A study about the possible characteristics of cardiovascular histamine receptors of the turtle.

Histamine produced pressor, depressor or biphasic responses in the systemic blood pressure of the turtle. The pressor response or component reversed to depressor effect after the injection of mepyramine, an H1 receptor blocker. It was thought that this response is mediated through the H1 receptors. The depressor response or component was abolished by the treatment of metiamide, a new histamine H2 receptor blocker. Also, the effect reversed after metiamide. These results indicated the presence of H2 receptors in the turtle.