1

We evaluated the expression of glucose transporter (glut) isoforms and its function in RD cells, human rhabdomyosarcoma, which retain the potential to differentiate into muscle. Gluts 1, 3, and 4 were expressed in RD cells, as detected by reverse-transcription polymerase chain reaction and immunocytochemistry. Supraphysiological concentration (1 microM) of insulin treatment increased 2-deoxy glucose transport by up to 1.68-fold together with concomitant tyrosine phosphorylation of the insulin receptor beta subunit and of insulin receptor substrate 1. Suppression of glut 1 mRNA by 38% by antisense oligonucleotide transfection led to a reduction of basal and insulin-stimulated 2-deoxy glucose transport by 38 and 55%, respectively. Suppression of gluts 3 and 4 by antisense oligonucleotide transfection did not affect both basal and insulin-stimulated 2-deoxy glucose transport. Thus, glut 1 accounts for the major part of basal and insulin-stimulated glucose transport in RD cells. Next, we transfected expression vectors carrying human gluts 1 and 4 cDNAs into RD cells to add further support for the role of glut 1 in glucose transport. Overexpression of glut 1 stimulated basal and insulin-stimulated 2-deoxy glucose transport by 1.66- and 1.43-fold, respectively. Glut 4 overexpression did not affect basal and insulin-stimulated 2-deoxy glucose transport. Western blot analysis using glut 1 antibody showed that glut 1 was redistributed from intracellular membrane to plasma membrane. These observations support the notion that RD cells, with the potential to differentiate into muscle, retain insulin responsiveness. As human muscle cell lines are not available at this point, RD cells can serve as a useful alternative to human muscle for studies related to insulin signal transduction and glucose transport.

Participation of PI3K and atypical PKC in Na+-K+-pump stimulation by IGF-I in VSMC

The activity of the Na+-K+-pump is intricately linked to the maintenance of vascular tone. Here we demonstrate that insulin-like growth factor I (IGF-I) increases Na+-K+-pump activity in the vascular smooth muscle cell (VSMC) clone A7r5 in a time- and dose-dependent manner. This stimulatory effect of IGF-I was prevented by the tyrosine kinase inhibitor genistein (5 microM) and by the specific phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY-294002 (25 microM). IGF-I activated a wortmannin-sensitive PI3K and its purported effector, the atypical protein kinase C (PKC)-zeta. Stimulation of PKC-zeta was prevented by the generic PKC inhibitor GF109203x (bisindolylmaleimide, 10 microM). Downregulation of diacylglycerol-sensitive (conventional and novel) PKCs by 24-h pretreatment with 1 microM phorbol 12-myristate 13-acetate had no effect on IGF-I-stimulated Na+-K+-pump activity. Similarly, inhibition of only conventional and novel PKCs with GF109203x (1 microM) had no effect on IGF-I-stimulated Na+-K+-pump activity. In contrast, a concentration of GF109203x (10 microM) that also inhibits the atypical PKCs abolished Na+-K+-pump stimulation by IGF-I. Neither the Na+-K+-2Cl- cotransporter inhibitor bumetanide (100 microM) nor the Na+/H+ exchanger inhibitor HOE-694 (5 microM) affected the Na+-K+-pump stimulation by IGF-I, suggesting that a rise in intracellular Na+ concentration is not necessary for increased Na+-K+-pump activity. These results suggest that IGF-I directly stimulates the Na+-K+ pump via a signaling pathway involving PI3K and atypical PKC (zeta).

Cyclic stretch regulates autocrine IGF-I in vascular smooth muscle cells: implications in vascular hyperplasia

Vascular smooth muscle cells (VSMC) subjected to acute or chronic stretch display enhanced growth rates in vitro and in vivo. Clinical examples of vascular hyperplasia (e.g., systolic hypertension and postinjury restenosis) suggest that local insulin-like growth factor I (IGF-I) expression is enhanced. Therefore, we investigated the role of in vitro cyclic stretch on rat VSMC IGF-I secretion and cellular growth. In serum-free medium, cyclic stretch (1 Hz at 120% resting length for 48 h) stimulated thymidine incorporation approximately 40% above that seen in nonstretched cells. Graded stretch magnitude (100-125% resting length) yielded graded increases in VSMC growth. Exogenous IGF-I increased growth of serum-starved, nonstretched VSMC in a dose-dependent manner, with maximal growth seen with 10(-7) M. IGF-I secretion from stretched cells was 20- to 30-fold greater than from those cells cultured in a static environment. Stretch-induced increases in growth were completely blocked on addition of anti-IGF-I and partially blocked with platelet-derived growth factor (PDGF) antibodies and with a tyrosine kinase inhibitor (tyrphostin-1). Finally, blockade of stretch-activated cation channels with GdCl3 profoundly inhibited stretch-induced growth. We conclude that stretch increases VSMC IGF-I secretion and that such autocrine IGF-I is required for stretch-induced growth. PDGF and stretch-sensitive cation channels are likely additional components of a complex pathway that regulates stretch-induced VSMC seen in systolic hypertension and postinjury restenosis.

Insulin and insulin-like growth factor I differentially induce alpha1-adrenergic receptor subtype expression in rat vascular smooth muscle cells

Hyperinsulinemia has been implicated as an important risk factor for the development of accelerated cardiovascular disease. We wondered if insulin or IGF-I induced expression of alpha1 adrenergic receptors in vascular smooth muscle cells (VSMCs) which could enhance smooth muscle contraction and cell growth activated by catecholamines. Rat aortic VSMCs were incubated with insulin or IGF-I for various times and expression of alpha1 receptors was detected using [3H]prazosin binding. Both insulin and IGF-I increased alpha1 receptor number; also, these peptides increased expression of the alpha1D receptor gene with no change in expression of the alpha1B receptor gene as detected by RNase protection assays. Using Western blotting, we found that these peptides increased expression of the alpha1D receptor subtype in these cells. Increased expression of the alpha1D receptor mRNA was inhibited by the receptor tyrosine kinase inhibitor genistein and the PI 3-kinase inhibitor wortmannin but was not inhibited by protein kinase C inhibitor H7 or the L-type calcium channel blocker nifedipine. Preincubation of cells with insulin or IGF-I enhanced subsequent norepinephrine stimulation of mitogen activated kinase activity. These results suggest that insulin/IGF-I regulate expression of alpha1 receptors in VSMCs and potentially enhance the effects of catecholamines in settings of hyperinsulinemia.

Insulin-stimulated glucose transport inhibits Ca2+ influx and contraction in vascular smooth muscle

BACKGROUND

Insulin attenuates serotonin-induced Ca2+ influx, the intracellular Ca2+ transient, and contraction of cultured vascular smooth muscle cells from dog femoral artery. These studies were designed to test whether insulin-induced glucose transport was an early event leading to the inhibitory effects of insulin on Ca2+ influx, intracellular Ca2+ concentration, and contraction in these cells.

METHODS AND RESULTS

Insulin 1 nmol/L stimulated the 30-minute uptake of [3H]2-deoxyglucose in these cells via a phloridzin-inhibitable mechanism. Contraction of individual cells was measured by photomicroscopy, intracellular Ca2+ concentration was monitored by measuring fura 2 fluorescence by use of Ca(2+)-sensitive excitation wavelengths, and Ca2+ influx was estimated by the rate of Mn2+ quenching of intracellular fura 2 fluorescence when excited at a Ca(2+)-insensitive wave-length. In the presence of 5 mmol/L glucose, preincubation of cells for 30 minutes with 1 nmol/L insulin inhibited 10(-5) mol/L serotonin-induced contraction of individual cells by 62% (P < .01) and decreased the serotonin-stimulated component of Mn2+ influx by 78% (P < .05). Removing glucose from the preincubation medium or adding 1 mmol/L phloridzin completely eliminated these effects of insulin. Insulin lowered the serotonin-induced intracellular Ca2+ peak by 37% (P < .05), and phloridzin blocked this effect of insulin. When glucose uptake was increased to the insulin-stimulated level by preincubation of the cells for 30 minutes with 25 mmol/L glucose in the absence of insulin, serotonin failed to stimulate Mn2+ influx, the serotonin-induced Ca2+ peak was decreased by 46% (P < .05), serotonin-induced contraction was inhibited by 60% (P < .01), and addition of insulin did not further inhibit contraction.

CONCLUSIONS

Since the effects of insulin on serotonin-stimulated Ca2+ transport, intracellular Ca2+ concentration, and contraction were dependent on glucose transport and were duplicated when glucose transport was stimulated by high extracellular glucose concentration rather than insulin per se, it is concluded that insulin-stimulated glucose transport is an early event that leads to decreased Ca2+ influx and contraction in vascular smooth muscle.

Effects of pioglitazone on calcium channels in vascular smooth muscle

Pioglitazone, an insulin-sensitizing, antidiabetic agent, has blood pressure-lowering effects in insulin-resistant hypertensive rats and attenuates growth factor-induced increases of intracellular Ca2+ in rat aortic vascular smooth muscle cells. To determine whether modulation of voltage-dependent Ca2+ channels plays a role in this association, we investigated the effects of pioglitazone on voltage-dependent current in cultured rat aortic (a7r5) and freshly dissociated rat tail artery vascular smooth muscle cells. Both cell types were studied with whole-cell patch-clamp techniques. Current through L-type Ca2+ channels was elicited with a voltage ramp in the presence of Ba2+ substituted for Ca2+. T-type Ca2+ current was studied using a two-pulse protocol that enabled the isolation of transient current. In a7r5 vascular smooth muscle cells, 2-minute application of pioglitazone (5 and 10 mumol/L) reduced L-type current by 7.9 +/- 1.0% (n = 8) (mean +/- SEM, number of cells) and 14.5 +/- 3.0% (n = 9) (P < .01, two-tailed paired t test), respectively. In contrast, 2-minute application of pioglitazone had no significant effect on T-type Ca2+ current. In freshly dissociated tail artery vascular smooth muscle cells, 2-minute application of 10 mumol/L pioglitazone had an insignificant effect (4.8 +/- 5.6% reduction); however, 25 mumol/L pioglitazone reduced L-type current by 27.3 +/- 7.2% (n = 5) (P < .01). Two-minute application of 0.1% or 0.2% dimethyl sulfoxide (vehicle) alone had no significant effects on currents in either type of vascular smooth muscle cell.(ABSTRACT TRUNCATED AT 250 WORDS)