[The effect of the initial vascular tonus on the development of compensatory constrictor reactions in orthostatic exposures].

Under orthostatic actions, a decrease in cardiac output, systolic and diastolic pressure (the decrease of the latters being lesser by half) and an increase in total peripheral resistance occurred in cats. A decrease in the coronary blood flow only occurred at large angles of the inclination (45 and 60 degrees). The data obtained suggest compensatory vasoconstrictory responses to orthostasis. Against the background of an increased initial vascular tone (angiotensin administration), no significant changes occurred under orthostasis in diastolic pressure, peripheral resistance or coronary blood flow. The decrease in cardiac output and systolic pressure was the same as before the administration. Against the background of a decreased vascular tone (papaverin administration), orthostatic tests induced a slight increase in the depression of arterial pressure and a decrease in the cardiac output shifts. No significant decrease in peripheral resistance occurred except at the angles of 45 and 60 degrees. A decrease in the coronary blood flow only occurred at the inclination of 30 and 45 degrees. The data obtained suggest that vasomotor responses to orthostasis depend on the value of initial vascular tone.