Insulin-stimulated glucose transport inhibits Ca2+ influx and contraction in vascular smooth muscle
A M Kahn1, R A Lichtenberg, J C Allen
1Department of Medicine, University of Texas Medical School, Houston, TX 77025, USA.
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.
Insights
Insulin
Area of Science:
- Vascular smooth muscle physiology
- Endocrinology
- Cellular signaling
Background:
- Insulin is known to inhibit serotonin-induced calcium (Ca2+) influx, intracellular Ca2+ transients, and contraction in vascular smooth muscle cells.
- The precise mechanism by which insulin exerts these inhibitory effects requires further elucidation.
Purpose of the Study:
- To investigate whether insulin-stimulated glucose transport is an early event mediating insulin's inhibitory effects on Ca2+ influx, intracellular Ca2+ concentration, and contraction in dog femoral artery vascular smooth muscle cells.
- To determine the role of glucose transport in insulin's vascular effects.
Main Methods:
- Cultured vascular smooth muscle cells were used.
- Insulin's effect on [3H]2-deoxyglucose uptake was measured.
- Cell contraction was assessed using photomicroscopy.
- Intracellular Ca2+ concentration and Ca2+ influx were monitored using fura 2 fluorescence and Mn2+ quenching techniques, respectively.
- The role of glucose transport was examined by manipulating extracellular glucose concentrations and using phloridzin, a glucose transport inhibitor.
Main Results:
- Insulin (1 nmol/L) stimulated glucose uptake via a phloridzin-inhibitable pathway.
- Insulin pre-treatment significantly inhibited serotonin-induced cell contraction (62%) and Ca2+ influx (78%) in the presence of glucose.
- These insulin effects were abolished by removing glucose or adding phloridzin.
- Insulin reduced serotonin-induced intracellular Ca2+ peak by 37%, an effect blocked by phloridzin.
- Stimulating glucose uptake with high extracellular glucose (25 mmol/L) mimicked insulin's inhibitory effects on Ca2+ influx and contraction, even in the absence of insulin.
Conclusions:
- Insulin-stimulated glucose transport is an early and critical event in mediating insulin's inhibitory effects on vascular smooth muscle contraction.
- The observed effects of insulin on Ca2+ transport, intracellular Ca2+ concentration, and contraction are dependent on glucose transport.
- These findings suggest that enhancing glucose transport, either by insulin or high glucose concentrations, leads to decreased Ca2+ influx and subsequent reduction in vascular smooth muscle contraction.
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