Opiate receptor-mediated decrease in renal nerve activity during hypotensive hemorrhage in conscious rabbits

Effects of hemorrhage on renal nerve activity and of subsequent opiate receptor blockade with naloxone were studied in conscious rabbits. Mean arterial pressure remained constant at 77 ± 2 mm Hg through 17 ± 2 ml/kg hemorrhage, while renal nerve activity increased by 159±16%. After 25 ± 1 ml/kg hemorrhage, mean arterial pressure fell by 42 ± 3 mm Hg, and renal nerve activity decreased below the prehemorrhagic control level by 41 ±15%. Bolus injection of naloxone (3 mg/kg i.v.) increased mean arterial pressure to 79 ± 2 mm Hg, not significantly different from the prehemorrhagic control level. Renal nerve activity increased by 171 ±28%, comparable to the peak increase during nonhypotensive hemorrhage. On a different day, hemorrhage was repeated, and phenylephrine was infused during the subsequent hypotension. Phenylephrine increased mean arterial pressure to the prehemorrhagic control level. With increasing mean arterial pressure, renal nerve activity increased from its level during hypotensive hemorrhage and recovered toward the prehemorrhagic control level ( — 26±11%), but it did not return to the peak value reached during nonhypotensive hemorrhage. To further examine the blocking effects of naloxone on changes in mean arterial pressure and renal nerve activity induced by exogenous opiate peptides, methionine-enkephalin was injected both in th