What sets the long-term level of renal sympathetic nerve activity: a role for angiotensin II and baroreflexes?

Increasing evidence suggests elevated sympathetic outflow may be important in the genesis of hypertension. It is thought that peripheral angiotensin II, in addition to its pressor actions, may act centrally to increase sympathetic nerve activity (SNA). Without direct long-term recordings of SNA, testing the involvement of neural mechanisms in angiotensin II-induced increases in arterial pressure is difficult. Using a novel telemetry-based implantable amplifier, we made continuous recordings of renal SNA (RSNA) before, during, and after 1 week of angiotensin II-based hypertension in rabbits living in their home cages. Angiotensin II infusion (50 ng x kg(-1) x min(-1)) caused a sustained increase in arterial pressure (18+/-3 mm Hg). There was a sustained decrease in RSNA from 18+/-2 normalized units (n.u.) before angiotensin II to 8+/-2 n.u. on day 2 and 9+/-2 n.u. on day 7 of the angiotensin II infusion (P<0.01) before recovering to 17+/-2 n.u. after ceasing angiotensin II. Analysis of the baroreflex response showed that although angiotensin II-induced hypertension led to resetting of the relationship between mean arterial pressure (MAP) and heart rate, there was no evidence of resetting of the MAP-RSNA relationship. We propose that the lack of resetting of the MAP-RSNA curve, with the resting point lying near the lower plateau, suggests the sustained decrease in RSNA during angiotensin II is baroreflex mediated. These results suggest that baroreflex control of RSNA and thus renal function is likely to play a significant role in the control of arterial pressure not only in the short term but also in the long term.