Force-interval relations of twitches and cold contractures in rat cardiac trabeculae. Effect of ryanodine

The twitch force (Ft)-interval relation of cardiac muscle reflects recovery of calcium release from the sarcoplasmic reticulum (SR). The calcium content of the SR is thought to be reflected by force developed during a contracture (Fc), induced by rapid cooling to near 0°C. In right ventricular trabeculae of rat, under control conditions, the Ft-interval relation consisted of recovery of Ft to steady state (early recovery), followed by a secondary increase of Ft up to a maximum at an interval of-100 seconds (rest potentiation) and a decline of Ft at intervals> 100 seconds (rest depression). The mechanisms that may underlie recovery of force after the last twitch at short intervals are 1) time-dependent transport of Ca2 ' from the uptake compartment of the SR to the release compartment, 2) recovery of slow inward Ca2 ' current during the action potential, and 3) recovery of the Ca2 ' release channels in the SR. The Fc-interval relation was similar to the Ft-interval relation in that both a rest potentiation and a rest depression phase were present. However, at short interstimulus intervals (< 1 second), Fc was independent of time, suggesting that the mechanism underlying early recovery was bypassed. Ryanodine (0.1-10 nM) reduced rest potentiation in a dose-dependent manner and accelerated rest depression of both Ft and Fc. At high ryanodine concentration, a significant Fc could only be induced after short intervals. Significant acceleration of rest depression wa