Nitric oxide and renal perfusion in humans

The importance of the endothelium in modulating the activity of vascular smooth muscle, and therefore in regulating vascular tone, was first suggested by the pioneering studies of Furchgott and Zawadzki [1]. They reported that the damage or absence of endothelial cells curtails the vasodilator action of acetylcholine and other substances [2] normally observed when the endothelium is intact. The endothelium has been shown to exert its vasoregulatory action by releasing substances that induce smooth-muscle relaxation via enhanced production of intracellular cyclic guanosine monophosphate [3]. Among these substances is included nitric oxide (NO), which is continuously released both luminally and abluminally in normal blood vessels [4]. NO protects against platelet aggregation and adhesion [4] and influences vascular smooth muscle tone [4–7]. The effects of NO are mediated via the activation of cytosolic guanylate cyclase to elevate guanosine 3′,5′-cyclic monophosphate (cGMP) levels within the target cell [4,7,8]. NO is synthesized from l-arginine by the enzyme NO synthase (NOS), which exists in three isoforms of which two are constitutive and the other is an inducible form [4,7–9]. Constitutive NOS, responsible for the continuous basal release of NO under physiological conditions, require Ca2+/calmodulin for their activities, which are increased by mechanical forces associated with blood flow or by agonists such as acetylcholine and bradykinin [6,9,10]. Inducible NOS, slowly expressed in response to cytokines and lipopolysaccharides, can generate large amounts of NO even in the absence of Ca2+ [8,9]. The formation of NO by these enzymes can be inhibited by l-arginine analogues, such as NG-nitro-l-arginine methyl ester (l-NAME) or NG-monomethyl-l arginine (l-NMMA), which ultimately function as competitive inhibitors of NO-synthase [4,8,9]. The potential clinical implications of these discoveries were emphasized by studies showing an endothelium-dependent mechanism that causes relaxation of the large arteries in vitro [11,12] and of systemic resistance vessels in normal subjects [13]. There are also data showing that infusion of the NOS inhibitor l-NMMA produced a decrease of basal forearm blood flow and response to acetylcholine which was less in hypertensives than in normal subjects [14–16]. Apparently, these data suggest an abnormally low vascular production of NO in patients with hypertension. Is this phenomenon shared by the endothelium of other resistance vessels in hypertensive patients? In this issue of the journal, Jacobi and coworkers [17] attempt to answer this question. They show that systemic infusion of the NOS inhibitor l-NMMA resulted in a comparable reduction of renal plasma flow (RPF) in hypertensive and normotensive subjects. This led them to conclude that overall basal NO activity is not impaired in the renal circulation of patients with essential hypertension, which makes a difference compared to that which occurs in other peripheral vascular beds, such as the forearm vessels. Although these results appear convincing, they cannot be considered conclusive. Jacobi and coworkers only used a single dose of the NOS inhibitor l-NMMA to test the hypothesis of whether the NO-forming pathway was abnormal in patients with essential hypertension. At the dose employed, l-NMMA may have failed to completely inhibit intrarenal NO production, at least in the normotensive subjects, making difficult a formal comparison between renal haemodynamic changes induced by NOS inhibition in hypertensive patients and normotensive subjects. Previous studies in healthy men have shown that three increasing doses of the NOS inhibitor l-NAME induced a dose-dependent effect, both in magnitude and duration, on renal and systemic haemodynamics, as well as on diuresis and natriuresis [18]. A similar approach with the l-NMMA inhibitor would have provided a more reliable tool to demonstrate that the renal response to NOS inhibition was comparable in both hypertensive and normotensive subjects. Moreover, in the study by Jacobi et al., the mean arterial blood pressure was increased to a similar extent in hypertensive patients and normotensives, in parallel to a comparable reduction in RPF, after l-NMMA infusion. Indeed, a less marked increase in mean arterial blood pressure would be expected in patients with essential hypertension after NOS inhibition if, as suggested by Jacobi et al., basal NO production in the systemic vasculature of these patients were lower than normal. Difficulties in the interpretation of the results obtained by Jacobi et al. also derive from the finding that, despite comparable RPF decline, glomerular filtration rate (GFR) increased significantly over baseline in hypertensives but not normotensive subjects in response to l-NMMA. A relative preservation of GFR despite marked decrements in effective RPF after NO synthesis inhibition has been described in healthy subjects [18–20]. This is in agreement with experimental studies in rodents that also helped in understanding the mechanisms of GFR preservation during NO synthesis inhibition. Indeed, glomerular micropuncture studies in normal rats have shown a decrease in glomerular capillary ultrafiltration coefficient (Kf) in response to either local or systemic NO synthesis inhibition [21,22]. The decrease in Kf was mediated by a rise in mesangial cell tone, which would reduce the surface area available for filtration, ultimately lowering GFR [21]. Therefore, the observed relative preservation of GFR in these animals was caused by a rise in the hydrostatic pressure of the glomerular capillaries, indicating that NO synthesis inhibition elicited more vasoconstriction in the efferent than in the afferent arteriole of the glomerulus in normal conditions. Although it is difficult to demonstrate in humans, the difference in GFR changes after l-NMMA infusion observed by Jacobi et al. would imply a relatively different basal NO bioavailability at pre- and post-glomerular arterioles in hypertensive and normotensive subjects, eventually resulting in a higher increase in intraglomerular capillary pressure and, ultimately, a selective rise in GFR in the former patients after NO synthesis inhibition. The possible defect in glomerular NO bioavailability in patients with essential hypertension would question the conclusion of Jacobi et al. that basal NO synthesis of the renal vasculature is not impaired in hypertensive patients. Evidence for this may be obtained by measuring the urinary excretion of cGMP, for which synthesis is promoted by NO through the stimulation of soluble guanylate cyclase in the cytoplasm of the target cells [8]. However, this approach is not without limitations, given the fact that cGMP levels as a surrogate parameter of NO activity are neither specific nor particularly sensitive, since this secondary intracellular mediator may be generated after activation of other vasoactive substances, such as atrial natriuretic peptide, which also occur in hypertensive patients. Several animal studies have provided evidence of an increased impact of NO during sodium loading on systemic blood pressure and renal haemodynamics [23–25]. Moreover, in healthy subjects, a more pronounced relative increase in mean arterial blood pressure and relative decrease in RPF as a result of NO synthesis inhibition with l-NMMA was reported during high than low sodium intake [26]. Taken together, animal and human studies indicate that dietary sodium affects systemic and renal haemodynamic responses to NO inhibition. Because Jacobi et al. did not provide dietary guidelines regarding sodium intake to their patients, there is the possibility that differences in dietary sodium would have influenced arterial blood pressure and RPF changes occurring after l-NMMA infusion both in patients with essential hypertension and in normotensive subjects. However, daily urinary sodium excretion was similar in the two study groups, which would suggest a comparable sodium intake and also indicate that dietary sodium was not a major factor in determining the similarity of the renal haemodynamic response to l-NMMA infusion in hypertensive and normotensive subjects. Nevertheless, the reported direct association between individual sodium-sensitivity and the pressor response to l-NMMA [27] should be taken into account, which indicates the increased dependence of vascular tone on NO in subjects whose blood pressure is more sodium-sensitive. Thus, despite comparable sodium intake, sodium-sensitivity in individual hypertensive patients and normotensive subjects may be different, further complicating an interpretation of the study by Jacobi et al. In conclusion, this work provides new insights into the systemic and renal vascular reactivity involving the NO pathway in patients with essential hypertension. However, the available data cannot conclusively demonstrate a normal NO synthesis in the renal circulation, at variance with the forearm vascular beds, in hypertensive patients. Acknowledgements The authors are grateful to Dr Arrigo Schieppati for comments and criticisms.