Experimental hyperthyroidism III: Contractile responses to propranolol of the intact heart and of the isolated ventricular myocardium.

The effect of propranolol on cardiac mechanics and haemodynamics was examined on isolated papillary muscle as well as in situ in the closed thorax in euthyroid and hyperthyroid cats. Under propranolol there occurred on isolated papillary muscle significant decreases of muscle contraction, contraction velocity, isometric tension rise velocity and load and velocity values of simultaneously determined force-velocity relations. The 50% decrease of the values found was demonstrable at ca. 5--8 mu/ml for euthyroidal muscles, at 0.3--0.5 mug/ml for hyperthyroidal muscles. Contractility indices determined in situ (dp/dtmax, VCEmax, extrapolated Vmax) showed in hyperthyroidism at equal propranolol concentration a decrease about twice as great as in euthyroidism. The findings, showing a raised responsiveness of the ventricular myocardium in experimental hyperthyroidism to the negative inotropic effect of propranolol are discussed with regard to the therapeutic use of propranolol in hyperthyroidism.