Responses of the atrial myocardium to cardiac sympathetic nerve stimulation

The cardiac responses to sympathetic nerve stimulation were measured in open-chest, anesthetized dogs before and after infusions of cocaine, which were given to inhibit the neuronal uptake of norepinephrine. Cocaine did not augment the inotropic or chronotropic responses, but it did retard their decay after cessation of sympathetic stimulation. Before cocaine, the half-times for decay of the chronotropic, atrial inotropic, and ventricular inotropic responses were 28.4 +/- 2.2, 29.8 +/- 2.7, and 22.1 +/- 1.9 (+/- SE) s, respectively. After cocaine, however, the half-times were significantly greater (147 +/- 12.0, 51.4 +/- 3.9, and 32.9 +/- 2.5 s, respectively). The cocaine-induced prolongation of the decay time in a given cardiac tissue is a measure of the relative efficacy of neuronal uptake as a mechanism for dissipating the neurally released norepinephrine. Our data indicate, therefore, that among the various cardiac tissues that we studied, the neuronal uptake mechanism is least effective in the ventricular myocardium, somewhat more effective in the atrial myocardium, and most effective in the sinoatrial nodal region.