Differences in Ca2+-Handling and Sarcoplasmic Reticulum Ca2+-Content in Isolated Rat and Rabbit Myocardium

We made novel measurements of the influence of rest intervals and stimulation frequency on twitch contractions and on sarcoplasmic reticulum (SR) Ca2+-content (using rapid cooling contractures, RCCs) in isolated ventricular muscle strips from rat and rabbit hearts at a physiological temperature of 37 °C. In addition, the frequency-dependent relative contribution of SR Ca2+-uptake and Na+/Ca2+-exchange for cytosolic Ca2+-removal was assessed by paired RCCs. With increasing rest intervals (1–240 s) post-rest twitch force and RCC amplitude decreased monotonically in rabbit myocardium (after 240 s by 45±10% and 61±11%, respectively P<0.05, n=14). In contrast, rat myocardium (n=11) exhibited a parallel increase in post-rest twitch force (by 67±16% at 240 sP <0.05) and RCC amplitude (by 20±14%P<0.05). In rabbit myocardium (n=11), increasing stimulation frequency from 0.25 to 3 Hz increased twitch force by 295±50% (P<0.05) and RCC amplitude by 305±80% (P<0.05). In contrast, in rat myocardium (n=6), twitch force declined by 43±7% (P<0.05), while RCC amplitude decreased only insignificantly (by 16±7%). The SR Ca2+-uptake relative to Na+/Ca2+-exchange (based on paired RCCs) increased progressively with frequency in rabbit, but not in rat myocardium (66±2% at all frequencies). We conclude that increased SR Ca2+-load contributes to the positive force–frequency relationship in rabbits and post-rest potentiation of twitch force in rats. Decreased SR Ca2+-load contributes to post-rest decay of twitch force in rabbits, but may play only a minor role in the negative force–frequency relationship in rats. SR Ca2+-release channel refractoriness may contribute importantly to the negative force-frequency relationship in rat and recovery from refractoriness may contribute to post-rest potentiation.