Ryanodine modulation of 45Ca efflux and tension in rabbit aortic smooth muscle.

The effects of ryanodine on isometric tension development, net cellular Ca content and 45Ca efflux were measured in the isolated rabbit aorta. Pretreatment of the tissue with 10 microM ryanodine for 30 min reduced the norepinephrine (NE)-induced tension of the aortic rings bathed in the absence of extracellular Ca2+ in parallel with a reduction in the NE-stimulated 45Ca efflux. Ryanodine alone caused a delayed moderate increase in 45Ca efflux, slowly increasing the fractional loss from 0.02/min to 0.03/min, without any increase in tension. The increased 45Ca efflux was accompanied by a decrease in the net cellular Ca content. When ryanodine and NE were administered in sequence during 45Ca efflux, we found a quantitative correlation between the ryanodine induced loss of 45Ca and the inhibition of the NE-induced 45Ca release. We conclude that the inhibition of the NE-induced contraction by 10 microM ryanodine is the result of depletion of Ca from intracellular stores rather than inhibition of Ca2+ release.