Positive inotropic responses in cardiac muscles: influence of stimulation frequency and species.

Inotropic responses to cumulative additions of methoxamine (10(-7) to 3 X 10(-4) M), isoproterenol (10(-9) to 10(-5) M), or calcium (2 to 32 mM) were measured in isolated rat left atria and papillary muscles and rabbit right ventricular papillary muscles at three stimulation frequencies. Cardiac muscles were incubated in oxygenated Chenoweth-Koelle solution (2 mM calcium) at 37 degrees C. The basal developed force (BDF) before and maximum developed force (MDF) after challenge with methoxamine and isoproterenol were inversely related to stimulation frequency in rat preparations. BDF was directly related to stimulation rate in rabbit papillary muscles while MDF was independent of the rate. Drug-induced increases in force (MDF - BDF) were independent of stimulation frequency in rat and inversely related to stimulation frequency in rabbit. Responses to calcium were similar to the observed adrenergic responses. Also, force-frequency relationships of the rat and rabbit preparations were not similar in the absence and presence of these agonists. These data show that inotropic responses by rat and rabbit hearts are not affected similarly by stimulation frequency and this may reflect a species difference in the utilization of extracellular calcium for contraction.