Role of brain angiotensin II in the somatosensory induced antinatriuresis in the anaesthetized rat

1. The present study set out to explore the importance of angiotensin (Ang) II in the brain in allowing the somatosensory system to cause a reflex rena l nerve-mediated reduction in renal sodium and water excretion. 2. In chloralose-urethane-anaesthetized rats receiving saline i.c.v. (2 mu L + 1 mu L/h), the administration of capsaicin (0.5 mg, s.c.) increased blo od pressure by 14% (P < 0.001) and, while renal perfusion pressure was regu lated at an unchanged level, neither renal blood flow (RBF) nor glomerular filtration rate was changed. However, urine flow and absolute and fractiona l sodium excretion was reduced between 29 and 38% (P<0.05-0.01). All variab les had returned to control levels 30 min later. 3. The administration of captopril (40 mu g + 20 mu g/h i.c.v.) decreased b lood pressure and sodium excretion by 6 and 17%, respectively (both P < 0.0 5). Under these conditions, capsaicin s.c. increased blood pressure by 9% ( P < 0.05); however, with renal perfusion pressure regulated at a constant l evel, neither renal haemodynamics nor water nor sodium excretion were chang ed. 4. A final group of animals received AngII (100 ng + 50 ng/h) concomitantly with captopril i.c.v., which increased blood pressure, RBF and urine flow, absolute and fractional sodium excretions by 8 (P < 0.05), 22 (P < 0.001) and 52-149 % (P < 0.05-0.01), respectively. Capsaicin given s.c. under thes e conditions increased blood pressure by 6% (P < 0.05) and, while renal per fusion pressure was maintained at an unchanged value and renal haemodynamic s remained constant, urine flow and absolute and fractional sodium excretio n were reduced by 35-38% (all P < 0.05). 5. These data show that for the somatosensory system to induce a reflex inc rease in renal sympathetic nerve activity sufficient to cause an antinatriu resis and antidiuresis, the presence of AngII is necessary in the brain. Ho w AngII exerts this facilitatory action within the central nervous system r emains to be investigated.