Differential control of intrarenal blood flow during reflex increases in sympathetic nerve activity

The role of renal sympathetic nerve activity (RSNA) in the physiological re gulation of medullary blood flow (MBF) remains ill defined, yet regulation of MBF may be crucial to long-term arterial pressure regulation. To investi gate the effects of reflex increases in RSNA on intrarenal blood flow distr ibution, we exposed pentobarbital sodium-anesthetized, artificially ventila ted rabbits (n = 7) to progressive hypoxia while recording RSNA, cortical b lood flow (CBF), and MBF using laser-Doppler flowmetry. Another group of an imals with denervated kidneys (n = 6) underwent the same protocol. Progress ive hypoxia (from room air to 16, 14, 12, and 10% inspired O-2) significant ly reduced arterial oxygen partial pressure (from 99 +/- 3 to 65 +/- 2, 51 +/- 2, 41 +/- 1, and 39 +/- 2 mmHg, respectively) and significantly increas ed RSNA (by 8 +/- 3, 44 +/- 25, 62 +/- 21, and 76 +/- 37%, respectively, co mpared with room air) without affecting mean arterial pressure. There were significant reductions in CBF (by 2 +/- 1, 5 +/- 2, 11 +/- 3, and 14 +/- 2% , respectively) in intact but not denervated rabbits. MBF was unaffected by hypoxia in either group. Thus moderate reflex increases in RSNA cause rena l cortical vasoconstriction, but not at vascular sites regulating MBF.