Renal responses to increases in renal sympathetic nerve activity induced by brainstem stimulation in rabbits

We have stimulated the rostral ventrolateral medulla of the central nervous system to increase renal sympathetic nerve activity (RSNA), and measured the effect on renal blood flow, glomerular filtration rate, and urinary excretion. Increases in RSNA were produced by infusion of 0.02 M glutamate at a rate of 30-50 nl/min into the subretrofacial nucleus for 40 min, in 10 urethane anaesthetized rabbits. Changes in RSNA were quantified as the mean nerve activity per 1 s period and as the frequency and amplitude of individual discharges (reflecting the number of activated nerve fibres). Glutamate infusion increased RSNA 59 +/- 11% over control levels. This was predominantly due to a 65 +/- 15% increase in the frequency of discharges (3.0 +/- 0.35 to 4.6 +/- 0.4 Hz), rather than the amplitude of the discharges (+9 +/- 3% over control). The effects of these changes on the kidney were made against data collected in the last 20 min of the infusion and the 40 min pre-and post-stimulation periods, when arterial pressure and heart rate were unchanged from control levels. Renal blood flow fell significantly from 31.3 +/- 4.5 to 17.7 +/- 5.1 ml/min (47% decrease) and filtration fraction significantly increased from 12.7 +/- 1.1 to 15.7 +/- 2.1% (24% increase) during glutamate infusion. Each of these variables returned to their pre-stimulus levels after ceasing the central stimulation. Fluid, sodium and potassium excretion were not changed by this stimulus. In conclusion, the results in this study suggest that a selective increase in sympathetic nerve activity to the kidney without change in renal perfusion pressure can cause constriction of the renal vasculature without alteration in sodium and water excretion.