Abstract

Spontaneously contracting chick myocardial tissue culture cells, prepared by trypsin dispersal of five- and seven-day-old whole embryonic hearts, were used to investigate cellular autonomic mechanisms in a nerve-free environment. Catecholamines produced an immediate sustained increase in rate which was blocked by propranolol but not by phentolamine. Isoproterenol exhibited a typical log concentration-effect curve and appeared more potent than norepinephrine (10-fold) and phenylephrine (1000-fold), whereas tyramine and nicotine were inactive. These results indicate the functional presence of beta adrenergic receptors and the absence of endogenous, intracellular catecholamines. Acetyicholine decreased rate but the response was not related to the concentration used and the maximum effect occurred between 15 and 30 minutes after application. In some instances, acetylcholine initially increased rate; therefore, acetylcholine responses may represent the algebraic sum of two opposing actions. The duration of action of acetylcholine was concentration dependent and appears limited by cholinesterase hydrolysis. Scopolamine, but not hexamethonium or d-tubocurarine, blocked all the effects of acetyicholine and a scopolamine-like action was detected for physostigmine and, to a lesser extent, for neostigmine. It is postulated that muscarinic-like receptors exist in cultured chick heart cells although they may not, as yet, be fully developed.