Effects of histamine on the circulatory system.

Experiments have been made in anaesthetised cats and dogs and in healthy, human volunteers to compare the changes in blood pressure and heart rate during systemic administration of histamine. Histamine, 1 x 10(-9) to 1 x 10(-7) mol/kg/min, lowered blood pressure in a similar dose-dependent fashion in all three species. In man and the cat this was accompanied by clear dose-dependent tachycardia whereas in the dog heart rate changes were minimal. Pharmacological analysis of the depressor responses to histamine in all three species and the reduction in total peripheral resistance in the cat and dog showed that the immediate responses to histamine in all three species involved H1-receptors and that sustained responses involved H2-receptors. Abolition of responses to histamine throughout infusions required H1-and H2-receptor blockade. Histamine antagonists, used in doses which cause abolition of cardiovascular responses to large doses of histamine, do not cause any significant change in the resting cardiovascular system.