PARASYMPATHETIC RESPONSES IN HYPERTHYROID RATS

¹ Department of Pharmacology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

Electrical stimulation of the vagus nerve at different frequencies produced significantly less slowing of the heart in thyroxinetreated rats than in euthyroid animals. The decrease in myocardial phosphorylase a activity caused by vagal stimulation was also less in hyperthyroid rats than in euthyroid animals which had received the ganglion stimulant McN-A343 (100 µg/kg). In thyrotoxic animals the diminution in the effects of vagal stimulation on heart rate and cardiac phosphorylase a activity was not due to increased acetylcholinesterase, since measurements of myocardial cholinesterase activity in hyperthyroid rats were not significantly different from those in euthyroid animals. Intravenous injection of various doses of acetylcholine into euthyroid rats given McN-A343 and into thyrotoxic animals caused approximately equal depression in cardiac phosphorylase a, heart rate and mean arterial blood pressure. It was concluded that in hyperthyroid rats the decrease in metabolic and cardiovascular responses due to vagal stimulation could not be attributed to alterations in acetyicholinesterase activity or to a change in sensitivity of cardiac cholinoceptive sites.