Abstract

Contractions were recorded from isolated, spontaneously beating or electrically driven cat atria with vagal and sympathetic nerves attached. Cervical vagal nerve stimulation produced a marked initial inhibition, followed by a slight poststimulation increase in contractile force. Cardiac vagal stimulation also produced marked biphasic responses. Sympathetic nerve stimulation resulted in only the usual positive inotropic response. For initial inhibition by the vagus, optimal pulse duration and frequency were 0.01 to 0.1 msec and 4 to 15 cps. For excitation by sympathetic nerve, optimal conditions were 1 to 2 msec and 8 to 15 cps. The voltage used was 2 to 4 V for both nerves. The inotropic effect after cessation of cervical vagal stimulation occurred at the same pulse duration and frequency which were optimal for initial inhibition by the vagus. The positive inotropic effect evoked by stimulation of the cardiac vagus occurred at the same optimal stimulus conditions for the sympathetic nerve responses. Inhibition and poststimulation excitation by cervical vagus or inhibition by cardiac vagus were blocked by atropine and hexamethonium and were not influenced by reserpine, chronic stellate ganglionectomy and pronethalol. Poststimulation excitation by the cardiac vagus and sympathetic nerve was not blocked by atropine or hexamethonium but was diminished by reserpine, stellate ganglionectomy and pronethalol. Incubation with norepinephrine did not restore excitation by sympathetic nerve stimulation in the reserpine-treated atria but restored inotropic excitation by nicotine. It is concluded that the efferent vagal and sympathetic nerves are purely cholinergic and adrenergic in nature, respectively, and that there are no major interactions between cholinergic and adrenergic transmitters in cat atria.