Renal denervation does not reduce blood pressure in an animal model of polycystic kidney disease

Hypertension is a highly prevalent co-morbidity for patients with polycystic kidney disease (PKD), but difficult to treat. As a novel therapeutic strategy, renal denervation (RDN) offers an alternative approach; however, its efficacy in this patient cohort is not clear. There is evidence that the mechanism by which the removal of the renal nerves acts to reduce blood pressure could be mediated by both the renal sympathetic and sensory nerves and there is also a strong likelihood of the procedure impacting other homeostatic pathways regulated by the kidney such as the renin-angiotensin-aldosterone system (RAAS) and sodium balance. The issue of reinnervation is also still an open question, given that in humans there are proposed long-term benefits despite anatomical and functional evidence of reinnervation from experimental animal models. The overall aim of this thesis is therefore to investigate the effect of RDN on hypertension secondary to PKD, the individual contribution of renal sensory and sympathetic nerves to the RDN-induced effect, the potential effect of RDN on the RAAS and renal handling of sodium, and the potential impact of renal reinnervation. We addressed these questions employing a well-established rodent model of juvenile onset PKD, the Lewis polycystic kidney (LPK) rat in a series of three studies. In Study 1 (Chapter 3), we aimed to validate the effectiveness of two RDN procedures, being total RDN by stripping of the renal nerves and periaxonal application of phenol to destroy both renal sensory and sympathetic nerves in the two strains and the relatively new procedure of selective afferent RDN using periaxonal application of capsaicin. Experiments were undertaken in 6-week old LPK and their normotensive Lewis control strain. This study revealed that at one-week post total RDN, both the renal sympathetic nerve marker tyrosine hydroxylase (TH) and the sensory nerve marker calcitonin gene-related peptide (CGRP) were essentially absent, while after afferent RDN, CGRP immunoreactivity was significantly abolished while levels of the sympathetic nerve marker TH remained intact. Study 2 aimed to examine the effect of total and selective afferent RDN on cardiovascular, renal (Chapters 4) and autonomic function (Chapter 5) in Lewis and LPK rats over an 8-week follow up. To our surprise, neither total or afferent RDN affected the level of systolic (SBP), mean (MAP) and diastolic (DBP) blood pressure in the LPK while total, but not afferent RDN, caused a significant reduction in SBP, MAP and DBP in Lewis rats. The renal function of both strains as determined by plasma urea, creatinine, urine protein/creatinine ratio (UPC) and creatinine clearance rate (CCR) was not affected by either total or afferent RDN. Regarding the effect of RDN on autonomic function as determined by spectral analysis, in the Lewis animals, total RDN significantly reduced systolic blood pressure variability (SBPV) parameters but did not affect heart rate variability (HRV) or baroreceptor sensitivity (BRS) parameters. Afferent RDN increased the overall level of low frequency (LF) components of HRV, decreased very low frequency (VLF) of SBPV but did not affect BRS parameters. In the LPK animals, total RDN lowered the overall level of VLF HRV but did not affect SBPV and BRS parameters in LPK. Afferent RDN increased the HF components of SBPV but did not affect HRV and BRS parameters in LPK. The overall interpretation is that the RDN procedure did not provide any beneficial effect on autonomic function in the LPK. The impact of RDN on the RAAS was assessed by examining circulating and intra-renal RAAS components and determining any impact of the procedures on urine sodium excretion in 10 weeks old Lewis and LPK (Chapter 6). Plasma and kidney renin content as determined by radioimmunoassay were significantly lower in the LPK relative to Lewis animals and were not affected by RDN in either strain. Intra-renal gene expression levels of RAAS components in the LPK vs. Lewis were significantly lower for renin, angiotensinogen, angiotensin converting enzyme 2 (ACE2) and angiotensin type one receptor (AT1αR), except for ACE1, which was significantly higher in the LPK, however RDN had no impact on the intra-renal RAAS gene expression. With regard to sodium excretion, urine sodium concentration was significantly lower in the LPK relative to Lewis, and not impacted by either total or afferent RDN in both strains. When normalised to the 24h urine volume, the level was still significantly lower in the LPK relative to Lewis. There was an overall treatment effect (P=0.04) on 24h sodium excretion with post hoc analysis within each strain indicating no difference between treatment groups. In the course of study 2, substantial renal reinnervation of both renal sensory and sympathetic nerves was demonstrated immunohistochemically at 4 and 8 weeks post-denervation. Study 3 (Chapters 4 and 5) therefore aimed to assess the impact of a repeat RDN procedure. Only total RDN was performed given the lack of effect seen in response to afferent denervation. Cardiovascular, renal and autonomic function parameters were compared between animals that had two (one at 6 weeks, the other at 10 weeks) vs single (6 weeks only) total RDN procedures. In the Lewis, repeat total RDN had no additional impact on SBP, MAP or DBP, while in the LPK rats, repeat total RDN caused a transient increase in blood pressure. In the Lewis, the repeat procedure did not have any treatment effect on renal function while in the LPK rats, a repeat RDN was associated with worsening renal function. Repeating total RDN had a negative effect on autonomic function parameters in the LPK, evidenced by an overall decrease in HRV and BRS. In Lewis, a repeat total RDN caused a decrease in SBPV and an increase in BRS, consistent with improved cardiac autonomic control. Collectively, these studies indicated that neither total or afferent RDN has a beneficial effect in attenuating hypertension, renal dysfunction or autonomic dysfunction in a rodent model of PKD, and that the procedure has limited impact on circulating or intra-renal RAAS or urine sodium excretion that could explain the decrease in blood pressure seen in the Lewis animals. These results suggest that as a patient cohort, RDN in conditions such as juvenile onset PKD is not a recommended procedure.