Myocardial Contractile Response to Norepinephrine, Isoproterenol, and Calcium Chloride in Hyperthyroid Guinea Pigs

Abstract

Myocardial contractility, measured as the maximum left ventricular systolic pressure response to aortic constriction, is greater than normal in the hyperthyroid guinea pig. The effect of inotropic agents on this response was determined in anesthetized, open-chest, euthyroid, and hyperthyroid guinea pigs. Norepinephrine and isoproterenol in single doses, 1 |txg/kg, and isoproterenol infused intravenously, 1, 4 and 10 ^ig/kg/min, caused a smaller increase in myocardial contractile response to aortic constriction in hyperthyroid than in euthyroid animals. Hyperthyroid guinea pigs also responded to an infusion of calcium chloride, 11 mg/kg/min, with an increase in maximum systolic pressure which was less than normal. With each drug the maximum systolic pressure level attained during aortic constriction was greater in the euthyroid than in the hyperthyroid animals. Cardiac pacing of euthyroid guinea pigs at 430 beats/min, a rate similar to the spontaneous rate of the hyperthyroid guinea pig, did not alter the inotropic response to infused calcium, though single injections and intravenous infusions of isoproterenol had less of an effect than in the spontaneously beating normal heart. These results suggest that the increased myocardial contractility of the hyperthyroid guinea pig may have reached a near maximum level. As a result, the added inotropic effects of isoproterenol and calcium in these animals are less than in normal controls.ADDITIONAL KEY WORDS active inotropic agents tachycardia aortic constriction maximal systolic pressure cardiac pacing thyroxine pressure work afterload • Although early reports suggested that the cardiovascular system of thyroid-stimulated 18. CAIBOLI, V. J., AND CROUT, J. R. : Role of the autonomic nervous system in the resting tachycardia of experimental hyperthyroidism. J.

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

“…could contribute to the previously reported increased contractile response to aortic constriction and might explain the decreased myocardial responsiveness to inotropic agents in hyperthyroid guinea pigs (11).…”

“…Schematic superimposition of frequency-force curves of left atrial myocardium from Figure I with Norepinephrine and calcium produce an increment in left ventricular myocardial contractile response to aortic constriction in hyperthyroid guinea pigs in vivo which is less than normal (11). At frequencies below approximately 120 beats/min, the increments in developed tension in vitro following administration of norepinephrine or calcium chloride are less in atrial muscle of hyperthyroid animals than in that of normal controls.…”

“…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.…”

Η Επιδρασις Της Δακτυλιτιδος Και Της Ισοπροπυλαρτερενολης Επι Του Γεροντικου Μυοκαρδιου

“…Τέλος ό Goodkind (1969) Άργότερον ό Bowen (1904), κατόπιν επισταμένης και ενδελεχούς με λέτης τοϋ καρωτιδογραφήματος, περιέγραψεν τήν έπίδρασιν τής κοπώσεως επί τής περιόδου εξωθήσεως.…”