Biphasic effects of cell volume on excitation-contraction coupling in rabbit ventricular myocytes

We studied the effects of osmotic swelling on the components of excitation-contraction coupling in ventricular myocytes. Myocyte volume rapidly increased 30% in hyposmotic (0.6T) solution and was constant thereafter. Cell shortening transiently increased 31% after 4 min in 0.6T but then decreased to 68% of control after 20 min. In parallel, the L-type Ca2+ current ( I Ca-L) transiently increased 10% and then declined to 70% of control. Similar biphasic effects on shortening were observed under current clamp. In contrast, action potential duration was unchanged at 4 min but decreased to 72% of control after 20 min. Ca2+ transients were measured with fura 2-AM. The emission ratio with excitation at 340 and 380 nm (f340/f380) decreased by 12% after 3 min in 0.6T, whereas shortening and I Ca-L increased at the same time. After 8 min, shortening, I Ca-L, and the f340/f380 ratio decreased 28, 25, and 59%, respectively. The results suggest that osmotic swelling causes biphasic changes in I Ca-L that contribute to its biphasic effects on contraction. In addition, swelling initially appears to reduce the Ca2+ transient initiated by a given I Ca-L, and later, both I Ca-L and the Ca2+ transient are inhibited.