Effect of growth hormone and thyroxine on isometric contractile mechanics of cardiac muscle

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. </jats:p

Number of nuclei in mammalian cardiac myocytes

The numbers of nuclei per cardiac muscle cell were determined in adult mammalian hearts after previous enzymatic isolation of individual myocytes. A high percentage of binucleation was observed in rat (78 ± 1.8%), rabbit (78 ± 0.8%), guinea pig (81 ± 1.6%), cat (76 ± 1.5%), dog (47 ± 2.4%,), and beef (45 ± 1.1%), while the human myocytes were found predominantly mononucleated (90%) and only 10 ± 2.2% of them were binucleated. These findings are in agreement with our data obtained after serial sectioning of the myocardium of the above species and subsequent reconstruction of individual myocytes. We also found that in fetal and neonatal rat the cardiac myocytes are predominantly mononucleated (95% and 84%, respectively), that the binucleation develops during the early postnatal period, and that adult values are reached by 12–14 days of age. When the body growth and consequently also the heart growth were either accelerated or slowed down by nutritional intervention during the above period, the rate of appearance of binucleation correlated with the organ size rather than with the age of the rat. Early postnatal increase in binucleation was also observed in rabbit, cat, and dog during normal growth. The regional distribution of the above phenomenon was found to be uniform throughout both ventricles. </jats:p