INTRODUCTION

TOP ABSTRACT INTRODUCTION OVERVIEW OF NEURAL CONTROL... FUNCTIONALLY SPECIFIC RENAL... REFERENCES

THE UNDERSTANDING OF THE NEURAL control of renal function is an example of the importance of the relationship between structure and function. Before the early 1970s, although it was known that the renal arterial vasculature was innervated, neither the renal tubules nor the juxtaglomerular granular cells were thought to be innervated. Thus, in a study examining the influence of renal nerve stimulation on renin secretion, the unexpected observation that renal nerve stimulation decreased sodium excretion without changing renal perfusion pressure, renal blood flow, or glomerular filtration rate was dismissed as follows (20): "The renal nerves could have a direct action on tubular sodium reabsorption. This hypothesis seems unlikely, since the tubules are not thought to be innervated."

The unambiguous demonstration that the renal tubules and the juxtaglomerular granular cells, in addition to the renal arterial vasculature, were innervated stimulated research into the functional significance of this intrinsic renal innervation. In regard to the tubules, the observation was straightforward (31): "Varicose regions of the axons, containing clusters of dense-cored vesicles (norepinephrine-containing), are in contact with the renal tubules. At the point of contact, only the basement membrane separates the nerve endings from the tubule cells." The authors' conclusion was prophetic (31): "These studies provide an anatomical basis for a direct action of the autonomic nervous system on renal tubular function."

	OVERVIEW OF NEURAL CONTROL OF RENAL FUNCTION

TOP ABSTRACT INTRODUCTION OVERVIEW OF NEURAL CONTROL... FUNCTIONALLY SPECIFIC RENAL... REFERENCES

Although the role of the renal sympathetic nerves in control of renal function has been recently extensively reviewed (5, 6), a brief summary is relevant for introduction of the major focus of this review. Once the detailed intrinsic renal sympathetic innervation was more clearly understood, it was readily demonstrated in a variety of species that low-frequency renal sympathetic nerve stimulation, which did not affect renal blood flow or glomerular filtration rate, caused a reversible decrease in urinary sodium excretion. These results indicated that renal sympathetic nerves acting directly on the now known to be innervated renal tubules directly increased renal tubular sodium reabsorption. Subsequently, using combinations of free-flow micropuncture and continuous tubular microperfusion, it was shown that this effect occurred throughout the tubule: proximal convoluted tubule, ascending limb of Henle's loop, distal convoluted tubule, and collecting duct. The magnitude of this effect was proportional to the density of the renal tubular innervation, being greatest in the ascending limb of Henle's loop and least in the collecting duct. It was inhibited by antagonists specific for _1B-adrenoceptors, which are located on the peritubular basement membrane along the nephron. In vitro studies showed that the neurotransmitter in renal sympathetic nerve terminals, norepinephrine, increased Na⁺-K⁺-ATPase activity in cultured renal proximal tubular epithelial cells and that this was inhibited by _1B-adrenoceptor antagonists. Companion studies using renal denervation and either reflex increases or decreases in renal sympathetic nerve activity gave compatible results: when renal sympathetic nerve activity increased, renal tubular sodium reabsorption increased, resulting in antinatriuresis; when renal sympathetic nerve activity decreased, renal tubular sodium reabsorption decreased, resulting in natriuresis. These effects were easily demonstrable in conscious animals (27-30), serving to refute earlier claims that they were artifacts related to the stress of anesthesia and surgery (36, 37). Intact renal innervation was shown to contribute to homeostatic regulatory responses such as the normal renal responses to sodium depletion and sodium loading. When studies of the neural control of renal function were extended to pathophysiological states such as hypertension, further important observations were forthcoming. A variety of experimental animal models of hypertension are either completely prevented, reversed, or ameliorated by renal denervation; for example, the obesity model of hypertension in the dog is completely prevented by renal denervation in association with a 50% decrease in cumulative sodium retention (19). Approximately 30-40% of the renal sodium retention of edema-forming conditions, such as congestive heart failure (7), cirrhosis (7), and the nephrotic syndrome (17), are dependent on intact renal sympathetic innervation. On the afferent side, signals from renal sensory receptors coursing via afferent renal nerves to the neuraxis are involved in inhibitory renorenal reflexes and excitatory renosystemic reflexes, which, via peripheral sympathoexcitation, contribute to the hypertension of chronic renal disease (1, 2).

In summary, as shown in Fig. 1, increases in renal sympathetic nerve activity produce increases in renin secretion rate, decreases in urinary sodium excretion by increasing renal tubular sodium reabsorption, and decreasing renal blood flow (and glomerular filtration rate). Using graded frequency renal sympathetic nerve stimulation (Fig. 2), it was found that the frequency response curve for renin secretion rate was to the left of that for decreases in urinary sodium excretion, which, in turn, was to the left of that for decreases in renal blood flow. In practical terms, this means that an increase in renin secretion rate can be achieved without antinatriuresis or renal vasoconstriction, that antinatriuresis can be achieved without renal vasoconstriction but will be accompanied by increased renin secretion rate, and that renal vasoconstriction will be associated with an increase in renin secretion rate and antinatriuresis.

View larger version (9K): [in this window] [in a new window]   Fig. 1.   The major effects of increased renal sympathetic nerve activity on juxtaglomerular granular cells, tubules, and arterial vessels.

View larger version (16K): [in this window] [in a new window]   Fig. 2.   Relationships of effects of direct renal sympathetic nerve stimulation (RNS) on responses of renin secretion rate (increases), urinary sodium excretion (decreases), and renal blood flow (decreases).

	FUNCTIONALLY SPECIFIC RENAL SYMPATHETIC NERVE FIBERS

TOP ABSTRACT INTRODUCTION OVERVIEW OF NEURAL CONTROL... FUNCTIONALLY SPECIFIC RENAL... REFERENCES

It was not known whether these three effector target structures (juxtaglomerular granular cells, arterioles, tubules) in the kidney are each supplied by a functionally distinct group of renal sympathetic postganglionic neurons that could be the prerequisite for an independent neuronal influence on each of them. Although Muller and Barajas (31) noted that the same renal sympathetic nerve fiber could synapse "en passant" with all three structural elements, there were also examples of renal sympathetic nerve fibers synapsing with only one of them. As shown in Fig. 3, left, there are renal sympathetic nerve fibers that make sequential contact with each of the three intrarenal effectors, usually starting with a vascular element. As shown in Fig. 3, right, there are also unique renal sympathetic nerve fibers that only make contact with one of the three intrarenal effectors and not any of the others.

View larger version (18K): [in this window] [in a new window]   Fig. 3.   Patterns of innervation of intrarenal effectors. T, tubule; V, vessel. On the left, a single renal sympathetic nerve fiber makes sequential contact with each of the effectors, beginning with the vessel. On the right, separate and selective single renal sympathetic nerve fibers make contact with only 1 of the 3 effectors.

The situation where a renal sympathetic nerve fiber makes sequential contact with multiple effectors is consistent with each effector having a different response threshold (e.g., exhibiting frequency dependence at supramaximal amplitude). However, it is also possible that each effector could respond by virtue of effector-specific information being encoded in the renal sympathetic nerve discharge pattern (e.g., variations in frequency, amplitude, duration; regular vs. irregular). This suggests that each effector may possess unique response characteristics in either the time or frequency domain. The situation where unique renal sympathetic nerve fibers specifically and selectively innervate arterioles, tubules, and juxtaglomerular granular cells suggests that they might be coupled to separate central neuron pools with specific and selective afferent reflex inputs.

Additional evidence for a differentiated innervation of effectors comes from the work of Luff et al. (21, 22), who noted two different types of sympathetic axons innervating the juxtaglomerular arterioles and the intralobular artery of the rabbit and rat kidney that were structurally different from those supplying other arteries.

Evidence for the existence of functionally specific sympathetic nerve fibers was obtained by Folkow and colleagues (16). They showed that differences in stimulation threshold of preganglionic fibers were associated with the activation of functionally differentiated effectors. As shown by stimulation of the abdominal sympathetic trunk of the cat, the fibers to the cutaneous arteriovenous anastomoses, the nictitating membrane, and the pupil had a lower threshold than the constrictor fibers to the ordinary vessels in the skin and tongue, whereas the sympathetic vasodilator fibers to the skeletal muscles had the highest threshold. By analogy with somatic nerves (16), this was explained by an inverse relationship between stimulation threshold and fiber diameters.

Anatomic features of renal nerve fibers. When characterized as to myelination and size, the renal nerves appear to be rather homogeneous (11). Approximately 96% are unmyelinated fibers with a size range of 0.4-2.5 µm. However, when the distribution of fiber diameters was examined, it was found to be bimodal with a primary mode at 1.1 µm and a secondary mode at 1.6 µm. This suggested at least two populations of renal sympathetic nerve fibers, possibly subserving different functions (i.e., functional nonuniformity).

Neurophysiological features of renal nerve fibers. The conduction velocity averaged 2.10 ± 0.10 m/s, consistent with unmyelinated C-type fibers (11). Strength-duration analysis (at constant frequency) indicated that, for any stimulus duration, a lower stimulus strength (volts) was needed to elicit an antidiuretic response than a vasoconstrictor response. These results indicated that the renal sympathetic nerve fibers producing the effect on the renal tubules (increased sodium and water reabsorption) were not the same as those producing the effect on the intrarenal arterioles (constriction). By analogy with somatic nerves where stimulation threshold is inversely related to fiber diameter, this suggests that the diameters of the nerve fibers involved in the antidiuretic response are greater than the diameters of the nerve fibers involved in the vasoconstrictor response. In addition, these results confirmed the earlier observations, shown in Fig. 2, demonstrating that during graded renal nerve stimulation, the response of the renal tubules occurs at a lower level of stimulation than that of the renal arterial vasculature.

Reflex responses in single units. Heterogeneity (i.e., functional nonuniformity) of renal sympathetic nerve fibers was more firmly demonstrated by recording the responses of single postganglionic renal sympathetic nerve units to a variety of reflex stimuli (11). The majority of single units had spontaneous activity and exhibited the expected responses to stimulation of arterial baroreceptors and central and peripheral chemoreceptors and peripheral thermal receptors. However, there was a minority population of single units that lacked spontaneous activity. These silent single units were shown to be renal by virtue of their response to stimulation of the splanchnic nerve, which contains the preganglionic input to postganglionic renal sympathetic nerves. These single units did not respond to stimulation of arterial baroreceptors and central and peripheral chemoreceptors but responded robustly to stimulation of peripheral thermal receptors. If the postganglionic renal sympathetic nerve fibers were homogeneous (i.e., functional uniformity), a consistent response pattern to all reflex stimuli would be expected. The observed different response patterns to the reflex stimuli indicate the existence of at least two populations of renal sympathetic nerve fibers, likely with functional specificity. Although evidence for homogeneity in renal single-unit responses was found in the rabbit by Dorward et al. (13) and in the cat by Stein and Weaver (38), Riedel and Peter (35) found two populations of renal single units in the rabbit that could be separated based on differences in their spike amplitude and their responses to norepinephrine and angiotensin II.

Peripheral thermal receptor stimulation. The increase in total integrated voltage of renal sympathetic nerve activity that occurs with peripheral thermal receptor stimulation (heat) decreases renal blood flow, and the renal vasoconstriction is prevented by prior renal denervation (34). As it was this stimulus that identified a unique subset of single renal sympathetic nerve fibers, we sought to determine quantitative aspects of the renal sympathetic neural discharge seen in multifiber recordings that were produced by peripheral thermal receptor stimulation. Postganglionic multifiber renal sympathetic nerve activity occurs in synchronized sympathetic discharges (bursts, peaks) with distinct coupling to the cardiac cycle. These synchronized renal sympathetic peaks may be characterized by their amplitude, duration, and frequency. Total integrated voltage encompasses the product of voltage under the curve of each peak (governed largely by peak amplitude as peak duration changes little) and peak frequency. Therefore, changes in total integrated voltage of renal sympathetic nerve activity can be due to changes in peak amplitude, peak frequency, or both. With the development of a method to quantitatively analyze these components of synchronized renal sympathetic discharge (25), interest in their significance has increased (23, 24). Peak amplitude is influenced by the number of active renal sympathetic nerve fibers wherein increased peak amplitude reflects the recruitment of previously silent fibers to fire and decreased peak amplitude reflects the silencing of previously firing fibers. Peak duration is influenced by the firing synchrony and the dispersion of the mass discharge due to different conduction velocities of the multiple renal nerve fibers. Peak frequency is a reflection of the rhythm of central oscillators and is subject to baroreceptor modulation. Using this approach, it was observed that peripheral thermal receptor stimulation increased total integrated voltage of renal sympathetic nerve activity solely by increase in peak amplitude with little to no change in peak frequency (3). When expressed as percent control, 95% of the increase in total integrated voltage of renal sympathetic nerve activity could be accounted for by an increase in peak amplitude. Therefore, peripheral thermal receptor stimulation elicits activity in previously silent renal nerve fibers, which likely explains the unmasking of a unique subgroup of single renal sympathetic nerve fibers by this stimulus. Similarly, intravenous volume loading, which decreased total integrated voltage of renal sympathetic nerve activity, did so predominantly by decreasing peak amplitude (84%) with a far lesser decrease in peak frequency. Therefore, intravenous volume loading causes previously discharging single renal sympathetic nerve fibers to become silent. In addition to efficiency in regulating total integrated voltage of renal sympathetic nerve activity, this regulatory system enables the selective activation and deactivation of functionally specific renal sympathetic nerve fiber groups.

View larger version (16K): [in this window] [in a new window]   Fig. 4.   Responses to tail compression (pinch) and peripheral thermal receptor stimulation (heat). A: total integrated voltage of renal sympathetic nerve activity (RSNA) was increased to the same magnitude by pinch and heat. B: both the decrease in renal blood flow (RBF, solid bar) and the increase in renal vascular resistance (RVR, open bar) was significantly greater in heat than pinch. * P < 0.05. Adapted from Ref. 6.

Pattern of renal sympathetic nerve activation. These results served to focus our attention on the morphological differences between the renal sympathetic nerve burst pattern recorded from multifiber preparations in vivo and the pattern commonly employed in studies where the renal sympathetic nerves are electrically stimulated. The in vivo pattern of a multispike burst is diamond shaped, with the amplitude of the initial and terminal spikes being less than those in the middle of the burst; as noted above, these bursts vary in their amplitude, duration, and frequency. When studying the influence of renal sympathetic nerve activity on renal function, investigators have generally used constant voltage electrical stimulation to apply stimuli that were square wave in nature to a multifiber renal sympathetic nerve preparation, consisting of single pulses of fixed amplitude, duration, and frequency. By means of this approach, considerable information has been acquired concerning the renal sympathetic neural control of renal function. Results using direct electrical renal sympathetic nerve stimulation have been in qualitative agreement with those using reflex stimuli to physiologically activate renal sympathetic nerve fibers with regard to the control of the renal circulation, renal tubular sodium and water reabsorption, and renin secretion rate. This qualitative agreement has supported the validity of using results from artificial square wave stimulation in understanding the physiological effects of renal sympathetic nerve activity on renal function. However, quantitative comparison has been difficult because of substantial differences in experimental design involved in these two quite different approaches.

View larger version (14K): [in this window] [in a new window]   Fig. 5.   A: effect of diamond vs. square wave stimuli at identical steps of integrated voltage (voltage × time, V × ms) delivered on RBF. n = 11. B: effect of diamond vs. square wave stimuli at identical steps of amplitude (voltage, V) on RBF. n = 10. For both panels, stimuli were 150 ms in duration, delivered at a frequency of 1 Hz; renal nerves were stimulated for 30 s at each step. , square wave pattern; , diamond wave pattern. Data are means ± SE. In each panel, the curves are significantly different from each other at P < 0.05. Adapted from Ref. 7.

View larger version (11K): [in this window] [in a new window]   Fig. 6.   Effect of variations in rest time on RBF responses to intermittent stimulation compared with continuous stimulation. Square wave stimulation at 5 pulses/s. Data are means ± SE, n = 14. Data are presented as %change in area under the curve for RBF (AUC RBF) for intermittent stimulation vs. continuous stimulation with number of pulses being constant at 5/s (C5). All points are significantly different (P < 0.05) from no (0%) change. Adapted from Ref. 7.

Nonlinear dynamic analysis of renal sympathetic nerve activity. In pathophysiological states, such as the later stages of decompensated congestive heart failure, renal sympathetic nerve activity is increased, often associated with impairment in central and/or peripheral portions of arterial and/or cardiac baroreflex control mechanisms (8). This is manifest as increases in peak amplitude of synchronized renal sympathetic bursts, reflecting an increase in the number of active fibers by virtue of recruitment of previously silent fibers to fire. In addition, there are more cardiac cycles that have a defined burst (i.e., fewer cardiac cycles are burst free), and, with increases in heart rate, this gives the appearance of continuous bursting with a reduction in the length of silent diastolic periods (15). It is intuitive as to how this shift in activity pattern would lead to greater renal sympathetic neural input to the intrarenal effectors resulting in the well-documented reductions in renal blood flow and glomerular filtration rate, increased fractional renal tubular sodium reabsorption with antinatriuresis, and increased renin secretion rate that characterize the later stages of decompensated congestive heart failure (14).

Perspectives

Similar to sympathetic fiber groups elsewhere, the renal sympathetic nerves are likely functionally differentiated. The concept of functionally specific renal sympathetic nerve fibers provides an additional dimension of control and regulation of the individual effectors within the kidney. A more complete knowledge of these specific and selective neuroeffector relationships would greatly improve our understanding of how the kidney responds to various stimuli in both health and disease.