Differential arterial baroreflex regulation of renal, lumbar, and adrenal sympathetic nerve activity in the rat

Abstract

Lumbar (LSNA), renal (RSNA), or adrenal sympathetic nerve activity (ASNA) is most commonly used as an index of sympathetic nerve activity in investigations of arterial baroreflex control in the rat. Although differential regulation of sympathetic outputs to different organs has been extensively studied, no direct and simultaneous comparisons of the full range of baroreflex reactivity have been described for these sympathetic outputs. Therefore, we compared steady-state sigmoidal baroreflex stimulus-response curves (via phenylephrine-nitroprusside infusion) for RSNA recorded simultaneously with LSNA or ASNA in urethan-chloralose-anesthetized male Sprague-Dawley rats. Characteristics of the baroreflex curves differed significantly between all three sympathetic outputs. ASNA exhibited the greatest range of baroreflex regulation, the highest upper level of activity, and the widest distribution of the gain over a broad range of mean arterial pressure (MAP). RSNA exhibited greater gain than LSNA. LSNA showed the smallest range and maximal inhibition in comparison to other sympathetic outputs. However, all three nerves responded similarly to baroreflex stimulation and unloading in the range in MAP close to the operating point. We conclude that baroreflex regulation of sympathetic activity shows wide regional variability in gain, range, and maximal inhibition. Therefore, the entire stimulus-response relationship should be considered in comparing regional sympathetic responses.

Baroreflex control of lumbar and renal sympathetic nerve activity in conscious rats

“…With the normalization procedure used, the range of SNA variation was smaller for LSNA than for RSNA, which accorded with the visual inspection of raw data. As a consequence, and although the slope coefficient P 2 did not differ between the two SNAs, the LSNA gain was markedly lower than the RSNA gain, which is in agreement with previous studies in anesthetized rats (29,30).…”

“…Both methods for assessing sympathetic baroreflex control revealed a significantly lower BRS for LSNA than for RSNA, which is in agreement with earlier observations in anesthetized rats (29,30). It is known that most, if not all, renal sympathetic nerve fibers are responsive to the stimulation of arterial baroreceptors (9,25).…”

“…However, it should not be ignored that a small proportion of cutaneous nerve fibers is present in the lumbar chain at this level (11,31). Regarding the post-vs. preganglionic nature of LSNA, it is of note that the residual LSNA recorded after ganglionic blockade amounted to almost 10% of baseline activity, which would point to a significant contribution from preganglionic axons to overall LSNA, in accordance with a previous study (29). However, this contribution is probably overestimated because of the reflex increase in preganglionic SNA that was elicited by the large AP fall caused by ganglionic blockade.…”

“…In the meantime, evidence has accumulated suggesting a differential baroreflex control of regional SNAs in several animal species (22). In the rat, the comparison of the baroreflex control of regional SNAs has been carried out in urethane-chloralose anesthetized animals by using the vasoactive drug injection technique, i.e., the so-called pharmacological method (29,30). In these studies, it was reported that the characteristics of the baroreflex function curves differed between RSNA, lumbar SNA (LSNA), and adrenal SNA.…”

Recording sympathetic nerve activity chronically in rats: surgery techniques, assessment of nerve activity, and quantification

“…However, such manipulations cannot be routinely performed throughout the experimental paradigm or may not be possible due to the unavailability of an external IV line. Furthermore, ganglionic blockade removes only about 90% of lumbar SNA when recorded at the level of L3-L5 because some of the fibers are preganglionic (30,38). Alternatively, the electrical signal between SNA bursts can be used to estimate background noise.…”