Abstract

Transmeinbrane action potentials were recorded from cells in the electrically driven left atrial appendage of the guinea-pig heart bathed in Tyrode's solution at 30°C. Epinephrine, isoproterenol and norepinephrine produced modifications in the plateau and the phase of rapid repolarization without changing the resting potential. All of the catecholamines increased the duration of the plateau phase of the action potential. Modest concentrations (<10^-6 M) of epinephrine and norepinephrine increased action potential duration (measured when repolarization was 80% complete), whereas large concentrations (>10^-6 M) tended to decrease the 80% duration. At all concentrations examined. isoproterenol did not appreciably change the 80% duration. Characteristically, the actions of isoproterenol on the action potential were associated with an increase in developed tension and propranolol completely prevented the changes in electrical and mechanical activity caused by this agent. Large concentrations of epinephrine also increased developed tension whereas modest concentrations of this agent were usually ineffective in this regard. Moreover, the ability of modest concentrations of epinephrine to increase the 80% duration was insensitive to blockade by propranolol and was antagonized by phentolamine and dihydroergotamine. It is concluded that atrial muscle cells in the guinea pig display two pharmacologically distinct adrenoceptive sites.