[Seasonal characteristics of the emergence from inhibition by and desensitization to acetylcholine of the frog heart muscle].

Seasonal changes in the sensitivity to acetylcholine (I/KACh), the time of recovery of half-amplitude (T50), as well as the ACh desensitization were studied in ventricles of the frog Rana temporaria. Cardiac arrest could be induced with 10(-7) to 10(-6) M ACh in winter whereas in spring small rhythmic contractions were observed even at 10(-3) M ACh. A correlation between the heart sensitivity to ACh and the time of the heart beat resuming was found: a decrease in the I/KACh corresponded to the 2-3-fold decrease in the T50 in spring and summer. After repeated ACh application the heart sensitivity decreased by 1.5-2.0-fold, i.e. a desensitization occurred. The data obtained are discussed in respect to the phenomenon of escape and involvement of putative modulators of the cholinergic responses: polyphosphates, lysolecithin, hormones.