SUMMARY Two major groups of renal chemosensory neural elements have been identified in the rat: one specifically activated by renal ischemia, the previously described "R" chemoreceptors, and the other by backflow of nondiuretic urine into the renal pelvis. The latter group is the object of the present investigation. In anesthetized, male Sprague-Dawley rats, single-unit recordings were obtained by dissection of the centrally cut nerves of the right kidney. The responses of single units to backflow into the renal pelvis of nondiuretic urine, diuretic urine, and solutions containing urea, mannitol, or inorganic ions were compared. The excitatory effect of the backflow of nondiuretic urine was due to its chemical composition rather than to changes in pelvic pressure and pelvic distension. The same units were activated markedly by renal ischemia. The resting discharge rate of the units was very high in nondiuretic conditions, and it declined progressively when diuresis was induced by expansion of the extracellular fluid volume. It is concluded that this group of sensory elements responds to the chemical environment in the renal interstitium as modified by ions crossing the pelvic epithelium, by leakage of ions out of ischemic cells, and by alterations in the excretory function of the kidney and renal blood flow. This group of renal sensory nerve endings has been termed "R2" chemoceptive receptors, to distinguish them from the previously described group of renal "R" chemoreceptors. Circ Res 46: 395-405, 1980LIKE OTHER visceral organs, the kidneys have a profuse sensory innervation. From recordings of impulses conducted along afferent fibers in the renal nerves, several functional classes of renal sensory receptors have been identified so far. In cats, there are receptors sensitive to alterations in ureteral pressure and to venous or arterial perfusion pressure (Beacham and Kunze, 1969;Astrom and Crafoord, 1968); in rabbits, receptors affected by changes in ureteral and arterial perfusion pressure, but not changes in venous pressure (Niijima, 1971); in dogs, units responsive to changes in venous, ureteral, or arterial perfusion pressure (Uchida et al., 1971); in rats, receptors sensitive to increase in venous pressure (Astrom and Crafoord, 1967). In the renal nerves of the rat at least two other populations exist. The fibers of one group are silent under control conditions and are activated only by renal ischemia; those of the other population exhibit a resting discharge and respond markedly to backflow of urine into the renal pelvis (Recordati et al., 1978 characteristics of the first group of receptors which, because of their sensitivity to ischemia and unresponsiveness to mechanical stimuli, were termed renal, "R," chemoreceptors. The second group, on the other hand, was analyzed only superficially. They were considered to be a population of renal mechanoreceptors because previous investigators (Beacham and Kunze, 1969;Astrom and Crafoord, 1968;Niijima, 1971) had thought that changes in pelvic pressure and pelvic d...

A study of the renal receptors and types of stimuli which give origin to supraspinal and spinal-mediated autonomic reflexes is presented. Multiunit and single unit recordings from the afferent renal nerves of male Sprague-Dawley rats have revealed two groups of renal chemosensitive receptors (chemoreceptors). These we have called renal R1 and R2 "chemoceptive" receptors. R1 receptors do not have a resting discharge but are activated after 38.7 +/- 3.3 (S.E) sec (n = 40) of complete renal ischemia (occlusion of the renal artery). Other activating stimuli are associated with a marked impairment in renal blood flow (prolonged occlusion of the renal vein and the hypotension of systemic asphyxia or hemorrhage). Their discharge is characterized by trains of impulses which cease abruptly upon re-entry of blood into the kidney. They are not responsive to increases or decreases in renal perfusion pressure or to increases in renal venous or ureteral pressure. In contrast, R2 receptors have a resting discharge and respond vigorously to backflow of normal urine (nondiuretic) into the renal pelvis. The results of the backflow into the pelvis of different test solutions (diuretic and nondiuretic urine, 1 M urea, 1 M mannitol and solutions of NaCl and KCl) indicate that this response is dependent upon the composition of the fluid bathing the renal pelvis rather than the increase in pelvic pressure or pelvic distension. The resting discharge rate is highest in nondiuretic conditions and declines substantially after diuresis is induced by extracellular volume expansion. R2 receptors are also activated by renal ischemia produced by clamping the renal artery. It is concluded that these two groups of afferent sensory units are renal chemosensitive receptors, (chemoreceptors) which respond to the chemical environment of renal interstitium.

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