Effect of rapid cooling on toad and guinea pig cardiac muscles

When bathing solution temperature was lowered rapidly to below 5 degrees C, contracture was observed in toad and guinea pig cardiac muscles (Rapid Cooling Contracture, RCC). RCC in toad cardiac muscle was observed even in the presence of TTX and Mn, and enhanced by reducing [Na]o and caffeine. RCC in guinea pig cardiac muscle showed two components; phasic component was dependent on stimulation frequency before cooling, stimulation period, [Ca]o, and [Na]o; tonic component was not dependent on these factors, but was enhanced by reducing [Na]o and in high [K]o solution. From these results, the possible role of Ca ion accumulated at intracellular sequestered sites in cardiac muscle was discussed in relation to excitation-contraction (E-C) coupling.