This study showed that resistance artery relaxation via K+ channels was impaired in moderate experimental CRI, while the vasorelaxation mediated by the NO pathway was preserved.
Furthermore, in advanced CRI the impaired endothelium-dependent dilatation of resistance vessels featured deficiency of both NO- and K+ channel-mediated vasorelaxation.
The endothelium-dependent relaxations induced by ACh in NA-precontracted resistance arterial rings were reduced in rats with moderate CRI, the finding of which corresponds to previous studies in microvessels from patients (Morris et al. 2001, Wang et al. 2000a) and laboratory animals (Bagi et al. 2003) with renal disease. The vasodilatation induced by ACh is mediated by NO, PGI2, and endothelium-derived hyperpolarization, and the relative roles of these components were addressed by NOS inhibition, COX inhibition, and K+ channel blockade, respectively. The NOS inhibition using L-NAME moderately diminished the relaxations to ACh similarly in both NTX and sham rats, suggesting, that the NO-mediated component of the relaxation in small mesenteric arteries was similar between the groups. The sensitivity of arterial smooth muscle to cGMP, as examined by the relaxation to the NO-donor SNP, was also similar in the NTX and sham groups after the 12-week follow-up. Furthermore, COX inhibition with diclofenac did not have significant influence on the relaxation to ACh in the study groups. This shows that COX-derived compounds did not significantly contribute to the endothelium-dependent relaxation in resistance arteries of these rats, and suggests also that CRI did not have detectable effect on that pathway.
The major mechanism that contributes to the vasodilatation in resistance arteries is via the activation of K+ channels (Coats et al. 2001). In agreement with this, the inhibition of KCa with charybdotoxin and apamin markedly reduced the relaxation to ACh, whereas the reduction in the relaxation was clearly less marked in CRI rats than in sham group, and the remaining response was similar between the study groups (Cohen and Vanhoutte 1995). This finding suggests an attenuated vasorelaxation via K+ channel-mediated hyperpolarization in rats with moderate CRI. The impaired endothelium-derived hyperpolarization could result from reduced sensitivity to, or decreased endothelial release of, hyperpolarizing factors (Roman 2002). Therefore, endothelium-independent relaxations to EET (KCa agonist) and levcromakalim (KATP opener) were performed. Interestingly, both of these responses were clearly reduced in the rats with moderate CRI, showing that the impaired arterial relaxation could be attributed to reduced vasodilatation via K+ channels in vascular smooth muscle. It is of note that since the defective K+ channel-mediated resistance artery relaxation was detected already at normotensive stage of CRI, the observed changes were presumably caused by renal insufficiency per se. Previously, PTH has been found to increase the synthesis of the KCa blocker 20-hydroxyeicosatetraenoic acid in renal tubular cells (Roman 2002).
Thus, SH observed in NTX rats could contribute to the defective K+ channel mediated relaxation in vascular smooth muscle.
When compared with findings observed in moderate CRI, the advanced stage of renal disease featured clearly further disturbed regulation of resistance artery tone: mesenteric resistance vessels of NTX rats followed for 27 weeks showed poor KCa channel mediated relaxation, and clearly deteriorated endothelium-dependent relaxation via NO pathway was also detected. In study IV, the relative roles of NO, PGI2 and endothelium-derived hyperpolarization were evaluated by the AUC changes of the relaxation responses to ACh induced by NOS inhibition (L-NAME), COX inhibition (diclofenac) and KCa blockade (charybdotoxin and apamin), respectively (Gschwend et al. 2002).
NOS inhibition reduced the relaxations to ACh, and AUC analyses showed that contribution of NO to the ACh response was lower in NTX rats than in sham group. Thus, NO-mediated endothelium-dependent relaxation was impaired in rats with advanced CRI. Several putative mechanisms can explain impaired vasorelaxation via endothelium-derived NO in uraemia. Increased concentrations of endogenous NOS inhibitors, like asymmetric dimethylarginine, may reduce NO synthesis (Fliser et al. 2003). Endothelial cells may feature deficiency of NO precursor L-arginine, the transport of which into the endothelium is inhibited by increased urea nitrogen (Wagner et al. 2002). NOS activity could also be affected by SH, since parathyroidectomy has been reported to improve NO production in NTX rats (Vaziri et al. 1998a).
As in moderate CRI, the sensitivity of arterial smooth muscle to cGMP, as examined using the NO-donor SNP, was similar in the NTX and sham rats also at advanced stage of CRI. In contrast, the endothelium-independent relaxations induced by the KCa agonist EET (Plane et al. 1997) were impaired in NTX rats after both 12 and 27 weeks of follow-up. Therefore, vasorelaxation via KCa in smooth muscle was decreased in earlier as well as in more advanced stages of CRI. High levels of PTH can increase the synthesis of the KCa blocker 20-hydroxyeicosatetraenoic acid in renal tubular cells (Roman 2002) and therefore suppress K+ channel activity. Moreover, the activation of type 1 PTH receptors has a relaxing effect on tracheal smooth muscle via the stimulation of cAMP and activation of BKCa (Shenberger et al. 1997). PTH can also act as a vasodilator via type 1 PTH receptors, whereas expression of these receptors is down-regulated in uraemia (Disthabanchong et al. 2004), the mechanism of which may contribute to the attenuated vasodilatation. For instance, downregulation of type 1 PTH receptor mRNA has been reported in human osteoblasts in end-stage renal failure (Picton et al. 2000). Furthermore, the expression of these receptors has been reported to be decreased in renal blood vessels of spontaneously hypertensive rats, which may partially explain the elevated renovascular resistance in these animals (Massfelder et al. 2002). The putative CRI-induced downregulation of type 1 PTH receptors in vascular smooth muscle, and its possible relationship to attenuated K+ channel mediated vasorelaxation, remains to be studied.
Despite the low activity of systemic RAS in NTX rats, possible changes of RAS in vascular wall may contribute to the changes in vascular function. Therefore, we evaluated the contractile sensitivities and maximal wall tensions in mesenteric resistance arteries in response to Ang II.
These vasoconstrictor responses were not altered in rats with moderate CRI 12 (weeks after the operations). However, in the same study, the maximal wall tensions induced by Ang II were clearly increased in the NTX rats that were followed for 24 weeks and featured more advanced renal insufficiency. These findings together with the observed increase of aortic ACE content suggest that CRI seems to have an enhancing influence on vascular local RAS, which may contribute to the disturbed regulation of arterial tone in CRI.
The sensitivities and maximal wall tensions to NA and KCl were not altered in both moderate and advanced stage of CRI. These results agree with a previous clinical study, where forearm vasoconstriction to NA was not altered in end-stage renal insufficiency patients (Passauer et al.
2003). Since no contractile changes were observed in the responses to NA and KCl, the differences observed in vasorelaxation could not result from alterations in the responses to the precontractile agents used in this study.