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⁺]_i transient was increased without significant modifications of the basal [Ca²⁺]_i and 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⁺]_i was 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⁺]_i transients 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.

rat cardiac myocyte; calcium channel; microgravity