Activation of L‐arginine transport (system y+) and nitric oxide synthase by elevated glucose and insulin in human endothelial cells.

Abstract

1. Modulation of L-arginine transport (system y+) and release of nitric oxide (NO) and prostacyclin (PGI2) by elevated glucose and insulin were investigated in human cultured umbilical vein endothelial cells. 2. Elevated glucose induced a time-(6-12 h) and concentration-dependent stimulation of L-arginine transport, which was reversible and associated with a 3-fold increase in intracellular cGMP accumulation (index of NO synthesis) and 75 % decrease in PGI2 production. 3. Elevated glucose had no effect on the initial transport rates for L-serine, L-citrulline, L-leucine, L-cystine or 2-deoxyglucose. 4. Resting membrane potential was unaffected by elevated glucose whereas basal intracellular [Ca2P] increased from 65 + 5 nm to 136 + 16 nM.5. Insulin induced a protein synthesis-dependent stimulation of L-arginine transport and increased NO and PGI2 production in cells exposed to 5 mm glucose. 6. In cells exposed to high glucose, insulin downregulated elevated rates of L-arginine transport and cGMP accumulation but had no effect on the depressed PGI2 production. 7. Our findings suggest that insulin's normal stimulatory action on human endothelial cell vasodilator pathways may be impaired under conditions of sustained hyperglycaemia.Endothelium-dependent relaxation is impaired in longlasting diabetes mellitus (reviewed by Cohen, 1993;Poston & Taylor, 1995), yet in the early stages of the disease blood flow is actually increased in various organs (reviewed by Tooke, 1989). The generalized vasodilatation in diabetic patients appears to be influenced by the degree of hyperglycaemia and is attenuated upon lowering blood glucose levels (see Tooke, 1989). As endothelium-derived nitric oxide (NO) plays a key role in the regulation of vascular tone (Moncada, Palmer & Higgs, 1991), it seems likely that prolonged hyperglycaemia may modulate the L-arginine-NO signalling pathway. We have recently shown that gestational diabetes induces phenotypic changes in human fetal endothelial cells, which are associated with a membrane hyperpolarization, activation of the L-arginine transporter (system y+), elevation of basal NO synthesis and decreased basal and histamine-stimulated PGI2 release (Sobrevia, Cesare, Yudilevich & Mann, 1995). Recent evidence also implicates insulin as a regulator of the L-arginine-NO vasodilator pathway in man (see Baron, 1994). Moreover, insulin has been reported to enhance the synthesis and expression of a high-affinity cationic amino acid transporter (system y+/ murine cationic amino acid transporter-1 (MCAT-1): Kim, Closs, Albritton & Cunningham, 1991) in rat liver cells (Wu, Robinson, Kung & Hatzoglou, 1994 b).In the present study we have investigated the effects of elevated glucose and human insulin on L-arginine transport and generation of NO and PGI2 in human umbilical vein endothelial cells isolated from normal pregnancies. In addition, we examined whether elevated glucose altered the resting membrane potential and intracellular Ca2+ levels. Our findings establish that both hyperglycaemia and human ...

Acute effects of glucose and insulin on vascular endothelium

“…Moreover, we have demonstrated that the effect of the eNOS substrate L-arginine on NO release and vasodilation is enhanced by D-glucose in a concentration-dependent manner. This finding is in accordance with a report of Sobrevia et al [39] which reported that D-glucose potentiates the uptake of L-arginine in vascular endothelial cells via the sodium-independent y + -transport system. However, this mechanism of eNOS activation may only be relevant at high levels of of L-arginine, and is unlikely to occur at physiological plasma concentrations with baseline levels of about 90 µmol/l and post-prandial peaks of about 130 µmol/l [40,41].…”

The effects of insulin and short-term hyperglycaemia on myocardial blood flow in young men with uncomplicated Type I diabetes

“…Insulin induces a dose and time-dependent vasodilation in the peripheral arteries [9,10,11] by a endothelium-dependent mechanism including the L-arginine-nitric oxide pathway [12,13] and the sympathetic nervous system [13]. Insulin induces vasodilation also in myocardial vasculature in healthy humans [14].…”

Acute hyperglycaemia in the forearm induces vasodilation that is not modified by hyperinsulinaemia

“…Their medical history, physical examination, and routine laboratory tests showed no evidence of diabetes, cardiovascular or other diseases. Their age was 22.5 ± 0.9 years (M ± SE, range 19-28), body mass index 22.8 ± 1.0 kg/m 2 (20)(21)(22)(23)(24)(25)(26)(27)(28)(29), serum cholesterol 5.0 ± 0.4 mmol/l (3.9-7.1), and HbA 1c 5.0 ± 0.1% (4.5-5.3). None of the subjects took any medication for at least 2 weeks before the study.…”

“…7,16,22,23 Others observed no effect on NO production 18 or even the opposite, namely that hyperglycaemia may in fact stimulate the production of NO. 24,25 There is no doubt that the induction of systemic hyperglycaemia in healthy lean and obese men results in increases of leg blood flow. 26 In view of the literature findings discussed, it seems that hyperglycaemia acutely induces vasodilation in the peripheral resistance vessels of the forearm and leg.…”

“…The mechanism of this acute, glucose-induced vasodilation was not investigated in this study. Acutely, it seems that glucose may stimulate the production of nitric oxide, 24,25 however, it may also act indirectly via increased production of insulin. Longer lasting high glucose levels may reduce vascular tone via several mechanisms like, reduced production of nitric oxide, 7 generation of reactive oxygen species, 8 production of vasoconstrictive prostanoids, 9 formation of advanced glycosylation end-products, 10 or release of endothelin.…”