Cellular Calcium Turnover in the Perfused Rat Heart : Modulation by Caffeine and Procaine Douglas

Pool A is a rapidly exchangeable cellular pool of Ca whose release is triggered by a mechanism involving extracellular Ca (Hunter et al., 1981). We have now found that pool A is rapidly released from the perfused rat heart when 10 mM caffeine is added to the perfusate. Pool A release by caffeine was demonstrated during a Ca free perfusion. When the perfusate contained 2.5 mM Ca, caffeine induced an immediate contractile failure. The steady state level of pool A (normally 69 ± 10 nmol Ca /g wet wt heart) was also decreased by 60%. Pool A was similarly depleted in hearts perfused with medium containing 0.2 mM Ca. Procaine (5 mM) inhibited by 50% the release of pool A triggered by caffeine and inhibited by 86% the release triggered by extracellular Ca. The inhibition of Ca-induced release of pool A by procaine was partially relieved by externally stimulating the hearts. External stimulation also decreased the inhibition by procaine of Ca uptake by pool A from 83% to 28%. These results are further evidence that pool A is located in the sarcoplasmic reticulum and that release of pool A to the myofibriles is triggered by excitationdependent Ca influx. (Ore Res 51: 363-370, 1982)