INOTROPIC EFFECTS OF HISTAMINE ON DEVELOPING CHICK HEART - RELEASE OFTRANSMITTERS FROM AUTONOMIC NERVE-TERMINALS

Inotropic effects of histamine were examined in isolated ventricular p reparations from late embryonic and hatched chick hearts. In 19 day-ol d embryonic preparations, histamine had little effect on the contracti le force. In preparations from 1 to 2 day-old hatched chick, histamine produced a transient decrease in contractile force followed by a sust ained increase. The negative and positive responses were antagonized b y atropine and propranolol, respectively, but not by histamine antagon ists terfenadine, cimetidine or thioperamide. Acetylcholine produced p ositive inotropic responses in the embryo while negative responses wer e observed after hatching. In myocardium of hatched chicks, compound 4 8/80, which releases histamine from mast cells, produced a transient d ecrease in contractile force followed by a sustained increase with a s imilar magnitude and time course to the case of exogenously applied hi stamine. The negative and positive responses were inhibited by atropin e and propranolol, respectively, but not by terfenadine, cimetidine or thioperamide, which was similar to the case with the responses to his tamine. The present results suggest that histamine, either applied exo genously or released from myocardial store sites, produces negative an d positive inotropic responses in hatched chick myocardium which are d ue to release of acetylcholine and norepinephrine, respectively, from autonomic nerve terminals.