Abstract
The effect of rapidly lowering bathing solution temperature from 36.5 +/- 0.5 degrees C to various low temperatures was examined in guinea-pig ventricular muscle to explore the possible role of intracellular Ca2+ store sites in excitation-contraction coupling. Rapid cooling from 36.5 +/- 0.5 degrees C to below 18 degrees C caused contracture (rapid cooling contracture, r.c.c.) with subthreshold depolarization for contraction, if electrical stimulation was applied before cooling. R.c.c. peak tension depended on cooling temperature, and pre-cooling stimulation frequency and duration. R.c.c. induced after pre-cooling stimulation was enhanced by increased extracellular Ca2+ ( [Ca2+]o) and decreased by reduction of [Ca2+]o. Co2+ (2-4 mM) added to the HEPES-buffered Krebs solution, which suppressed the action potential plateau and inhibited twitch response, did not abolish r.c.c. after pre-cooling stimulation at high frequency. Reduction of extracellular Na+ concentration ( [Na+]o) before cooling enhanced r.c.c., and even in non-stimulated preparations, incubation in low [Na+]o below 68.8 mM for 20 min produced r.c.c. R.c.c. was superimposed on the tonic component of the K+ contracture, after a quiescent preparation has been depolarized beyond -40 mV by addition of solid KCl to normal Krebs solution. The relation between r.c.c. tension and membrane potential was shifted to the left along the voltage axis by reducing [Na+]o and shifted to the right by decreasing [Ca2+]o. Results suggest that well-developed intracellular Ca2+ store sites could sequester enough Ca2+ to generate tension by an energy-dependent process which had been loaded mainly by a voltage-dependent Na+-Ca2+ exchange mechanism and Ca2+ current, and that rapid cooling could cause Ca2+ release from the intracellular store sites with little contribution to membrane excitation in the guinea-pig ventricular muscle.