Role of Ca2+ in the rapid cooling-induced Ca2+ release from sarcoplasmic reticulum in ferret cardiac muscles

Rapid lowering of the solution temperature (rapid cooling, RC) from 24 to 3A degrees C within 3 s releases considerable amounts of Ca2+ from the sarcoplasmic reticulum (SR) in mammalian cardiac muscles. In this study, we investigated the intracellular mechanism of RC-induced Ca2+ release, especially the role of Ca2+, in ferret ventricular muscle. Saponin-treated skinned trabeculae were placed in a glass capillary, and the amount of Ca2+ released from the SR by RC and caffeine (50 mM) was measured with fluo-3. It was estimated that in the presence of ATP about 45% of the Ca2+ content in the SR was released by RC. The amount of SR Ca2+ released by RC was unchanged by the replacement of ATP by AMP-PCP (a non-hydrolysable ATP analogue and agonist for the ryanodine receptor but not for the Ca2+ pump of SR), suggesting that the suppression of the Ca2+ pump of SR at low temperature might not be a major mechanism in RC-induced Ca2+ release. The free Ca2+ concentration of the solution used for triggering RC-induced Ca2+ release was estimated to be only about 20 nM with fluo-3 or aequorin. When this solution was applied to the preparation at 3A degrees C, only a small amount of Ca2+ was released from SR presumably by the Ca2+-induced Ca2+ release (CICR) mechanism. Thus, in mammalian cardiac muscles, RC releases a part of the (< 50%) stored Ca2+ contained in the SR, and the mechanism of RC-induced Ca2+ release may differ from that of CICR, which is thought to play a role in frog skeletal muscle fibres that express ryanodine receptors of different types.