Renal neural response to ischemic renal failure in chronic hypoxic rats

The sympathetic nervous system in acute kidney injury

Acute kidney injury (AKI) is frequently accompanied by activation of the sympathetic nervous system (SNS). This may result from pre-exisiting chronic diseases associated with sympathetic activation prior to AKI or it may be induced by stressors that ultimately lead to AKI such as endotoxins and arterial hypotension in circulatory shock. Conversely, sympathetic activation may also result from acute renal injury. Focusing on studies in experimental renal ischaemia and reperfusion (IR), this review summarizes the current knowledge on how the SNS is activated in IR-induced AKI and on the consequences of sympathetic activation for the development of acute renal damage. Experimental studies show beneficial effects of sympathoinhibitory interventions on renal structure and function in response to IR. However, few clinical trials obtained in scenarios that correspond to experimental IR, namely major elective surgery, showed that peri-operative treatment with centrally acting sympatholytics reduced the incidence of AKI. Apparently, discrepant findings on how sympathetic activation influences renal responses to acute IR-induced injury are discussed and future areas of research in this field are identified.

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 beta-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.

Renal afferent signaling diuretic response is impaired in streptozotocin-induced diabetic rats

Renal afferent signaling diuretic response is impaired in streptozotocin-induced diabetic rats.

BACKGROUND

Renal insufficiency develops in diabetes and shows structural and functional abnormalities. Renal afferents, including chemoreceptors and mechanoreceptors located in the vascular and ureteropelvic portions of the kidney, may reflect changes in the environment and trigger an afferent nerve-mediated regulatory function that is known as the renorenal reflex. In this study, the involvement of these renal sensory receptors during the early diabetic state is defined.

METHODS

Diabetes was induced in rats after a tail vein injection of streptozotocin (STZ; 60 mg/kg intravenously). Four groups of rats, control (C), diabetic (DM), diabetic with acute insulin treatment (DMAI, 9 U/rat, subcutaneously, on the experimental day), and chronic insulin treatment (DMCI, 9 U/rat, subcutaneously, daily) were studied. Spontaneous firing type 2-renal chemoreceptor (CR2), arterial mechanoreceptor (MRa), ureteropelvic mechanoreceptor (MRu), and venous mechanoreceptor (MRv) were identified by single-unit analysis of renal afferent nervous activity. The receptor activities were confirmed by their response patterns to stimuli elicited by renal arterial occlusion (RAO), backflow of urine, increasing arterial pressure, increasing ureteropelvic pressure (UP), or renal venous occlusion (RVO). The response of these afferent receptors to a challenge of volume expansion and their functional activities on renorenal reflexes were also examined. Immunostaining with PGP 9.5 was applied for examination of the nerve distribution in the diabetic kidney. The tissue level of histamine in the renal pelvis was determined. We explored the effect of histamine on renal receptor activity in these animals to address the possible role of histamine in MRu receptor activity.

RESULTS

In early diabetics, signaling activities in MRa and MRv were maintained; however, activity in CR2 and MRu was depressed. For CR2, the reduced basal discharge and the repressed responses to RAO, backflow of urine, and volume expansion found in DM rats were recovered by acute insulin treatment to restore glucose levels to near normal. For MRu, the depressed response to increasing UP and volume expansion was not restored by acute correction of hyperglycemia in DMAI rats. However, antihistamine treatment or chronic insulin treatment recovered the MRu response to mechanical stimuli in DM rats. Because of the desensitized CR2 and MRu activity, renorenal reflexes elicited by backflow of urine and increasing UP were depressed in DM rats.

CONCLUSION

Despite a lack of structural changes, the operating system, signaling ability, and renorenal reflex regulatory function of two renal afferent nerve receptors, CR2 and MRu, are altered in the early diabetic state.

Attenuated response of renal mechanoreceptors to volume expansion in chronically hypoxic rats

Multifiber renal afferent nerve activity responds to volume expansion in sea level rats but not in chronically hypoxic (high altitude) rats. We performed single-unit recordings of renal afferent nerve activity to characterize renal sensory receptors and their responses to volume expansion in these animals. Hypoxia was induced by placing Wistar rats in an altitude chamber (380 Torr, 5,500 m) for 4 wk. Spontaneously firing renal R2 chemoreceptor and arterial, ureteropelvic, and venous mechanoreceptors were identified. The basal activity of each receptor was similar among these rats. In response to specific stimulus, the increasing impulse of R2 chemoreceptor was similar between two groups of rats, but the increasing activity of each mechanoreceptor was less in hypoxic rats. When challenged with saline load, R2 chemoreceptor activity decreased, but all mechanoreceptors activated in all rats. Despite similar increases of arterial, renal ureteropelvic, and venous pressure during saline load, the increasing activity of each mechanoreceptor was significantly less in hypoxic rats. These results indicated chronic hypoxia attenuates the sensitivity of renal mechanoreceptors in response to the stimulation of saline load.