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 [H-3]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 Ca2+-sensitive excitation wavelengths, and Ca2+ influx was estimated by the rate of Mn2+ quenching of intracellular fura 2 fluorescence when excited at a Ca2+-insensitive wavelength. 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.