Threshold effects of acetylcholine on primary pacemaker cells of the rabbit sino-atrial node

Leading or primary pacemaker cells located within the rabbit sino-atrial node have been identified by using electrophysiological and pharmacological techniques. Stable intracellular recordings lasting 20-30 min from cells within the s.a. node reveal three distinct patterns of spontaneous intracellular responses: (i) leading or primary pacing; (ii) follower or subsidiary pacing; and (iii) `anomalous' pacemaker discharge. Our main objective was to measure the first detectable effect, or effects, of acetylcholine on the spontaneous intracellular electrical activity in mammalian primary pacemaker cells. Trains of brief `field' stimuli were applied to evoke transmitter release from endogenous nerve varicosities. Systematic variations in the amplitude and duration of each stimulus, and in the train length; in conjunction with application of $\beta $ blockers (l-pindolol (10$^{-6}$ M); l-propranolol, (2 $\times $ 10$^{-7}$ M)) yielded small and transient, but very consistent negative chronotropic effects. These electrophysiological changes were blocked by atropine (1 $\times $ 10$^{-7}$ M) and were mimicked by bath application of low doses of acetylcholine (10$^{-7}$-10$^{-6}$ M) or muscarine chloride (10$^{-8}$-10$^{-7}$ M). In primary cells the first, or threshold effect of vagal excitation is a decrease in the slope of the pacemaker potential, without a detectable (less than 2 mV) hyperpolarization or change in action potential duration. A reduction in the dV/dt$_{\max}$ of the initial depolarization is also quite consistently observed. Application of longer stimulus trains yield the classical hyperpolarizing response, which is often assumed to be the major electrophysiological correlate of the negative chronotropic effect. These data provide a detailed electrophysiological description of the `physiological' effects of the vagus nerve excitation on primary or leading pacemaker cells of the mammalian s.-a. node. A plausible explanation for the absence of hyperpolarization is suggested; and a working hypothesis is presented for the changes in ionic current or currents, that underlie this negative chronotropic effect.