Hyperosmotic but not hyposmotic Stress evokes a rise in cytosolic Ca2+ concentration in endothelium of intact rat aorta

The effects of osmotic stress on the cytosolic Ca2+ concentration ([Ca2+](i )) in the endothelium of excised intact rat aorta were investigated using t he indicator fura-2 and the patch clamp technique. Hyperosmotic stress evok ed a reversible rise in endothelial [Ca2+](i) in the presence but not absen ce of extracellular Ca2+, indicating that it evoked Ca2+ entry without rele ase from intracellular stores. Hyposmotic stress was without significant ef fect. Cytochalasins B and D reduced the effect of hyperosmotic stress but n ot acetylcholine on endothelial [Ca2+](i). In endothelium isolated from und erlying smooth muscle, hyperosmotic stress produced a rise in the [Ca2+](i) , and depolarisation of the endothelial membrane potential whereas hyposmot ic stress was without effect. Mechanosensitive cationic channels recorded i n cell-attached patches were activated by hyperosmotic solutions applied to the endothelium and inhibited by hyposmotic solutions. These data suggest that shrinkage of endothelial cells evokes an increase in [Ca2+](i) by open ing a pathway for Ca2+ entry from the extracellular space. The mechanosensi tive ionic channels which we have previously described may be responsible f or this response.