Renal Sympathetic Nerve Activity and Heart Rate Responses to Renal Pelvic Infusion of Bradykinin and Capsaicin in Rats Exposed to Intermittent Hypoxia

Abstract

Renal oxidative stress and inflammation are evident in individuals with obstructive sleep apnea and in animals exposed to intermittent hypoxia (IH). It is known that kidney injury alters renal afferent nerve activity disrupting reno-renal reflex mechanisms. This study investigated reno-renal reflex changes in response to intra-renal pelvic infusion of bradykinin and capsaicin in rats exposed to IH. Wistar rats were exposed to 96 cycles of IH (6% O2, n=10) or to normoxia (sham, 21% O2, n=11) daily for 14 days. On day 15, rats were anesthetized (induction: urethane, α-chloralose and sodium pentobarbitone mixture i.p. (416, 27 and 33mg/kg), maintenance: urethane and α-chloralose mixture i.v. (62 and 4mg/kg)) to record cardiovascular parameters, renal sympathetic nerve activity (RSNA), and urine flow (UF). Intra-renal pelvic infusion of three doses of bradykinin (53µg/kg, 141µg/kg, 247µg/kg, for 5 min at 20µl/min in random order) was performed, and was repeated following the blockade of bradykinin receptors 1 (BK1R) and 2 (BK2R). Finally, an intra-renal pelvic infusion of capsaicin (0.133µg/kg) was performed. During infusions, changes in blood pressure, heart rate (HR) and contralateral RSNA were recorded. Urine samples were collected from the contralateral kidney. Data are expressed as mean±SD and were analyzed using t-test or ANOVA, where appropriate. IH rats were hypertensive (128±17 vs. 112±22mmHg) with elevated HR (440±37 vs. 403±42bpm), and increased basal UF (56±18 vs. 26±12µl/min/kg); all p<0.05. IH rats had increased resting RSNA, but this was not statistically significant (% of max: 19.6±6.4 vs. 14.8±7.2, p=0.105). All intra-renal pelvic doses of bradykinin augmented RSNA in IH and sham rats (all p<0.05), without changes in blood pressure during infusions. However, UF during bradykinin infusion was unchanged in IH and sham rats. Increases in RSNA and HR in response to 247µg/kg bradykinin were blunted in IH rats compared with shams (RSNA p=0.005; HR p=0.021), with diminished increase in RSNA most evident during the last three mins of infusion (% of baseline: 126±20 vs. 108±5%, p=0.016) and attenuated increase in HR during the last 30s of infusion (∆HR: +7±7 vs. +18±15bpm, p=0.05). This was also evident by a decrease in the area under RSNA curves (AUC) in IH rats compared with shams (1884±970 vs. 5437±3950%.s, p=0.046). AUC analysis revealed that increases in RSNA in response to bradykinin were attenuated during BK1R blockade (p=0.012), and greater suppression of bradykinin-induced increase in RSNA was observed following concurrent BK2R blockade (p=0.005). Intra-pelvic infusion of capsaicin induced a similar increase in HR, RSNA and UF in IH and sham rats. Protein expression of bradykinin and TRPV1 receptors in kidney tissue was equivalent between groups. However, in isolated renal pelvic wall, BK2R expression was 53% less in IH (n=4) compared with sham (n=4), but not statistically significant (p=0.120), which might be related to the small sample size. Our results indicate a blunted reflex increase in RSNA and HR in response to intra-pelvic bradykinin in IH rats, which could be due to down-regulation of the BK2R pathway.