ENDOGENOUS OPIOID MECHANISMS IN HYPOTHALAMIC BLOOD-FLOW AUTOREGULATION DURING HEMORRHAGIC HYPOTENSION AND ANGIOTENSIN-INDUCED ACUTE HYPERTENSION IN CATS

The influence of naloxone-induced general opiate receptor blockade on hypothalamic blood flow autoregulation was investigated in anaesthetiz ed, artificially ventilated, temperature controlled cats. In order to study the changes of the hypothalamic blood flow (H-2-gas clearance te chnique) at the lower limit of auroregulation systemic arterial pressu re was reduced in a stepwise manner to 100, 80, 60 and 40 mmHg, by hae morrhage. Autoregulatory mechanisms of the hypothalamic vessels remain ed effective and hypothalamic blood flow showed no significant reducti on until the arterial pressure was reduced to 60 mmHg in the vehicle-t reated control cats. General opiate receptor blockade by 1 mg kg(-1) m L(-1) i.v. injected naloxone resulted in a significant reduction of th e autoregulatory capacity oi the hypothalamic vessels: the blood flow followed passively the arterial pressure fall already from 100 mmHg me an arterial pressure. The effect of opiate receptor blockade on the up per limit oi the autoregulation was studied during acute arterial hype rtension, induced by angiotensin-II infusion (25 mu g 0.1 mL(-1) min(- 1) i.v.). Hypothalamic blood flow remained remarkably steady following angiotensin-II infusion in the saline-treated control animals. Naloxo ne pretreatment (1 mg kg(-1) mL(-1) i.v.), however, induced a signific ant downward shift of the upper limit of the autoregulation, and hypot halamic blood flow started to increase in the 125-145 mmHg arterial pr essure range. The narrowing oi the autoregulatory range following gene ral opiate receptor blockade suggests an important role of endogenous opioid peptides in hypothalamic blood flow autoregulatory mechanisms b oth in hypotensive and in hypertensive conditions.