Effects of hindlimb suspension on cytosolic Ca2+ and [H-3]ryanodine binding in cardiac myocytes

Effects of a 14-day hindlimb suspension were examined on [H-3]ryanodine bin ding to rat ventricular microsomes and on cytosolic Ca2+ concentration ([Ca 2+](i)) and voltage-dependent Ca2+ channels in isolated ventricular myocyte s. In suspended rats, the amplitude of the twitch [Ca2+](i) transient was i ncreased without significant modifications of the basal [Ca2+](i) and sarco plasmic reticulum content. Because cell capacitance, L-type Ca2+-current de nsity, and Ca2+-channel gating were not significantly modified after suspen sion, the increase in [Ca2+](i) was expected to reside in a change in ryano dine receptors. Scatchard analysis of [H-3]ryanodine binding revealed that suspension enhanced binding by increasing the affinity of the receptors for [H-3]ryanodine without affecting the maximal binding capacity. Both Ca2+-r elease channel activity and [H-3]ryanodine binding are modulated by Ca2+. H owever, the Ca2+ sensitivity of [H-3]ryanodine binding remained unchanged a fter suspension. Taken together, these results suggest that the increase in twitch [Ca2+](i) transients after suspension may result from a change in t he intrinsic properties of the ryanodine receptors but not from a change in the expression level of these receptors.