The kidney receives a major fraction of the cardiac output and consequently any change in renal blood flow (RBF) can importantly influence systemic arterial pressure. The kidney receives a rich supply of sympathetic nerves and changes in renal sympathetic nerve activity (RSNA) may play a role in the regulation of renal vasomotor tone. However, the contribution of RSNA in regulating RBF during normal daily activity has not been completely elucidated. The purpose of the present study was to determine whether a quantitative relationship existed between RSNA and RBF during normal daily activity. Wistar rats (n = 8) were anaesthetized with pentobarbitone sodium (45 mg kg^-1 I.P.) and chronically implanted with catheters, for the measurements of systemic arterial (Pa) and central venous (Pcv) pressures, and with electrodes for the measurement of RSNA to the left kidney and with a Doppler flow cuff on the left renal artery for RBF under aseptic condition. At least 3 days after the surgery, Pa, Pcv, RSNA and RBF were measured continuously during natural changes in behaviour, classified as REM sleep, non-REM sleep (NREM), quiet awake, moving and grooming states. Comparisons were made between the last 90 s of the basal state and the first 90 s of the new state. Renal vascular conductance was calculated by dividing mean RBF by the systemic pressure gradient (Pa-Pcv). Experiments accorded with national guidelines. Data (means ± S.E.M.) were subjected to repeated measures ANOVA and Fisher’s least significant test. Significance was taken when P < 0.05. Rats were killed humanely with an overdose of pentobarbitone sodium at the end of the experiments.

The transition from NREM to REM sleep resulted in a step reduction of RSNA by -29 ± 3 % (P < 0.05) associated with a parallel increase in RBF from basal values (NREM) of 9.3 ± 0.1 to 9.7 ± 0.1 kHz (P < 0.05) while Pa rose from 106 ± 1 to 112 ± 2 mmHg (P < 0.05). During transition from quiet awake to moving states, RSNA increased immediately by 55 ± 8 % (P < 0.05), associated with a parallel decrease in RBF from basal values (quiet awake) of 9.4 ± 0.1 to 8.7 ± 0.1 kHz (P < 0.05) but Pa did not change (105 ± 1 versus 106 ± 1 mmHg). There was a linear association between the increases in averaged RSNA and activity level in the order of REM, NREM, quiet awake, moving and grooming states. Conversely, the averaged RBF decreased linearly in association with increases in activity level. There was a significant inverse linear relationship between RSNA and RBF (P < 0.05) as well as between RSNA and renal vascular conductance (P < 0.05) across all the behavioural states.

These results indicate that a moderate or mild change in RSNA induced by the natural behaviour changes had an important influence on RBF. Therefore it is likely that the kidney plays an important role in buffering the distribution of systemic blood flow such that systemic arterial pressure is controlled during natural behavioural changes in the rat.

This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan.