Effects of Alpha-adrenoceptor Agonists on Cardiac Output and Its Regional Distribution in the Pithed Rat

Cardiac output, its distribution and tissue blood flows were determined with tracer microspheres in pithed rats during pressor responses elicited by either alpha 1-adrenoceptor agonists (cirazoline, phenylephrine) or alpha 2-adrenoceptor agonists (xylaxine, B-HT 933). Two doses were used for each of cirazoline and B-HT 933 and phenylephrine was investigated in the presence of propranolol (3 mg kg-1). The rats were pithed under halothane anaesthesia. Cardiac output was increased by xylazine, the higher dose of B-HT 933 and phenylephrine. Heart rate was increased by phenylephrine and the higher doses of both cirazoline and B-HT 933. Stroke volume was greater in those groups given xylazine, phenylephrine and the higher dose of B-HT 933 but was decreased in those animals given the higher dose of cirazoline. Both alpha 2-adrenoceptor agonists increased the number of microspheres trapped in the lungs and the proportion of the cardiac output passing through the hepatic artery but decreased that flowing through the spleen and gastrointestinal tract. The higher dose of B-HT 933 also decreased the fraction of cardiac output flowing to the kidneys but kidney blood flow was maintained as a result of the increased cardiac output. Also, this treatment reduced blood flow in the epididimal fat pads. Both alpha 1-adrenoceptor agonists increased the fraction of cardiac output received by the coronary vasculature but the only other effect on distribution common to these agents was an increase in the percentage of the cardiac output passing to the hepatic artery. Cirazoline decreased the proportion of cardiac output distributed to the gastrointestinal tract and spleen but the total fraction of cardiac output passing to the hepatosplanchnic region was maintained as a result of the increase to the hepatic artery. Cirazoline markedly reduced the proportion of the cardiac output received by the kidneys and absolute flow in these organs was only 1.4% of control after the higher dose of this agonist but flow at the lower dose was maintained by the higher cardiac output. It is concluded that there is a significant contribution to the pressor responses elicited by alpha-agonists resulting from an alpha-adrenoceptor-mediated increase in cardiac output that may result from greater heart rates or stroke volumes. Also, there is a differential distribution of alpha-receptor subtypes throughout the vasculature which is especially noticeable in the kidneys.