Intracellular calcium and mechanical function in isolated perfused hearts from rats and guinea pigs

We tested the hypothesis that the variable functional properties of rat versus guinea pig hearts are due to differences in intracellular Ca2+ handling. Hearts isolated from rats and guinea pigs were perfused with physiological saline, and isovolumic left ventricular (LV) pressure as well as coronary perfusion pressure were recorded simultaneously with Ca2+ transients from aequorin-loaded cells. Guinea pig hearts developed 47% less LV pressure than rat hearts, and the time to peak pressure was prolonged by 71% at similar heart rates. Diastolic and systolic levels of myoplasmic Ca2+ were approximately the same in both species at normal external Ca2+ concentration (1 mM); however, at low Ca2+ concentration (0.5 mM), guinea pig hearts maintained a higher level of myoplasmic Ca2+ than rat hearts, and the relative depression of LV systolic pressure was less. Guinea pig hearts also exhibited higher resistance to the negative inotropic effect of caffeine and did not respond to increments in perfusion pressure with increases in LV-developed pressure and systolic Ca2+ levels as did rat hearts. These contrasting findings with regard to intracellular Ca2+ handling may be attributed to a different organization of the ionic transport system with higher dependence of rat cardiomyocytes on normal function of the sarcoplasmic reticulum.