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Refractory period fluid stimulation of right atria: a method for studying presynaptic receptors in cardiac autonomic transmission.

A method is described for measuring the atrial period or beat interval from the surface electrogram in isolated spontaneously contracting guinea pig right atria. Stimulation of parasympathetic and sympathetic nerve endings can be readily obtained by applying electrical field pulses across the atria during the atrial refractory period to prevent arrhythmia. In the presence of atropine, the atria respond to a single field pulse with a slight tachycardia that can be taken as a measure of the released transmitter norepinephrine. Because the effector response is mediated by a beta-adrenoreceptor, this preparation is particularly suited for the study of the pharmacology of presynaptic alpha-adrenoreceptors. The fall in period (tachycardia) to one-, two-, or four-field pulses delivered one per consecutive refractory period is linear, and the responses are reproducible for many hours. This biological system offers advantages in sensitivity and stability over methods employing radiolabeled norepinephrine in the study of presynaptic receptors in cardiac autonomic transmission.