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 degreesC, 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 = II) exhibited a parallel increase in post-rest twitch force (by 67 <plus/minus> 16% at 240 s; P < 0.05) and RCC amplitude (by 20 <plus/minus> 14%; P < 0.05). In rabbit myocardium (n = 11), increasing stimulation frequency from 0.25 to 3 Hz increased twitch force by 295 <plus/minus> 50% (P < 0.05) and RCC amplitude by 305 <plus/minus> 80% (P < 0.05). In contrast, in rat myocardium (n = 6)! twitch force declined by 43 <plus/minus> 7% (P < 0.05), while RCC amplitude decreased only insignificantly (by 16 <plus/minus> 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 (similar to 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, (C) 2000 Academic Press.