The pharmacological characteristics of two types of cholinoreceptors in the membrane of dialyzed neurons

The effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine responses in Helix neurons

1. Using internal perfusion and concentration-clamp procedures applied to Helix neurons, the effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine Cl-dependent responses were determined. 2. Intracellular cAMP (10(-4) M) depressed those acetylcholine responses which were blocked by ouabain but had no effect on ouabain-insensitive acetylcholine responses. In the presence of elevated intracellular cAMP, ouabain had no further depressant effect on these acetylcholine responses. Both elevated cAMP and ouabain reduced the acetylcholine response without altering the current-voltage curves. 3. An increase in intracellular Ca2+ concentration depressed the amplitude of current induced by application of acetylcholine in neurons with ouabain-sensitive responses and shifted the dose-response relationship to the right. However, elevated Ca2+ did not reduce the maximal response induced by acetylcholine, nor did it prevent the reduction of that response by ouabain. 4. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent stimulator of protein kinase C activity, caused depression of both the ouabain-sensitive and the ouabain-insensitive acetylcholine responses. The inhibitory effect of TPA was markedly enhanced after addition of ATP to the intracellular medium and was greatly reduced by cooling to 5 degrees C. The blocking effect of ouabain, however, reexamined in the presence of TPA. 5. These observations are consistent with the hypothesis that the depression of acetylcholine induced Cl--responses in Helix neurons is a result of an increase in intracellular cAMP concentration but is unrelated to activation of protein kinase C or increases in intracellular Ca2+.

Ouabain blocks some rapid concentration-induced clamp acetylcholine responses on Helix neurons

1. The effects of ouabain, a potent inhibitor of Na(+)-K+ ATPase, were determined on the transmembrane responses of internally dialyzed Helix neurons to rapid acetylcholine (ACh) application using the "concentration clamp" technique. 2. Ouabain selectively depressed "A"-type responses to ACh, which are due to a selective increase in membrane permeability to chloride. In contrast, the "B"-type responses, due primarily to an increase in monovalent cation permeability, was unaffected. 3. The blockade of the Cl- responses was not associated with a change of the reversal potential of the response. Ouabain depressed the maximal response without shifting the dose-response curve. 4. Ouabain caused an increase in the time constant of decay of the ACh current, but the value in the presence of ouabain was not different from that of a lower concentration of ACh determined so as to give a response of the same peak amplitude. Therefore, the effect of ouabain is not on the process of receptor desensitization directly.