CONTRACTILE ACTIVATION PROPERTIES OF VENTRICULAR MYOCARDIUM FROM HYPOTHYROID, EUTHYROID AND JUVENILE RATS

Contractile activation properties of intact and chemically skinned ventricular myocardium preparations were studied in juvenile (3-4 weeks old), adult euthyroid and adult hypothyroid rats. The rats were made hyperthyroid by treatment with iodine-131 and propylthiouracil. The ventricular muscle of euthyroid rats contains a mixture of isozymes of myosin while the myocardium of juvenile and hypothyroid rats are relatively pure in regard to V1 and V3 types of myosin respectively. No significant differences were found in either the maximum Ca2+ activated or rigor force developed by ''chemically skinned'' preparations in either the juvenile or hypothyroid groups compared with euthyroid adults, suggesting that there is no difference between myocardia with different isozymes of myosin in the intrinsic capacity to generate force. In the hypothyroid (V3) preparations there was a significant shift in the force/pCa relation to the left compared with the euthyroid adult (mixture of V1 and V3 isozymes). The force/pCa relation for the juvenile lay in between that for the hypothyroid and euthyroid adults. The greater apparent Ca2+ sensitivity to activation in the hypothyroid group may relate to a slower cross-bridge cycling rate or altered Ca2+ kinetics in ventricular myocardium with exclusively V3 isozyme. In intact papillary muscles differences were found in the dependence of force on extracellular [Ca2+] such that a higher extracellular [Ca2+] was required for muscles from hypothyroid animals to attain maximum twitch force than those from juveniles. The force/frequency relations also differed, with the hypothyroid group being better able to sustain force as stimulation frequency increased than the juvenile group. Also, in the hypothyroid group, the contraction following a 3-min period of quiescence was potentiated in relation to the preceding steady-state contractions, whereas in the juvenile group it was not. These results indicate that the thyroid state may influence the pattern of calcium. translocation as well as the myosin isozyme type and that both of these factors may influence contractile properties. Furthermore, the pattern of responses seen depends not only on the pattern of isozymes present but may also depend on the age of the animal because there was no simple relationship between the apparent sensitivity of the contractile apparatus for calcium and proportion of isozyme in the myocardium.