Key result
A 14-day hindlimb suspension in rats increased the amplitude of the twitch [Ca2+]i transient by enhancing the affinity of ryanodine receptors without changing their maximal binding capacity.
Hindlimb suspension alters the intrinsic properties of ryanodine receptors, increasing their affinity and enhancing twitch [Ca2+]i transients in rat ventricular myocytes.
No clinical implications yet; hypothesis-generating for ryanodine receptor changes in cardiac unloading, pending human data.
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.
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Halet et al. (1999) studied this question. 14-day hindlimb suspension was evaluated on [3H]ryanodine binding to rat ventricular microsomes and cytosolic Ca2+ concentration ([Ca2+]i) and voltage-dependent Ca2+ channels. A 14-day hindlimb suspension in rats increased the amplitude of the twitch [Ca2+]i transient by enhancing the affinity of ryanodine receptors without changing their maximal binding capacity.
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