We examined 45Ca2+ influx in A7r5 vascular smooth muscle cells under cyclical stretch and static conditions and compared the results obtained at resting membrane potential (2.5 mM [K+]o, Em = -58 mV according to uptake of [3H]tetraphenylphosphonium) with those under depolarizing conditions (70 mM [K+]o, Em = -27 mV). Application of 10% average strain (24% maximum) in cycles of 3 s on, 3 s off at resting Em caused a 5-fold increase in Ca2+ influx rate to a level similar to depolarized cells and depolarized, stretched cells. 1 μM (+)-isradipine blocked 90% of the stretch-or depolarization-activated Ca2+ uptake. When the cells were stretched under Na+-free conditions, a reduction, not activation, of Ca2+ influx rate occurred. Our results suggest that stretching of cultured aortic vascular smooth muscle cells enhances Ca2+ uptake through a voltage-dependent, dihydropyridine-sensitive Ca2+ entry pathway, whose activation by stretch is dependent upon extracellular Na+.
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