ACETYLCHOLINE-ACTIVATED IONIC CURRENTS IN PARASYMPATHETIC NEURONS OF BULLFROG HEART

1. The electrical and pharmacologic properties of acetylcholine (ACh)-induced current (I(ACh)) were studied in the parasympathetic neurons isolated from bullfrog heart with the use of the concentration-clamp technique, which allows intracellular perfusion and rapid change of external solution within 2 ms under the single-electrode voltage-clamp condition. 2. The I(ACh) consisted of an initial transient peak component and a successive steady-state plateau component. Both currents increased in a sigmoidal fashion with increasing ACh concentration. The dissociation constant (K(d) value) and the Hill coefficient for each component were 2.2 x 10-5 M and 1.6, respectively. 3. In the K+-free solution, the reversal potential (E(ACh)) of I(ACh) was close to the Na+ equilibrium potential (E(Na)). The current-voltage (I-V) relation showed inward rectification at positive potentials. 4. Nicotine mimicked only the peak component of I(ACh). However both peak and steady-state components were blocked nonselectively by the nicotine blockers d-tubocurarine and hexamethonium. 5. Carbamylcholine (CCh) mimicked the steady-state component of I(ACh). The steady-state component was selectively inhibited by atropine at concentrations 1,000 times lower than that required for inhibition of the peak component. The steady-state was blocked equally by either pirenzepine (M1 blocker) or AF-DX-116 (M2 blocker). 6. It was concluded that the I(ACh) consisted of a peak component having double exponential activation and inactivation, mediated through the nicotinic actions, and a steady-state component having no inactivation, mediated through the muscarinic action.