Modulation of stimulation frequency responses and calcium dependency of functional parameters in hyperthyroid rat ventricular papillary muscles.

The effects of stimulation frequency (0.2-1.5 Hz) and extracellular calcium concentration ([Ca2+]o) (0.6-15.0 mM) on the contractile function of thin papillary muscles of euthyroid and hyperthyroid rats were studied. Hyperthyroidism led to a decrease in developed tension (DT) and time to peak tension (TPT), but it exhibited no influence on the maximal rates of contraction (+dT/dt) and relaxation (-dT/dt). Also, the mean rates of contraction were similar in euthyroid and hyperthyroid muscle groups. The increase in stimulation frequency brought about a marked decrease in DT, +dT/dt, and -dT/dt of euthyroid papillary muscles at lower frequencies in comparison to papillary muscles in the hyperthyroid group. At stimulation frequencies above 1.0 Hz, the absolute and relative levels of DT and -dT/dt of hyperthyroid myocardium were elevated over euthyroid preparations. At the same time, TPT was unchanged in any of the muscle groups. Hyperthyroidism modulated the relationships between contractile parameters and [Ca2+]o. At a [Ca2+]o of 1.0-4.0 mM, the DT of hyperthyroid papillary muscles was lower than in euthyroid muscle. At 4.0 and 8.0 mM of [Ca2+]o, the equal values of maximal DT were registered for euthyroid and hyperthyroid papillary muscles, respectively. An increase in the [Ca2+]o in the range of 1.0-15.0 mM was accompanied by an increase in TPT of both muscle groups, but to a greater extent in hyperthyroid myocardium. In conclusion, the myocardium of hyperthyroid rat appeared to exhibit decreased sensitivity to calcium as well as to the negative inotropic effect of enhanced stimulation frequency. Alterations of the processes of transsarcolemmal movement and intracellular recycling of Ca2 may be implicated.