Left Ventricular Myocardial Contractile Response to Aortic Constriction in the Hyperthyroid Guinea Pig

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Abstract
Maximum left ventricular systolic pressure response to aortic constriction served as a measure of myocardial contractility in anesthetized euthyroid and hyperthyroid guinea pigs. This response in hyperthyroid guinea pigs (155.4 ± 1.40 mm Hg) was significantly greater than in euthyroid animals (147.3 ± 1.33 mm Hg) (P<.001) and developed at a lower end-diastolic pressure in hyperthyroid (11.27±0.418 mm Hg) than in euthyroid guinea pigs (14.15±0.521 mm Hg) (P<.001). Beta-adrenergic receptor blockade with propranolol decreased the heart rate in both groups of animals but did not alter the contractile response. After propranolol, the heart rate of the hyperthyroid animals, though decreased, was still greater than that of euthyroid controls. Cardiac stimulation at a rate similar to that of the hyperthyroid animals (430 beats/min) did not change the left ventricular contractile response of normal guinea pigs. It is concluded that myocardial contractility in hyperthyroid guinea pigs is greater than normal. Increased activity of the sympathetic nervous system is probably present in hyperthyroidism, is reflected in the tachycardia, but does not appear to increase myocardial contractility. The increased myocardial contractility therefore is probably due in large part to a direct effect of thyroid hormone on cardiac muscle.