2 unusual responses of the myocardium in the frog Rana temporaria to acetylcholine

When studying seasonal variations in acetylcholine sensitivity of isolated frog ventricle, two forms of unusual responses have been observed: 1) tonic response induced by supermaximal concentration of acetylcholine, and 2) double response to the increasing acetylcholine concentrations with two different threshold and maximum doses (KAch values differ by 1-2 orders). The tonic response is blocked by mytolone. Both effects are of a seasonal nature. Similar pattern of these reactions in frogs and molluscs suggests a possibility of restoration of evolutionary more ancient form of cholinergic response, hormonal influences being involved into this process.