Relaxation in rabbit and rat cardiac cells: species‐dependent differences in cellular mechanisms.

The roles of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase and Na(+)-Ca2+ exchange in Ca2+ removal from cytosol were compared in isolated rabbit and rat ventricular myocytes during caffeine contractures and electrically stimulated twitches. Cell shortening and intracellular calcium concentration (Ca2+i) were measured in indo-1-loaded cells. Na(+)-Ca2+ exchange was inhibited by replacement of external Na+ by Li+. To avoid net changes in cell or SR Ca2+ load during a twitch in 0 Na+ solution, intracellular Na+ (Na+i) was depleted using a long pre-perfusion with 0 Na+, 0 Ca2+ solution. SR Ca2+ accumulation was inhibited by caffeine or thapsigargin (TG). Relaxation of steady-state twitches was 2-fold faster in rat than in rabbit (before and after Na+i depletion). In contrast, caffeine contractures (where SR Ca2+ accumulation is inhibited), relaxed faster in rabbit cells. Removal of external Na+ increased the half-time for relaxation of caffeine contractures 15- and 5-fold in rabbit and rat myocytes respectively (and increased contracture amplitude in rabbit cells only). The time course of relaxation in 0 Na+, 0 Ca2+ solution was similar in the two species. Inhibition of the Na(+)-Ca2+ exchange during a twitch increased the Ca2+i transient amplitude (deltaCa2+i) by 50% and the time constant of Ca2+i decline (tau) by 45% in rabbit myocytes. A smaller increase in tau (20%) and no change in deltaCa2+i were observed in rat cells in 0 Na+ solution. Ca2+i transients remained more rapid in rat cells. Inhibition of the SR Ca(2+)-ATPase during a twitch enhanced deltaCa2+i by 25% in both species. The increase in tau after TG exposure was greater in rat (9-fold) than in rabbit myocytes (2-fold), which caused Ca2+i decline to be 70% slower in rat compared with rabbit cells. The time course of Ca2+i decline during twitch in TG-treated cells was similar to that during caffeine application in control cells. Combined inhibition of these Ca2+ transport systems markedly slowed the time course of Ca2+i decline, so that tau was virtually the same in both species and comparable to that during caffeine application in 0 Na+, 0 Ca2+ solution. Thus, the combined participation of slow Ca2+ transport mechanisms (mitochondrial Ca2+ uptake and sarcolemmal Ca(2+)-ATPase) is similar in these species. We conclude that during the decline of the Ca2+i transient, the Na(+)-Ca2+ exchange is about 2- to 3-fold faster in rabbit than in rat, whereas the SR Ca(2+)-ATPase is 2- to 3-fold faster in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)