Are there any presynaptic 5-HT receptors in frog atria?

The effect of 5-hydroxytryptamine (5-HT) on the stimulation-induced inotropic responses was investigated in the isolated frog atria and ventricles. 5-HT (1 X 10(-5)-2 X 10(-4) M) inhibited the contractile responses elicited by intramural nerve stimulation (INS) in atria. The inhibitory action of 5-HT was antagonized by raising the external Ca++ concentration. Pizotifen and methysergide (1 X 10(-6) M) antagonized the inhibitory action of 5-HT on high frequency INS responses. Higher concentrations of methysergide by itself depressed the responses to low frequency INS. Ketanserin, metoclopramide and phentolamine (1 X 10(-6) M) failed to antagonize the inhibitory action of 5-HT. The results suggest that the inhibitory effect of 5-HT in frog atria is mediated through a subtype of 5-HT receptors which is neither of 5-HT2 nor of "M" type but may correspond to the 5-HT1 subtype of serotoninergic receptors. In contrast with atria, 5-HT-induced inhibition of INS responses was not observed in frog ventricles.