Influence of altered thyroid state on the inotropic potency of isoproterenol--studies with isolated rat left atria paced at different frequencies.

The influence of thyroid status upon the inotropic effect of isoproterenol has been examined using left atrial preparations isolated from hypothyroid, control (untreated), and hyperthyroid rats. Atria were paced at 5 Hz with submaximal field pulses, and isoproterenol was added cumulatively. Mean maximum inotropic responses to isoproterenol were 1.4 mN (hypothyroid), 1.2 mN (control), and 0.3 mN (hyperthyroid). Analysis of log concentration-fractional response curves revealed that isoproterenol was approximately 10 times less potent in preparations from hypothyroid rats. In preparations from hyperthyroid rats, the potency of isoproterenol was similar to that observed in control preparations. These results cast doubt upon the functional significance of the increased responsiveness to isoproterenol observed in other studies in which atria were paced at low frequencies. When applied at a concentration approximating the appropriate EC50 to atria paced at varying frequencies (1, 3, 5, and 5.8 Hz), isoproterenol enhanced contractile force developed in response to stimulation at the lower frequencies in atria from hypothyroid rats, and depressed the response of atria from hyperthyroid rats to stimulation at the higher frequencies.