Ca2+ entry and vasoconstriction during osmotic swelling of vascular smooth muscle cells

Exposure of aortic strips from guinea-pigs to hypotonic extracellular fluid is followed by marked vasoconstriction, which is inhibited by D-600 (3 microM), a blocker of voltage-sensitive Ca2+ channels. Conventional electrophysiology, patch-clamp studies, pH determination with 2',7' bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and Ca2+ measurements with Fura-2 have been performed on smooth muscle cells cultured either from rat or human aorta to further elucidate the underlying mechanisms. Exposure of the cells to a 25% hypotonic extracellular fluid leads to a rapid and fully reversible depolarization, paralleled by an increase of the selectivity and conductance of the cell membrane to Cl-, an acidification of the cytoplasm and an increase of intracellular Ca2+ concentration ([Ca2+]i). The latter is inhibited by the Ca2+ channel blocker D-600 (1-3 microM). It is concluded that osmotic cell swelling leads to the activation of an anion channel. The subsequent depolarization of the cell membrane activates voltage-sensitive Ca2+ channels which increases [Ca2+]i, thus stimulating the contraction of vascular smooth muscle cells.