Stretch-induced increases in intracellular calcium of isolated vascular smooth muscle cells

Vascular smooth muscle responds to stretch with an increase in active force development. To investigate the role of Ca2+ in this response, we used the fluorescent dye fura-2 to quantitate changes in cytosolic Ca2+ in single, vascular smooth muscle cells during rapid stretch. Cells were enzymatically dispersed from pig coronary arteries, loaded with fura-2/AM, and studied using a digital-imaging microscope. Stretch of individual cells was accomplished by attachment with suction to two patch-type micropipettes to apply force to the ends of the cell. Stretch induced the release of Ca2+ from intracellular stores as well Ca2+ influx across the plasma membrane. In physiological saline solution containing 1.5 mM Ca2+, intracellular calcium increased with cell stretch in a sigmoidal fashion. This relationship was shifted upward in 10 mM Ca2+ bath solution and abolished after several minutes in Ca(2+)-free solution. The dihydropyridine Ca2+ channel blocker nifedipine, in doses sufficient to completely block inward Ca2+ current, produced only a partial block of the sustained stretch-induced intracellular Ca2+ response. It is concluded that in isolated pig coronary arterial smooth muscle cells, stretch-induced Ca2+ influx occurs in part via a nifedipine-resistant pathway, which may be a stretch-activated cation channel.