[The effect of carnosine on synaptic transmission in the frog neuromuscular junction].

The influence of carnosine on quantal content of the e.p.p. of the frog cutaneous pectoris muscle was studied. The e.p.p. quantal content decreased in presence of carnosine (8 mmol/l). Removal of carnosine from the incubation medium potentiated synaptic transmission as compared with its level prior to addition of carnosine to the medium (after-potentiation). Depressant presynaptic action of carnosine did not depend on external Ca+2 concentration or initial level of evoked quantum secretion whereas the after-potentiation only occurred in conditions of decreased Ca+2 concentration in the medium. In the blockade of cyclic nucleotides phosphodiesterase with theophylline, carnosine induced the same effects as in the experiments without theophylline. However, the presence of theophylline in the medium during removal of carnosine prevented the after-potentiation. Possible mechanisms of presynaptic effects of carnosine are discussed.