Histamine receptors in rabbit atria

In order to characterize the types of histamine receptors (H₁ or H₂) in rabbit atria, isolated atrial preparations were exposed to selective histamine receptor agonists, alone and in the presence of selective histamine receptor antagonists. The rate of isolated right atria was increased by histamine (H₁ and H₂), 4-methylhistamine (H₂), and impromidine (H₂). 2-Pyridylethylamine (PEA, H₁) had very little effect on atrial rate. Cimetidine pretreatment (selective H₂ receptor blockade) competitively antagonized the positive chronotropic effects of histamine, 4-methylhistamine, and impromidine. Promethazine pretreatment (selective H₁ receptor blockade) competitively blocked the chronotropic effects of histamine, but had no effect on the responses to 4-methylhistamine or impromidine. The force of isolated left atria was increased by all four agonists. Cimetidine pretreatment competitively blocked the inotropic effects of histamine, 4-methylhistamine, and impromidine, and had no effect on the response to PEA. Promethazine pretreatment competitively blocked the inotropic responses to histamine and to PEA, and had no effect on the responses to 4-methylhistamine or impromidine. The phosphodiesterase inhibitor theophylline (3 x 10⁻⁴M) potentiated the inotropic but not the chronotropic effects of 4-methylhistamine. (1 x 10⁻⁴M , impromidine (1 x 10⁻⁸M) and PEA (1 x 10⁻⁴M) . Histamine (1 x 10⁻⁵M) exposure for 20 seconds did not alter the cyclic AMP levels of isolated rabbit right and left atria. We report that both types (H₁ and H₂) of histamine receptors are present in rabbit right and left atria. However, I cannot confirm reports (Hughes,1978) that both H₁ and H₂ histamine receptor agonists act through the generation of cyclic AMP.Pharmaceutical Sciences, Faculty ofGraduat