Nitric oxide (NO) plays an important role in various physiological processes in the kidney. In vivo experiments first suggested that the natriuretic and diuretic effects caused by NO may be due to decreased NaCl and fluid absorption by the nephron. In the last 10 years, several reports have directly demonstrated a role for NO in modulating transport in different tubule segments. The effects of NO on proximal tubule transport are still controversial. Both stimulation and inhibition of net fluid and bicarbonate have been reported in this segment, whereas only inhibitory effects of NO have been found in Na/H exchanger and Na/K-ATPase activity. The effects of NO in the thick ascending limb are more homogeneous than in the proximal tubule. In this segment, NO decreases net Cl and bicarbonate absorption. A direct inhibitory effect of NO on the Na-K-2Cl cotransporter and the Na/H exchanger has been reported, while NO was found to stimulate apical K channels in this segment. In the collecting duct, NO inhibits Na absorption and vasopressin-stimulated osmotic water permeability. An inhibitory effect of NO on H-ATPase has also been reported in intercalated cells of the collecting duct. Overall, the reported effects of NO in the different nephron segments mostly agree with the natriuretic and diuretic effects observed in vivo. However, the net effect of NO on transport is still controversial in some segments, and in cases like the distal tubule, it has not been studied.

Abstract-Recent studies have shown that angiotensin-(1-7) (Ang- [1][2][3][4][5][6][7]), which is generated endogenously from both Ang I and II, is a bioactive component of the renin-angiotensin system and may play an important role in the regulation of blood pressure. However, little is known about its role in regulating the reactivity of the afferent arteriole or the mechanism(s) involved. We hypothesized that Ang-(1-7), acting on specific receptors, participates in the control of afferent arteriole tone. We first examined the direct effect of Ang-(1-7) on rabbit afferent arterioles microperfused in vitro, and we tested whether endothelium-derived relaxing factor/NO and cyclooxygenase products are involved in its actions. To assess the vasodilator effect of Ang-(1-7), afferent arterioles were preconstricted with norepinephrine, and increasing concentrations of Ang-(1-7) were added to the lumen. We found that 10 Ϫ10 to 10 Ϫ6 mol/L Ang-(1-7) produced dose-dependent vasodilatation, increasing luminal diameter from 8.9Ϯ1.0 to 16.3Ϯ1.1 m (PϽ0.006). Indomethacin had no effect on Ang-(1-7)-induced dilatation. N G -nitro-L-arginine methyl ester, a NO synthesis inhibitor, abolished the dilatation induced by Ang-(1-7). We attempted to determine which angiotensin receptor subtype is involved in this process. We found that 10, a potent and selective Ang-(1-7) antagonist, abolished the dilatation induced by Ang-(1-7). An angiotensin II type 1 receptor antagonist (L158809) and an angiotensin II type 2 receptor antagonist (PD 123319) at 10 Ϫ6 mol/L had no effect on Ang-(1-7)-induced dilatation. Our results show that Ang-(1-7) causes afferent arteriole dilatation. This effect may be due to production of NO, but not the action of cyclooxygenase products. Ang-(1-7) has a receptor-mediated vasodilator effect on the rabbit afferent arteriole. This effect may be mediated by Ang-(1-7) receptors, because angiotensin type 1 and type 2 receptor antagonists could not block Ang-(1-7)-induced dilatation. Thus, our data suggest that Ang-(1-7)opposes the action of Ang II and plays an important role in the regulation of renal hemodynamics. Key Words: arterioles Ⅲ angiotensin Ⅲ nitric oxide Ⅲ prostaglandins Ⅲ receptors, angiotensin A ngiotensin II (Ang II) is believed to be the principal bioactive end product of both the circulating system and tissue renin-angiotensin system (RAS). Recent reports have suggested that important central and peripheral actions of the RAS may be conveyed by shorter sequences of Ang peptides, including Ang III, Ang-(3-8), and Ang-(1-7). 1,2 Among the putative RAS mediators, the heptapeptide Ang-(1-7) is particularly interesting because of its physiological actions. It has been shown to activate several subtypes of angiotensin receptors in neural, endothelial, and vascular smooth muscle cell preparations and to exert biological actions in the brain and peripheral vessels that are both complementary to and distinct from those of Ang II. 3 A major target organ of the RAS is the kidney, where Ang II plays a pivotal rol...

Abstract-NO plays a role in the regulation of blood pressure through its effects on renal, cardiovascular, and central nervous system function. It is generally thought to freely diffuse through cell membranes without need for a specific transporter. The water channel aquaporin-1 transports low molecular weight gases in addition to water and is expressed in cells that produce or are the targets of NO. Consequently, we tested the hypothesis that aquaporin-1 transports NO.In cells expressing aquaporin-1, NO permeability correlated with water permeability. NO transport was reduced by 71% by HgCl 2 , an inhibitor of aquaporin-1. Transport of NO by aquaporin-1 saturated at 3 mol/L NO and displayed a K 1/2 (the concentration of NO that produces half of the maximum transport rate) of 0.54 mol/L. Reconstitution of purified aquaporin-1 into lipid vesicles increased NO influx by 316%. In endothelial cells, lowering aquaporin-1 expression with a small interfering RNA (siRNA) blunted aquaporin-1 expression by 54% and NO release by 44%. We conclude that NO transport by aquaporin-1 may allow cells to control intracellular NO levels and effects. NO transport by aquaporin-1 may play a role in central nervous system, vascular and renal function, and consequently blood pressure. Disruption of NO transport by aquaporin-1 offers an alternate cause for diseases currently explained by inadequate NO bioavailability.

The effect of endothelin-1 (ET-1) on the proximal tubule remains unclear. This may be due to a biphasic effect on transport in this segment. We hypothesized that ET-1 has a biphasic effect on fluid absorption (Jv) in the proximal straight tubule and that its inhibitory effect is superimposed on its stimulatory effect. ET-1 (10-13 M) stimulated Jv from 0.68±0.07 to 1.11±0.20 nl/mm/min, a 60% increase (P < 0.04). 10-12 and 1010 M ET-1 had no significant effect. 10-' M ET-1 reduced Jv from 0.81±0.19 to 0.44±0.15 nl/mm/min (P < 0.009).Staurosporine (STP, 10-s M) prevented both 10-9 and 10-13 M ET-1 from altering Jv significantly indicating that protein kinase C (PKC) is involved. Indomethacin (10 -M) blocked the inhibition produced by 10-9 M ET-1. ETI (10-' M), a lipoxygenase inhibitor, also blocked ET-1 inhibition of Jv. Interestingly ET-1 (10-9 M) stimulated Jv in the presence of both indomethacin and ETI. When 10-9 M ET-1 was added in the presence of 10-' M quinacrine, a phospholipase (PL) inhibitor, Jv also increased from 1.02±0.20 to 1.23±0.22 nl/mm/ min (P < 0.03). STP blocked this increase. We conclude that (a) 10-13 M ET-1 stimulates fluid absorption by activating PKC; (b) 10-9 M ET-1 decreases Jv by PKC-, PL-, cyclooxygenase-, and lipoxygenase-dependent mechanisms; and (c) the inhibitory effect of ET-1 on Jv is superimposed on the stimulatory effect. (J. Clin. Invest. 1994. 93:2572-2577