Abstract
Effects of the histamine H1- or H2-receptor antagonists mepyramine (Mep) and cimetidine (Cim) on neuroendocrine and cardiovascular responses to 50 degrees head-up tilt were evaluated in seven human males. Central hypovolemia was characterized by two phases. The first is a normotensive phase with increases in heart rate (HR), total peripheral resistance (TPR), and decrease in cardiac output. Plasma adrenocorticotropic hormone, beta-endorphin, cortisol, catecholamines, and renin activity increased moderately. Normotension lasted 39 +/- 7 min during infusion of saline but was reduced by Mep [18 +/- 3 min, F(2,12) = 9.60, P less than 0.01] and was unaffected by Cim (44 +/- 4 min). The second is a hypotensive phase associated with presyncopal symptoms (hypovolemic shock) and decreases in HR and TPR and a further increase in pituitary-adrenal and sympathoadrenal activity. Decreases in mean arterial pressure and TPR were augmented by Mep, which inhibited release of norepinephrine. Cim inhibited epinephrine release without affecting the development of hypovolemic shock. It is concluded that histaminergic mechanisms are involved in activation of the sympathoadrenal system but not in the pituitary-adrenal axis during central hypovolemia in humans.
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