RENAL NERVES AFFECT RATE OF ACHIEVING SODIUM-BALANCE IN SPONTANEOUSLYHYPERTENSIVE RATS

The spontaneously hypertensive rat (SHR) has an elevated efferent symp athetic nerve activity, suggesting that the renal handling of sodium a nd water may be altered. This study evaluated the renal neurogenic inf luence on the rate of achieving sodium balance in adult SHRs and Wista r-Kyoto (WKY) rats after either a step increase or step decrease in fi xed sodium intake. Conscious, unrestrained rats with either innervated or denervated kidneys were initially placed on a low-sodium (0.3 mEq/ d) or high-sodium (5.0 mEq/d) intake by intravenous infusion. Hourly u rinary sodium excretion was determined 24 hours before and 72 hours af ter sodium intake had been increased from low to high or decreased fro m high to low. After either step change in fixed sodium intake, both i nnervated SHRs and innervated WKY rats achieved sodium balance within 24 hours. Similarly, the time course of achieving sodium balance was n early identical between WKY rats with innervated and denervated kidney s after either switch in sodium intake. In SHRs receiving a step incre ase in sodium intake, both innervated and denervated kidneys increased urinary sodium excretion equally for 9 hours; however, at this time, innervated SHRs continued to increase sodium excretion rapidly, wherea s denervated rats were delayed in a further response. Thus, innervated SHRs achieved sodium balance approximately 18 hours sooner than dener vated SHRs. Differences in urinary sodium excretion did not result fro m concomitant changes in plasma renin activity or mean arterial pressu re. These data suggest that in sodium depletion of SHRs and WKY rats, the mechanism for immediate regulation of urinary sodium excretion app ears to be independent of renal sympathetic outflow, whereas the renal nerves do provide a rapid sodium excretory response to a step increas e in sodium intake.