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

Effects of a 14-day hindlimb suspension were examined on [3H]ryanodine binding to rat ventricular microsomes and on cytosolic Ca2+ concentration ([Ca2+]i) and voltage-dependent Ca2+channels in isolated ventricular myocytes. In suspended rats, the amplitude of the twitch [Ca2+]itransient was increased without significant modifications of the basal [Ca2+]iand sarcoplasmic reticulum content. Because cell capacitance, L-type Ca2+-current density, and Ca2+-channel gating were not significantly modified after suspension, the increase in [Ca2+]iwas expected to reside in a change in ryanodine receptors. Scatchard analysis of [3H]ryanodine binding revealed that suspension enhanced binding by increasing the affinity of the receptors for [3H]ryanodine without affecting the maximal binding capacity. Both Ca2+-release channel activity and [3H]ryanodine binding are modulated by Ca2+. However, the Ca2+ sensitivity of [3H]ryanodine binding remained unchanged after suspension. Taken together, these results suggest that the increase in twitch [Ca2+]itransients after suspension may result from a change in the intrinsic properties of the ryanodine receptors but not from a change in the expression level of these receptors.