Impaired vasorelaxation in hypertension: beyond the endothelium

The endothelium plays a critical role in modulating vascular relaxation by release of endothelial-derived nitric oxide (NO), stimulation of vascular smooth muscle cell soluble guanylate cyclase (sGC) and the subsequent increase in intracellular cyclic guanosine monophosphate (cGMP) [1]. Altered vascular tone, a characteristic feature in various cardiovascular diseases, including hypertension, is associated with impaired endothelium-dependent vasodilation due, in large part, to reduced NO signalling [2–7]. Among the mechanisms implicated in perturbed endothelial-derived bioavailability of NO in hypertension are (i) reduced expression/abundance of endothelial nitric oxide synthase (eNOS), the major endothelial NO-generating enzyme system [8,9], (ii) decreased activation of eNOS [5], possibly due to tetrahydrobiopterin (eNOS cofactor) deficiency [10] and (iii) increased quenching of NO by abundant oxygen free radicals [7,11,12]. The importance of endothelial-derived NO for the regulation of vascular tone and blood pressure is suggested by the observation that mice deficient in eNOS develop hypertension [9]. Experimental studies also show that NO increases intracellular cGMP and decreases blood pressure by relaxation of resistance arteries. For a long time, the endothelium was considered to be the major regulator of vascular relaxation through the NO/cGMP pathway. However, recent studies have challenged this concept. Growing evidence indicates that endothelium-independent, NO/cGMP-mediated processes also play a role in the modulation of vascular tone and that abnormalities in these events contribute to aberrations in vasorelaxation in hypertension. This is supported by studies demonstrating that endothelium-independent vasodilators release NO, activate guanylate cyclase and generate cGMP in the vascular wall [12–16]. In addition, vascular smooth muscle relaxes in response to classical endothelium-independent agents, such as adenosine, prostacyclin, forskolin and β-receptor agonists [12–16]. Mechanisms that are activated by NO/cGMP in vascular smooth muscle are unclear and processes contributing to abnormal endothelium- independent derived cGMP in hypertension are ill-defined. Some studies suggest that decreased activity of cGMP-dependent protein kinase type I alpha (cGKI), a downstream effector of cGMP, plays a role in reduced NO/cGMP-mediated smooth muscle relaxation in hypertension [17,18]. This is supported by the findings that deletion of the gene for cGKI leads to high blood pressure in mice [17]. In aorta from spontaneously hypertensive rats (SHR), mRNA expression of cGKI is reduced and activation of the enzyme is decreased [18]. Other studies suggest that processes upstream from cGKI, particularly dysregulation of soluble guanylate cyclase, a heterodimer composed of α1 and β1 subunits, is important in impaired endothelium-independent vasodilation in hypertension [19,20]. This is highlighted in the current issue of the journal [21]. The study by Lopez-Farre et al. [21] investigated whether endothelium-independent relaxation (sodium nitroprusside-induced) is altered in aortic rings from adult stroke-prone SHR (SHRSP) and whether these changes are associated with defective NO/sGC/cGMP signalling. Since these rats have increased plasma catecholamine levels and increased sympathetic tone, the authors questioned the putative regulatory role of α1-adrenergic receptors on the NO/cGMP relaxing system using the α1-receptor blocker doxazosin. Endothelium-dependent relaxation to acetylcholine, as well as sodium nitroprusside-induced dilation, were blunted in SHRSP. This was accompanied by reduced expression of soluble guanylate cyclase (β1 subunit) in the aortic wall and decreased generation of vascular cGMP. These data suggest that, independent of the endothelial NO-generating system, reduced vasorelaxation in hypertension could also result from abnormal vascular smooth muscle function. Although this study is not the first to report alterations in regulation of soluble guanylate cyclase in hypertension, the data provide further support for the importance of vascular smooth muscle dysregulation in impaired vasodilation in hypertension. Decreased expression of soluble guanylate cyclase and blunted NO/cGMP signalling have also been demonstrated in aorta of hypertensive TGR (mREN2) 27 rats [20] and in SHR [18]. A novel aspect of the present study is the demonstration that doxazosin preserved endothelium-independent vasorelaxation, suggesting that α1-adrenergic receptors play an important role in processes underlying endothelium-independent NO/cGMP-mediated relaxation in SHRSP aorta. Unfortunately, precise mechanisms for this are not elaborated upon in the study and it is unclear whether the α1-adrenergic effects are vascular smooth muscle cell-specific. Nevertheless, this phenomenon certainly warrants further consideration. Despite recent data implicating endothelium-independent processes in impaired vasodilation in experimental hypertension, there is little convincing data to support these findings in human hypertension. In fact, clinical studies demonstrate, for the most part, that vasodilatory responses to acetylcholine, but not to sodium nitroprusside, are attenuated in human hypertension [5,10,22–24]. These findings have been shown in brachial arteries by forearm venous occlusion plethysmography and intra-arterial infusions of acetylcholine or sodium nitroprusside [7,23,24], as well as in skin microcirculation measured by laser Doppler flowmetry following iontophoretic delivery of acetylcholine and sodium nitroprusside [25]. In-vitro studies of resistance arteries, dissected from gluteal subcutaneous biopsies of essential hypertensive patients, exhibited blunted vasodilatory responses to acetylcholine, but not to sodium nitroprusside [22,26,27]. Only endothelium-dependent relaxation seems to be positively influenced by antihypertensive therapy [27,28]. A few studies failed to demonstrate alterations in endothelium-dependent vasodilation in patients with essential hypertension [29–32]. Accordingly, it is difficult to reconcile that hypertension is invariably characterized by impaired endothelial-mediated vasorelaxation [3,29]. Indeed, it may be possible that factors other than the endothelium, such as vascular smooth muscle, could be important in altered vascular tone, as suggested in the present study [21]. However, the clinical significance of these experimental findings awaits clarification. Although the work of Lopez-Farre et al. [21] adds to the body of evidence demonstrating that endothelium-independent processes contribute to decreased vasodilation in hypertension, a number of aspects require further consideration. First, these processes need to be examined in resistance arteries, the vessels important in blood pressure regulation, rather than in large conduit arteries, such as the aorta which was examined by Lopes-Farre [21] and others [18,20]. Second, the importance of α1-adrenergic receptors in mechanisms regulating aortic relaxation in SHRSP should be regarded with caution because doxazosin actions on cGMP/sGC/vasodilation were maximally effective at concentrations that reduced mean arterial blood pressure, making it difficult to dissociate haemodynamic effects from direct α1-adrenergic actions. Nevertheless, because α1-receptor blockers are part of the therapeutic armamentarium for clinical hypertension, it is possible that some of the beneficial effects of these drugs may extend beyond blood pressure-lowering. Finally, it should be emphasized that besides the NO/sGC/cGMP pathway implicated in endothelium-independent vasodilation, there are many other complex and interacting processes that also play an important role in vasodilator pathways in vascular smooth muscle, such as receptor-mediated activation of adenylate cyclase, formation of cAMP and activation of protein kinase A and myosin light-chain kinase [33,34]. Additionally, endothelium-produced vasodilators other than NO, such as prostacyclin and endothelium-derived hyperpolarizing factor should be considered in vascular dysregulation in hypertension [17]. The study by Lopez-Farre et al. [21] highlights the possibility that processes underlying impaired vasodilation in hypertension extend beyond the endothelium and that altered vascular smooth muscle behaviour may be of importance. While the exact mechanisms remain elusive, the suggestion that α1-adrenergic receptors might play a role is significant, especially since α1-receptor blockers are effective antihypertensive agents. Clearly, understanding the complexities contributing to vascular dysfunction may provide opportunities for improved management and possibly prevention of hypertension. The role of the cGMP vasorelaxing system in the vascular wall should be examined further, both in the physiological regulation of vascular tone and in pathophysiological processes contributing to increased vascular resistance in hypertension.