Regulation of adrenal blood flow: response to hemorrhagic hypotension.

Adrenal blood flow was measured during hemorrhagic hypotension in pentobarbital-anesthetized dogs using the radioactive-microsphere technique. A decrease in adrenal vascular resistance occurred during hemorrhage that was sufficient to maintain adrenal blood flow at prehemorrhage levels even with a decrease in mean arterial blood pressure (MABP) to as low as 50 mmHG. Blockade of alpha- and beta-adrenergic receptors, H1- and H2-histamine receptors, and prostaglandin synthesis all decreased resting adrenal blood flow. However, blood flow did not decrease further with hemorrhage subsequent to these interventions. Neither cholinergic blockade (muscarinic) alone nor ganglionic blockade subsequent to cholinergic blockade changed adrenal blood flow. However, ganglionic blockade alone increased adrenal blood flow. Hemorrhage subsequent to hypophysectomy also did not change blood flow to the adrenal. These data suggest that the maintenance of adrenal blood flow during hemorrhagic hypotension to 50 mmHg MABP is independent of the autonomic nervous system, prostaglandin synthesis, histamine receptors, or an intact pituitary-adrenal axis, even though all of these systems do influence adrenal blood flow.