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

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. ADDITIONAL KEY WORDSpropranolol hemodvnamic function beta-receptor blockade pressure work tachycardia cardiac stimulation • Many attempts have been made to define the relationship of the sympathetic nervous system to the cardiovascular changes produced by excess thyroid hormone. For years the hyperkinetic circulation of thyrotoxicosis was thought to be related to a hypersensitivity to catecholamines (1). However, recent reports have shown that thyroxine does not

Effect of Thyroid Hormone on the Frequency-Force Relationship of Atrial Myocardium from the Guinea Pig

“…However, recent reports have demonstrated that thyroid hormone does not increase the sensitivity of adrenergic receptor sites (5-11) and /3-receptor blockade with drugs does not alter the contractile response to aortic constriction in the hyperthyroid guinea pig (4). Since the frequencyforce curve observed in muscle from reserpinetreated hyperthyroid animals did not differ from hyperthyroid animals, the role of endogenous norepinephrine appears insignificant in the production of an altered frequencyforce curve in atrial muscle from hyperthyroid animals.…”

“…Changes in heart rate have a similar though smaller effect on contractile force in vivo (2,3). Both increased myocardial contractility and tachycardia are present in the intact hyperthyroid guinea pig (4). The increased contractility of hyperthyroidism does not appear to be caused by increased sympathetic nervous system activity or by hypersensitivity of the myocardium to catecholamines (4)(5)(6)(7)(8)(9)(10)(11), but by a direct effect of the thyroid hormone on the heart.…”

“…Both increased myocardial contractility and tachycardia are present in the intact hyperthyroid guinea pig (4). The increased contractility of hyperthyroidism does not appear to be caused by increased sympathetic nervous system activity or by hypersensitivity of the myocardium to catecholamines (4)(5)(6)(7)(8)(9)(10)(11), but by a direct effect of the thyroid hormone on the heart. The studies reported in this paper were designed to investigate in vitro the influence of frequency of contraction on myocardial contractility (developed tension) and on the response to inotropic agents in hearts from hyperthyroid animals.…”

Biochemical and morphological study of cardiac hypertrophy. Effects of thyroxine on enzyme activities in the rat myocardium

“…Hyperthyroidism is characterized by a cardiovascular hyperdynamic state with reduction in the duration of the active state and an augmentation in the rate of tension development and rate of relaxation (9,20,25,28). In hyperthyroidism, the increased energy demand on the heart has been demonstrated in terms of increased oxygen consumption, and cardiac hypertrophy eventually develops (18).…”

Anaerobic performance and metabolism of the hyperthyroid heart

“…Myocardial hypertrophy, increased heart rate, and increased mvocardial contractility are Received for publication '8 April 1969 and in revised forin 13 June 1969. all consequences of hyperthyroilism (7)(8)(9). These changes are accompanied by increased cardiac oxygen consumption and free fatty acid utilization, whereas glucose uptake and oxidation are decreased (10,11).…”