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Insulin inhibits serotonin-induced Ca2+ influx in vascular smooth muscle
A M Kahn1, J C Allen, C L Seidel
1Department of Medicine, University of Texas Medical School, Houston 77025.
Background:
Insulin in physiological concentrations attenuates the agonist-induced intracellular Ca2+ ([Ca2+]i) transient and inhibits contraction in individual nonproliferated cultured canine femoral artery vascular smooth muscle cells (VSMCs). In the present study, we wished to define the effects of insulin on individual components of Ca2+ transport in vascular smooth muscle.
Methods And Results:
Insulin (40 microU/mL) attenuated the 5-hydroxytryptamine (5-HT, serotonin; 10(-5) mol/L)-induced [Ca2+]i transient (measured by fura 2 fluorescence) in primary confluent canine femoral artery VSMCs in the presence of extracellular Ca2+. In Ca(2+)-free media, the 5-HT-induced [Ca2+]i transient was reduced by 42% and was not affected by insulin. This finding suggested that insulin inhibits 5-HT-induced Ca2+ influx but does not affect sarcolemmal Ca2+ efflux or Ca2+ release from intracellular stores. In support of those conclusions, we found that insulin inhibited the 5-HT-induced component of Mn2+ (a Ca2+ surrogate) influx (measured by fura 2 fluorescence quenching at the Ca2+ isosbestic excitation wavelength). In addition, 5-HT stimulated the rates of 45Ca2+ efflux from intact cells (a measure of sarcolemmal Ca2+ efflux) and from saponin-permeabilized cells (a measure of Ca2+ release from intracellular stores), but insulin did not affect these rates of 45Ca2+ efflux.
Conclusions:
We conclude that a physiological insulin concentration attenuates the 5-HT-induced [Ca2+]i transient in confluent primary cultured canine femoral artery VSMCs by inhibiting the 5-HT-induced component of Ca2+ influx but not by affecting sarcolemmal Ca2+ efflux or Ca2+ release from intracellular stores.
Insights
Insulin reduces serotonin-induced calcium transients in smooth muscle cells by blocking calcium influx. It does not impact calcium release from stores or efflux from the cell membrane.
Area of Science:
- Vascular smooth muscle physiology
- Cellular calcium signaling
Background:
- Physiological insulin concentrations attenuate agonist-induced intracellular calcium ([Ca2+]i) transients and inhibit contraction in canine femoral artery vascular smooth muscle cells (VSMCs).
- Understanding insulin's precise effects on vascular smooth muscle calcium transport is crucial.
Purpose of the Study:
- To define the effects of insulin on individual components of calcium transport in vascular smooth muscle cells.
- To investigate insulin's mechanism in attenuating agonist-induced calcium transients.
Main Methods:
- Primary confluent canine femoral artery VSMCs were utilized.
- Fura 2 fluorescence was used to measure intracellular calcium ([Ca2+]i) transients and manganese (Mn2+) influx.
- 45Ca2+ efflux assays were performed on intact and saponin-permeabilized cells.
Main Results:
- Insulin attenuated serotonin (5-HT)-induced [Ca2+]i transients in the presence of extracellular calcium.
- In calcium-free media, insulin did not affect the 5-HT-induced [Ca2+]i transient, indicating it inhibits calcium influx, not release or efflux.
- Insulin inhibited the 5-HT-induced component of Mn2+ influx and did not alter 45Ca2+ efflux rates from cells or intracellular stores.
Conclusions:
- Insulin attenuates 5-HT-induced [Ca2+]i transients in VSMCs by inhibiting the 5-HT-induced calcium influx.
- Insulin's action does not involve affecting sarcolemmal calcium efflux or calcium release from intracellular stores.
- These findings clarify insulin's role in regulating vascular smooth muscle calcium dynamics.