Impaired renal sensory responses after renal ischemia in the rat

Renal sensory responses and reflex function were examined in rats 24 h after 45 min of ischemic injury caused by unilateral renal arterial occlusion (RAO). The integrity of renal pelvic mechanoreceptor (MRu)-mediated renorenal reflex was examined. An increase in ipsilateral afferent renal nerve activity (ARNA) and a reflex decrease in efferent renal nerve activity (ERNA) and contralateral diuresis and natriuresis produced by increasing the intrapelvic pressure were seen in sham-operated (Sham) rats, but it was largely attenuated in RAO rats. Using single-fiber recordings of the renal MRu discharge, graded increases in intrapelvic pressure or renal pelvic administration of substance P (SP) resulted in pressure-or concentration-dependent increases in ARNA in the control kidney of Sham rats, whereas attenuated responses were seen in the postischemic kidney of RAO rats. The unresponsiveness of renal MRus in RAO rats was accompanied by an insufficient release of SP. However, the baseline SP release is higher in RAO kidneys due to a reduced neutral endopeptidase (NEP) activity in the renal pelvis of the postischemic kidney. No changes in NK-1 receptor mRNA levels were demonstrated; however, the expression of NK-1 receptors in the plasma membrane of RAO pelvis were decreased, possibly resulting from the internalization of the receptors associated with β-arrestin trafficking. Renal excretory responses after saline loading were significantly lower in the postischemic kidney of RAO rats than in Sham rats. Responses of ARNA and ERNA were also lower. It is concluded that the defective activation of renal sensory mechanoreceptors in the postischemic kidney results from an inadequate release of SP after mechanostimulation and the reduced functional NK-1 receptors.