Abstract

Carbachol (025—1.0 µg), injected into the physostigmine-containing Ringer-Locke-perfused cat superior cervical ganglion, released acetylcholine (ACh; 19.0-29.0 mµg) into the effluent; ACh release was markedly reduced (to 7.0-8.0 mµg) by chronic (12-16 days) preganglionic denervation. N-Methylcarbachol (0.5-7.0 µg) caused the release of a maximum of 10.0 mµg and N,N-dimethylcarbachol produced no detectable release of ACh; the threshold doses of the N-methyl analogs for activation of ganglia with normal circulation were not increased by chronic preganglionic denervation. Continuous infusions of carbachol (10-6 or 10^-5 M) in Ringer-Locke solution containing hemicholinium (2 x 10^-5 M) for 1 hr produced no reduction in ganglionic ACh stores, whereas repeated injections of carbachol (12 x 1.0 µg) over the same period caused a reduction of approximately 25%. Increasing the Ca⁺⁺ concentration of the Ringer-Locke solution from 2.17 to 4.34 or 8.68 mM resulted in an increase in ACh release by preganglionic stimulation, but reduced or blocked ACh release by carbachol (1.0 µg); conversely, in the absence of Ca⁺⁺ from the perfusion solution ACh release by preganglionic stimulation was prevented, but ACh release by carbachol was not, even in the presence of ethylene glycol tetraacetic acid (10^-3 M). It is concluded that ACh release by carbachol can result from more than one mechanism or site, depending upon the experimental conditions.