Effect of growth hormone and thyroxinon isometric contractile mechanics of cardiac muscle.

Abstract

To classify the effects of hormonal interventions on myocardial contractility the descriptive mechanics of contraction of isolated ventricular papillary muscles of rats were investigated under isometric conditions at 27 °C. The maximum developed tension (MT), the maximum rate of tension development (dT/dt), the mean rate of tension development (ΔT/Δt), and time to maximum tension (TMT) were determined in "rested-state contractions". Two strains of animals were used (Wistar and Sprague–Dawley). In each case the animals were divided into four groups: (1) normal, (2) hypophysectomized (Hx), (3) hypophysectomized and subsequently treated with growth hormone (GH), and (4) hypophysectomized and subsequently treated with thyroxine (T4). MT decreased after hypophysectomy, was not changed after GH administration, and was improved after T4 administration. The decrease of MT in groups 2 and 3 was due to a decrease in dT/dt and ΔT/Δt and was partly compensated by prolonged TMT. Administration of T4 brought the values of dT/dt and ΔT/Δt to normal, and shortened the value of TMT below normal. These experiments indicate that although the decrease in MT after hypophysectomy was relatively small the pattern of the descriptive mechanics of contraction was profoundly altered. The results suggest that the degree of activation may have been decreased and that the contraction became slower and lasted longer. These changes were not influenced by GH but were reversed by T4 administration.