Nitric oxide synthase gene knockout mice do not become hypertensive during pregnancy.

OBJECTIVE: The purpose of this study was to test whether omitting the vasodilator nitric oxide that is derived from any 1 of the 3 isoforms of nitric oxide synthase results in hypertension during pregnancy. STUDY DESIGN: We measured systolic blood pressure before, during, and after pregnancy using an automated tail cuff method in 3 mutant (gene knockout) mouse strains in which the gene for neuronal nitric oxide, inducible nitric oxide, or endothelial nitric oxide was disrupted by gene targeting. RESULTS: In neuronal nitric oxide gene knockout mice (n = 10), blood pressure was 100 +/- 3 mm Hg, not significantly different from 101 +/- 3 mm Hg in matched wild-type control mice (n = 10). Pregnancy did not change blood pressure or heart rate in either group. In inducible nitric oxide gene knockout mice (n = 9), blood pressure was 110 +/- 3 mm Hg, the same as in the wild-type control mice (110 +/- 2 mm Hg; n = 14). Blood pressure was unaffected by pregnancy in either group of mice. However, heart rate was significantly less in knockout mice (647 +/- 11 beats/min vs 666 +/- 9 beats/min; P <.005); this difference persisted through pregnancy. In endothelial nitric oxide gene knockout mice (n = 8), blood pressure was higher before pregnancy (114 +/- 4 mm Hg vs 103 +/- 4 mm Hg; P <.05) than in wild-type control mice (n = 9), but this difference disappeared during pregnancy, returning only after delivery. Heart rates were not different before pregnancy and were unaffected by pregnancy. CONCLUSION: There was no apparent increase in systolic blood pressure in any of the 3 nitric oxide synthase gene knockout strains during pregnancy compared to the wild-type control mice. This suggests that, at least in the mouse, genetic deficiency of any 1 isoform of nitric oxide synthase does not result in pregnancy-induced hypertension.

Nitric oxide synthase and cGMP-mediated stimulation of renin secretion.

The interaction between nitric oxide (NO) and renin is controversial. cAMP is a stimulating messenger for renin, which is degraded by phosphodiesterase (PDE)-3. PDE-3 is inhibited by cGMP, whereas PDE-5 degrades cGMP. We hypothesized that if endogenous cGMP was increased by inhibiting PDE-5, it could inhibit PDE-3, increasing endogenous cAMP, and thereby stimulate renin. We used the selective PDE-5 inhibitor zaprinast at 20 mg/kg body wt ip, which we determined would not change blood pressure (BP) or renal blood flow (RBF). In thiobutabarbital (Inactin)-anesthetized rats, renin secretion rate (RSR) was determined before and 75 min after administration of zaprinast or vehicle. Zaprinast increased cGMP excretion from 12.75 +/- 1.57 to 18.67 +/- 1.87 pmol/min (P < 0.003), whereas vehicle had no effect. Zaprinast increased RSR sixfold (from 2.95 +/- 1.74 to 17.62 +/- 5.46 ng ANG I. h(-1) x min(-1), P < 0.024), while vehicle had no effect (from 4.08 +/- 2.02 to 3.87 +/- 1.53 ng ANG I x h(-1) x min(-1)). There were no changes in BP or RBF. We then tested whether the increase in cGMP could be partially due to the activity of the neuronal isoform of NO synthase (nNOS). Pretreatment with the nNOS inhibitor 7-nitroindazole (7-NI; 50 mg/kg body wt) did not change BP or RBF but attenuated the renin-stimulating effect of zaprinast by 40% compared with vehicle. In 7-NI-treated animals, zaprinast-stimulated cGMP excretion was attenuated by 48%, from 9.17 +/- 1.85 to 13.60 +/- 2.15 pmol/min, compared with an increase from 10.94 +/- 1.90 to 26.38 +/- 3.61 pmol/min with zaprinast without 7-NI (P < 0.04). This suggests that changes in endogenous cGMP production at levels not associated with renal hemodynamic changes are involved in a renin-stimulatory pathway. One source of this cGMP may be nNOS generation of NO in the kidney.

Effects of aging and alterations in dietary sodium intake on total nitric oxide production.

Animal studies suggest that nitric oxide (NO) deficiency is linked to salt-sensitive hypertension and that NO activity decreases during normal aging. This study investigates the impact of increasing age and manipulations in dietary salt intake on biochemical indices of the NO system in healthy humans. We measured NO(2) + NO(3) (NO(X); stable oxidation products of NO) and cyclic guanosine monophosphate (cGMP; major second messenger) in plasma and urine of 30 healthy subjects aged 22 to 77 years. Subjects were maintained on controlled low NO(X) and low-, normal-, or high-salt diets for 3 days. Salt sensitivity of blood pressure was seen only in the oldest subjects. Plasma renin activity was suppressed by a high salt intake in all age groups, and baseline values declined with advancing age. Neither age nor salt intake correlated with indices of NO activity over the third 24-hour period of controlled salt intake. In a subgroup of subjects aged 33 +/- 4 years challenged with ultrahigh sodium intake (400 mEq/24 h), again there was no increase in NO(2) + NO(3) or cGMP measures. In contrast to animal studies, there is no correlation in humans between either salt intake or age and total NO production and activity, indicated by NO(2) + NO(3) and cGMP measures. This does not preclude undetected alterations occurring in NO production and/or activity in strategic locations in the kidney and cardiovascular system. Limitations of blood and urine measurements of NO(2) + NO(3) and cGMP as indices of NO activity are discussed.

Role of neuropeptide Y in the development of two-kidney, one-clip renovascular hypertension in the rat.

OBJECTIVE: Along with the renin-angiotensin system, sympathetic stimulation may contribute to renovascular hypertension. The vasoactive peptide neuropeptide Y (NPY) is co-released with and potentiates the pressor effects of norepinephrine through the Y-1 receptor. NPY, by exaggerating sympathetic activity, may contribute to renovascular hypertension, possibly by augmenting adrenergic-mediated renin release. This was studied by determining the effect of continuous Y-1 blockade on the development of two-kidney, one-clip renovascular hypertension and the effect of NPY on in vitro renin release. METHODS: Mean arterial pressure and renal blood flow responses to NPY (10 microg/kg, administered intravenously) were measured in five anesthetized Sprague-Dawley rats before and after BIBO3304TF administration to test the Y-1 antagonist BIBO3304TF. In hypertension studies, 28 rats underwent left renal artery clipping. Of these, 13 were implanted with a mini-osmotic pump for continuous BIBO3304TF infusion (0.3 microg/h, administered intravenously); the other 15 underwent sham implantation. Systolic blood pressure was then monitored for 4 weeks. Finally, in vitro renin release was measured from renal cortical slices (n = 6-12) incubated with NPY (10(-8) to 10(-6) mol/L) or NPY plus the adrenergic agonist isoproterenol (10(-4) mol/L). RESULTS: BIBO3304TF attenuated the NPY-induced increase in mean arterial pressure by 54% (P <.02) and the NPY-induced decrease in renal blood flow by 38% (P <.05). In 4-week hypertension studies, systolic blood pressure in clipped controls increased from 130 +/- 3 mm Hg to 167 +/- 6 mm Hg (P <.01), whereas BIBO3304TF-treated rats had no significant increase (125 +/- 3 mm Hg to 141 +/- 8 mm Hg). Final systolic blood pressure was 26 mm Hg lower in BIBO3304TF-treated rats than in controls (P <.01). In renal cortical slices, no NPY effect was observed in basal or isoproterenol-stimulated renin release. CONCLUSIONS: The Y-1 receptor antagonist BIBO3304TF attenuated acute pressor responses to NPY and blunted the development of two-kidney, one-clip renovascular hypertension in rats. NPY may contribute to the hypertensive response in this renovascular hypertension model. Our in vitro data do not suggest that this is due to NPY enhancement of renin release.

Influence of nitric oxide derived from neuronal nitric oxide synthase on glomerular filtration.

The neuronal isoform of nitric oxide synthase (nNOS) has been localized to specific regions of the kidney, including the thick ascending limb of the loop of Henle and the macula densa. Because of this discrete localization in the renal cortex, nitric oxide (NO) produced by nNOS has been suggested to play an important role in the regulation of macula densa-mediated arteriole tone and therefore could play an important role in the regulation of whole-kidney glomerular filtration rate (GFR). We hypothesized that selective blockade of nNOS would decrease GFR. Renal hemodynamics were measured before and after acute selective blockade of nNOS by 50 mg/kg 7-nitroindazole (7-NI) in anesthetized rats. Administration of 7-NI had no significant effect on basal blood pressure (from 105 +/- 3 to 101 +/- 2 mm Hg), renal blood flow [from 6.08 +/- 0.39 to 6.31 +/- 0.33 ml/min/gram of kidney weight (gkw)], or total renal vascular resistance (from 18.1 +/- 1.6 to 16.4 +/- 1.0 mm Hg/ml/min/gkw) but decreased GFR by 26% (from 1.36 +/- 0.15 to 1.00 +/- 0.13 ml/min/gkw; p < 0.02), urinary flow rate by 28% (from 24.7 +/- 1.8 to 17.8 +/- 2.2 microl/min; p < 0.05), and sodium excretion by 22% (from 5.55 +/- 0.53 to 4.30 +/- 0.52 microEq/min; p < 0.05). However, fractional sodium excretion was not changed by nNOS inhibition. There were no such changes in vehicle-treated time controls. We conclude that, in the renal cortex, NO produced by nNOS plays an important role in the regulation of whole-kidney GFR and excretion in normal, sodium-replete rats.

Chronic cyclooxygenase-2 inhibition blunts low sodium-stimulated renin without changing renal haemodynamics.

BACKGROUND: Cyclooxygenase-2 (COX-2), the inducible isoform of cyclooxygenase, is found in the macula densa of the renal cortex and is upregulated by dietary sodium restriction. Because of this discrete cortical localization, we hypothesized that COX-2 plays a role in the chronic stimulation of renin via the macula densa pathway. METHODS: We examined the effect of the selective COX-2 inhibitor NS 398 in male Sprague-Dawley rats. RESULTS: A low sodium diet (0.02% NaCl) for 14 days elevated plasma-renin activity (PRA) nine-fold, from 6.1 +/- 2.0 to 54.9 +/- 6.5 ng angiotensin I (Ang I)/ml per h (P < 0.0001). Selective COX-2 inhibition with NS 398 had no effect on PRA in animals on normal sodium (5.1 +/- 1.3 ng Ang I/ml per h), but decreased PRA by 41% in sodium-restricted rats, to 33.3 +/- 3.6 ng Ang I/ml per h (P < 0.05). Chronic treatment with NS 398 did not decrease renal renin content (31.8 +/- 1.8 versus 33.5 +/- 2.6 ng Ang I/ mg per h, with NS 398 versus controls), nor did it influence systemic blood pressure or renal haemodynamics. Neither urinary sodium excretion nor prostaglandin (PG)E2 excretion was altered in rats given NS 398. Chronic treatment with the non-selective COX inhibitor indomethacin during sodium restriction over 5 days reduced PRA by 35%, from 29.36 +/- 4.81, to 19.13 +/- 2.88 ng Ang I/ml per h (P < 0.05). Indomethacin had no effect on blood pressure or renal blood flow but reduced urinary PGE2 excretion by 70%. CONCLUSIONS: One component of the chronic stimulation of PRA by dietary sodium restriction via the macula densa pathway appears to involve the induction of COX-2.

Response to angiotensin inhibition in rats with sustained renovascular hypertension correlates with response to removing renal artery stenosis.

PURPOSE: Sustained (late-phase) renovascular hypertension is associated with lower plasma renin activity than is the early phase. It is not clear to what extent this reduced plasma renin activity reflects diminished influence of the renin-angiotensin system. It also is not clear whether this change in the character of the disease influences the effectiveness of surgical removal of the renal artery stenosis in reversing hypertension. Using an animal model of sustained (> or =10 weeks after renal artery clipping) two-kidney, one-clip renovascular hypertension, we hypothesized that the magnitude of the depressor response to selective angiotensin II receptor blockade with losartan would reflect the influence of the renin-angiotensin system on hypertension and enable us to predict the depressor response to subsequent surgical removal of the clip. METHODS: The left renal arteries of 20 male Sprague-Dawley rats weighing 150 to 200 gm were fitted with a silver clip (0.23 mm internal diameter). Systolic blood pressure was measured by means of tail-cuff plethysmography for 10 weeks. Rats were then given losartan orally (30 mg/kg a day) for 1 week while blood pressure was monitored. After an additional week to allow recovery, 13 rats underwent surgical unclipping, and seven underwent sham repair. Blood pressure again was monitored over the final week. RESULTS: All two-kidney one-clip rats had hypertension 10 weeks after clipping (mean systolic blood pressure 206 +/- 10 mm Hg). Losartan decreased systolic blood pressure by 36 +/- 6 mm Hg. The response was variable, ranging from 3 to 66 mm Hg, and overall blood pressure did not normalize (170 +/- 8 mm Hg). Subsequent surgical unclipping decreased systolic blood pressure by 46 +/- 9 mm Hg. Again the response was variable, ranging from 10 to 99 mm Hg, although overall blood pressure did not normalize (164 +/- 7 mm Hg). The decrease in blood pressure after unclipping showed a high correlation with the blood pressure decrease after losartan administration (r = 0.861, p < 0.001). Resting plasma renin activity (before intervention) was 16 +/- 4 ng angiotensin I per milliliter per hour and was not predictive of the response to either losartan or surgical unclipping. The rats subjected to sham operations had no statistically significant changes in blood pressure. Histologic evaluation showed patent renal arteries without appreciable stenosis or intimal hyperplasia after removal of the clips. CONCLUSIONS: In sustained two-kidney, one-clip renovascular hypertension, the depressor response to angiotensin II receptor blockade is attenuated, suggesting that late-phase hypertension becomes increasingly angiotensin II-independent. In our model, the extent to which sustained renovascular hypertension becomes refractory to 7 days of angiotensin II blockade is highly predictive of the ultimate outcome of surgical repair of renal artery stenosis.

Beta-adrenoceptor-stimulated renin release is blunted in old rats.

Plasma renin activity (PRA) was similar in young versus old male Sprague Dawley rats under unstressed conditions (1.3 +/- 0.2 versus 1.8 +/- 0.3 ng angiotensin I/ml per min). Airjet stress increases PRA in young but not old rats (13.9 +/- 3.8 versus 2.9 +/- 0.8 ng angiotensin I/ml per min), respectively. This response is ablated in young rats by beta-adrenoceptor blockade, suggesting that the increased PRA is mediated by beta-adrenoceptors, and this response was blunted in old rats.

Influence of nitric oxide in the chronic phase of two-kidney, one clip renovascular hypertension.

Chronic two-kidney, one clip (2K1C) renovascular hypertension is characterized by a largely angiotensin-independent elevated blood pressure (BP). We hypothesized that the long-term effect of hypertension would compromise endothelium-derived nitric oxide (NO) and diminish its influence in controlling renal perfusion. We determined the influence of endothelium-derived NO on renal hemodynamics and the angiotensin-NO interaction regulation of renal perfusion in rats with chronic 2K1C hypertension. Renal blood flow (RBF) was measured by radioactive microspheres in rats with either early-phase (4 weeks after clipping, n=7) or chronic-phase (13 to 16 weeks after clipping, n=7) 2K1C hypertension. The systemic and renal response to NO synthesis inhibition was determined with 10 mg/kg body wt N omega-nitro-L-arginine methyl ester (L-NAME). In rats with early-phase 2K1C hypertension, BP was 149+/-3 mm Hg, which increased by 42+/-3 mm Hg with L-NAME (P<.001). L-NAME decreased RBF by 20% (P<.02) and 17% (P<.005) and increased renal vascular resistance (RVR) by 58% (P<.005) and 62% (P<.02) in the nonclipped and clipped kidneys, respectively. In rats with chronic 2K1C hypertension, BP was 166+/-3 mm Hg, and L-NAME increased this by 35+/-6 mm Hg (P<.001). In the nonclipped and clipped kidneys of chronic 2K1C hypertensive rats, L-NAME decreased RBF by 20% (P<.01) and 17% (P<.01) and increased RVR by 51% (P<.005) and 60% (P<.02), respectively. There were no differences in L-NAME-induced changes between early- and chronic-phase 2K1C hypertensive rats. Next, we treated seven chronic-phase 2K1C hypertensive rats with 10 mg/kg body wt losartan, which reduced BP by only 7.7% (P<.005). After losartan, L-NAME increased BP by 41+/-3 mm Hg (P<.001), decreased RBF to the nonclipped kidney by 44% (P<.05), and increased RVR by 110% (P<.005); the decrease in RBF was significantly greater compared with untreated chronic-phase controls (P<.05). In the clipped kidney, L-NAME decreased RBF by 26% (P<.05) and increased RVR by 76% (P <.05). Thus, angiotensin blockade did not attenuate the systemic or renal vasoconstriction to L-NAME. Our results suggest that in both early and chronic phases of 2K1C hypertension, NO contributes significant dilator tone to buffer the hypertension and maintains perfusion of both kidneys by counterbalancing angiotensin-independent vasoconstriction.

Evolution of chronic nitric oxide inhibition hypertension: relationship to renal function.

We conducted longitudinal measurements of blood pressure and renal function in the conscious, chronically catheterized rat before and during acute nitric oxide synthase inhibition (N-nitro-L-arginine methylester [L-NAME], 37 micromol/kg IV) and then chronic administration of oral L-NAME (approximately 37 micromol/kg per 24 hours). These studies specifically investigate the impact on plasma and renal renin as well as volume status during the evolution of this hypertension in rats not subjected to acute experimental stress. Blood pressure progressively increased with chronic administration of L-NAME and reached values greatly above those seen with acute administration of L-NAME. There were parallel increases in renal vascular resistance and development of proteinuria, and glomerular filtration rate began to decline at day 21, coincident with the appearance of renal damage. Twenty-four-hour urinary nitrite and nitrate excretion remained depressed, reflecting reduced nitric oxide synthesis. The plasma renin activity was variable and only increased transiently at 21 days, thus the angiotensin II dependence of this hypertension is not caused by stimulated plasma renin activity. Despite severe hypertension, sodium intake and excretion were unchanged over the 21 days of L-NAME administration. Plasma volume was significantly reduced at days 2 and 12 of L-NAME administration; thus the prolonged plasma volume contraction must result from the acute natriuretic response to the initial acute L-NAME administration.

Response of proximal tubules to angiotensin II changes during maturation.

The renin-angiotensin system changes with age, but it is unclear how renal responses to angiotensin II (Ang II) evolve as an animal matures. We hypothesized that Ang II exerts a greater effect on proximal nephron volume absorption (Jv), blood pressure (BP), renal blood flow (RBF), and renal vascular resistance (RVR) in young compared with adult rats. To test this hypothesis, we investigated the effects of Ang II on proximal nephron fluid absorption in response to 10(-10) mol/L Ang II in rats from three age groups: young (4 to 5 weeks old), intermediate (6 weeks old), and adult (7 weeks old). In proximal straight tubules from 7 young rats, Jv was 0.64+/-0.05 nL/mm per minute. Ang II in the bath increased Jv by 69+/-18% to 1.05+/-0.07 nL/mm per minute (P<.005). In tubules from five intermediate-aged rats, Jv was 0.60+/-0.10 nL/mm per minute and increased by 34+/-5% to 0.83+/-0.16 nL/mm per minute after Ang II (P<.02). In five adult rats, Jv was 0.69+/-0.06 nL/mm per minute and increased 20+/-6% to 0.85+/-0.13 nL/mm per minute after Ang II (P<.05). Next we tested whether the exaggerated effect of Ang II on proximal tubular Jv in young rats was due to Ang II-induced changes in cAMP. cAMP content of proximal tubules from eight young rats was 24.8+/-7.6 fmol/mm and fell by 29.7+/-9.8% (P<.025) after treatment with Ang II. In contrast, cAMP content of proximal tubules from nine adults was only 9.8+/-4.5 fmol/mm, 40% of baseline in young rats, and was unchanged by Ang II (9.2+/-4.5 fmol/mm). We finally determined whether the increased sensitivity to Ang II in tubules of young rats is mimicked by renal hemodynamics. Eleven adult rats had BP of 115+/-5 mm Hg, RBF of 6.99+/-0.42 mL/min per g kidney weight (kw), RVR of 16.82+/-0.95 mm Hg/mL per minute per g kw (resistance units), and plasma renin activity (PRA) of 11.2+/-2.3 ng Ang I/mL per hour. Seven young rats had BP of 98+/-7 mm Hg, 17 mm Hg lower than adults (P<.025). RBF was 4.94+/-0.23 mL/min per g kw, and RVR was 20.30+/-1.19 RU, 20% greater than in adults (P<.025). PRA was 9.2+/-2.2 ng Ang I/mL per hour. There were no differences between groups with regard to increased BP, decreased RBF, or increased RVR with graded doses of 8, 40, and 200 fmol Ang II/g body weight. Thus, Ang II increased Jv more in young rats but had a lesser effect in adults. This was coupled with a greater effect of Ang II on tubular cAMP in young rats, but no differences in systemic or renal hemodynamic responses to Ang II between adults and young. We conclude that during adolescent development, Ang II may be an important factor in the regulation of salt and water metabolism, but not renal hemodynamics.

Evaluation of the renin-angiotensin system in a congenic renin Dahl salt-sensitive rat.

When an approximately 30 centiMorgan (cM) region of chromosome 13 containing the renin gene from the Dahl salt-resistant rat (R) was introgressed into the Dahl salt-sensitive rat (S), the resulting congenic rat (designated S.R-Ren) had a systolic blood pressure on a 2% (w/w) salt diet that was 24 mmHg lower than that of its S counterpart. Due to the large size of the transferred segment (over 30 million bp), the question remained as to whether or not the renin gene was the cause of the blood-pressure difference between the strains. We evaluated the role of the renin-angiotensin system in S.R-Ren and S rats fed a 0.05% salt diet by examining differences between strains in (1) expression of renin in three tissue types, (2) the blood-pressure response to blockade of both angiotensin-converting enzyme and angiotensin II receptors, and (3) pressure natriuresis. No differences were found in renin levels in plasma, kidney or adrenal gland between strains. The blood-pressure responses to the angiotensin-converting-enzyme inhibitor captopril and to the angiotensin II-receptor blocker saralasin in conscious S and S.R-Ren rats were similar. Furthermore, renal function, evaluated by a pressure-natriuresis index that took into account both the time and the arterial pressure needed to excrete an acute salt load, did not differ between strains. Our findings therefore fail to demonstrate a role for the renin gene in conferring lower blood pressure in the congenic rat and suggest that there is an unknown arterial-pressure-regulating locus in this 30 cM region of chromosome 13.

Macula densa stimulation of renin is reversed by selective inhibition of neuronal nitric oxide synthase.

The neuronal isoform of nitric oxide synthase (nNOS) exists in the renal cortex predominantly in the macula densa, suggesting that nitric oxide (NO) derived from the macula densa plays a role in feedback regulation of renin in response to altered sodium metabolism. To determine if nNOS is a critical component in renin stimulation induced by dietary sodium restriction, rats received either normal sodium or a sodium-restricted diet (0.03%) for 7 days and subsequently were or were not treated with the selective inhibitor of nNOS 7-nitroindazole (7-NI) either acutely (50 mg/kg body wt ip) on the final day or chronically (20 mg/kg body wt ip 2 x/day) over the final 5 days. On the last day, rats were anesthetized with Inactin and fitted with arterial and renal venous catheters to collect blood and monitor blood pressure (BP) and a flow probe to measure renal blood flow (RBF). BP (105 vs. 108 mmHg) was similar in normal and low-sodium dietary groups, respectively, whereas RBF tended to be higher in the sodium-restricted group (6.5 +/- 0.3 vs. 7.6 +/- 0.4 ml.min-1.g kidney wt-1). Both renal venous renin (RR) and renin secretion rate (RSR) were elevated approximately fourfold by sodium restriction [RR = 5.8 +/- 0.8 vs. 20.5 +/- 2.7 ng angiotensin (ANG) I.ml-1.h-1; P < 0.001; RSR = 3.0 +/- 0.9 vs. 13.1 +/- 4.1 ng ANG I.h-1.min-1; P < 0.025]. Acute 7-NI did not change BP, RR, or RSR, but reduced RBF in sodium-restricted rats by 8% (P < 0.05). Chronic 7-NI had no effect on renin in rats on a normal diet, but reduced RR by one-half in the sodium-restricted group (to 9.9 +/- 1.6 ng ANG I-ml-1.h-1; P < 0.001) and reduced RSR to normal (diet) levels (to 3.9 +/- 1.4 ng ANG I.h-1.min-1; P < 0.05). Although selective NOS inhibition by 7-NI did not affect BP, RBF, or renin in control rats on a normal diet, chronic 7-NI reversed the stimulation of renin induced by dietary sodium restriction. These data suggest that nNOS-derived NO plays an important role in the macula densa during feedback stimulation of renin induced by dietary sodium restriction.

Cyclooxygenase-2 mediates increased renal renin content induced by low-sodium diet.

We hypothesized that neuronal nitric oxide synthase and cyclooxygenase-2, which both exist in the renal cortex, predominantly in the macula densa, play a role in the control of renal renin tissue content. We studied the possible role of neuronal nitric oxide synthase in regulating renal renin content by using mice in which the neuronal nitric oxide synthase gene has been disrupted (nNOS-/-) compared with its two progenitor strains, the 129/SvEv and the C57BL/6, to determine if the absence of neuronal nitric oxide synthase would result in decreased renal renin content or blunt the increase observed during low sodium intake. Renal renin content from cortical slices was determined in adult mice from all three strains maintained on a normal sodium diet. Renal renin content was significantly reduced in the nNOS-/- mice compared with the 129/SvEv and the C57BL/6 mice (3.11 +/- 0.23 versus 5.66 +/- 0.50 and 7.55 +/- 1.17 micrograms angiotensin l/mg dry weight, respectively; P < .005), suggesting that neuronal nitric oxide synthase may stimulate renal renin content under basal conditions. Neither selective pharmacological inhibition of neuronal nitric oxide synthase using 7-nitroindazole or disruption of the neuronal nitric oxide synthase gene affected the increase in renal content observed during dietary sodium restriction. The influence of cyclooxygenase-2 on renal renin content through a macula densa-mediated pathway was studied using a selective cyclooxygenase-2 inhibitor, NS398, in 129/SvEv mice. A low-sodium diet increased renal renin content from 6.97 +/- 0.52 to 11.59 +/- 0.79 micrograms angiotensin l/mg dry weight (P < .005); but this increase was blocked by NS398. In addition, treatment with NS398 reduced renin mRNA in response to a low-sodium diet. Thus, increased renal renin content in response to dietary sodium restriction appears to require the induction of cyclooxygenase-2, while neuronal nitric oxide synthase appears to affect basal but not stimulated renal renin content.

Selective neuronal nitric oxide synthase inhibition blocks furosemide-stimulated renin secretion in vivo.

The macula densa is a regulatory site for renin. It contains exclusively the neuronal isoform of nitric oxide synthase (NOS), suggesting NO could stimulate renin secretion through the macula densa pathway. To test whether neuronal NOS mediates renin secretion, renin was stimulated by either the renal baroreceptor or the diuretic furosemide (acting through the macula densa pathway). Renin secretion rate (RSR) was measured in 12 Inactin-anesthetized rats at normal (104 +/- 3 mmHg) and reduced renal perfusion pressure (65 +/- 1 mmHg), before and after selective blockade of the neuronal NOS with 7-nitroindazole (7-NI, 50 mg/kg ip). 7-NI had no effect on basal blood pressure (102 +/- 2 mmHg) or renal blood flow (RBF). Decreasing renal perfusion pressure doubled RSR from 11.8 +/- 3.3 to 22.9 +/- 5.7 ng ANG I.h-1.min-1 (P < 0.01) (ANG I is angiotensin I). Similarly, in 7-NI-treated rats, reduced perfusion doubled RSR from 8.5 +/- 1.8 to 20.5 +/- 6.2 ng ANG I.h-1.min-1 (P < 0.01). Renal hemodynamics and RSR were measured in response to 5 mg/kg iv furosemide in 12 control rats and 11 rats treated with 7-NI. Blocking neuronal NOS did not alter blood pressure (102 +/- 2 mmHg), RBF (5.8 +/- 0.4 ml.min-1.g kidney wt-1), or renal vascular resistance (18.7 +/- 1.4 mmHg.ml-1.min.g kidney wt).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurons in the dorsal motor nucleus of the vagus may integrate vagal and spinal information from the GI tract.

Previous investigations have provided evidence that the activity of parasympathetic efferent neurons in the dorsal motor nucleus of the vagus (DMNV) may be influenced by either vagal afferent or spinal input from the gastrointestinal (GI) tract. Many questions remain, however, regarding the nature of this input and its integration by the brain stem. The present study was designed to examine one important aspect of this issue: the potential contribution of the spinal input to the brain stem in the generation of the response properties of intestine-sensitive neurons in the DMNV. Using intracellular recording and labeling techniques in adult rats, we found that ascending spinal pathways were capable of conveying both low- and high-threshold visceral information to the DMNV. We also determined that the neurons in the nucleus of the solitary tract failed to respond to intestinal distention when the vagal afferents to the brain stem had been severed, suggesting that the spinal projections terminate directly on the DMNV neurons. These data lend support to the emerging hypothesis that the spinal afferents that accompany the abdominal splanchnics are capable of responding to both innocuous and noxious stimuli. The results also suggest that the neurons in the DMNV play a larger role in the integration of visceral sensory information than was previously realized.

Endothelium-derived constricting factor in renovascular hypertension.

We have reported that in two-kidney, one clip hypertensive rats, renal perfusion is maintained by a balance between the vasodilator endothelium-derived nitric oxide and the vasoconstrictor angiotensin II. Others have suggested that endothelium-derived constricting factor, reported to be thromboxane A2 and/or endoperoxide, contributes to increased blood pressure in angiotensin II-dependent hypertension. We hypothesized that in angiotensin II-dependent two-kidney, one clip hypertension, endothelium-derived constricting factor contributes to vasoconstriction of the clipped kidney following nitric oxide synthesis inhibition. Using radioactive microspheres, we studied renal blood flow to the stenotic kidney of two-kidney, one clip hypertensive rats 4 weeks after clipping. The influence of nitric oxide on systemic and renal hemodynamics was evaluated by determining the response to nitric oxide synthesis inhibition using 10 mg/kg N omega-nitro-L-arginine methyl ester in these rats, which were either not treated (n = 8) or treated (n = 8) with 4 mg/kg of the constricting factor receptor antagonist BMS 180,291. Mean basal blood pressure in rats was 167 +/- 9 mm Hg (mean +/- SEM). N omega-Nitro-L-arginine methyl ester increased blood pressure by 35 +/- 7 mm Hg (P < .001). In the clipped kidney, N omega-nitro-L-arginine methyl ester decreased renal blood flow by 40% (from 4.5 +/- 0.9 to 2.7 +/- 0.6 mL.min-1.g kidney-1; P < .01) and increased renal vascular resistance by 100% (from 51.9 +/- 9.6 to 105.0 +/- 19.2 mm Hg.mL-1.min-1.g kidney-1; P < .005).(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine vasopressin-induced renal vasodilation mediated by nitric oxide.

The vasoconstrictor vasopressin has been reported to induce paradoxical local vasodilation in the basilar vasculature through stimulation of the endothelium-derived relaxing factor nitric oxide (NO). We investigated the possibility that at subpressor doses, exogenous arginine vasopressin (AVP) might have a similar effect in the kidney. Ten Inactin-anesthetized rats were infused with sequential doses of AVP from 25 to 6,400 microU/min in 30-min increments. Subpressor infusion resulted in progressive renal vasodilation; renal blood flow (RBF) increased significantly going from 14 +/- 6% above basal at 200 microU/min (p < 0.02) to 27 +/- 5% (p < 0.01) at 1,600 microU/min accompanied by a 24 +/- 5% decrease in renal vascular resistance (RVR). At 6,400 microU/min, blood pressure (BP) increased 29 +/- 6 mm Hg and RVR increased. A second group of 8 rats were first given 10 mg/kg b.w. of the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME) before infusion of AVP. L-NAME increased BP 22 +/- 3 mm Hg (p < 0.001), and decreased RBF 16 +/- 3% (p < 0.005). After L-NAME, no dose of AVP had any further effect on either BP, RBF, or RVR. Continuous infusion of a single subpressor dose of 100 microU AVP resulted in a 26% increase in RBF (from 7.52 +/- 0.68 to 9.49 +/- 0.54 ml/min/g kidney weight, p < 0.001). AVP doubled urinary cyclic guanosine monophosphate excretion, a marker for renal NO synthesis, from 8.51 +/- 1.01 to 17.48 +/- 4.26 pM/min (p < 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthesis inhibition blocks reversal of two-kidney, one clip renovascular hypertension after unclipping.

It is well established that two-kidney, one clip renovascular hypertension can be rapidly reversed by unclipping. We hypothesized that rapid renal reperfusion and the subsequent fall in blood pressure are mediated in part by nitric oxide, the endothelium-derived relaxing factor. We tested whether the hypotensive response to unclipping could be blocked by nitric oxide synthesis inhibition using a bolus of 10 mg/kg body wt N omega-nitro-L-arginine methyl ester. Rats were made hypertensive by placing a silver clip on the left renal artery. After 4 weeks, they were anesthetized and either not treated (controls) or had nitric oxide synthesis blockade. After 10 minutes, the clip was removed and blood pressure monitored over 60 minutes. Initial pressure in controls was 157 +/- 8 mm Hg, and heart rate was 310 +/- 21 beats per minute. Unclipping resulted in pressure falling to 125 +/- 6 mm Hg within 45 minutes (P < .005). Heart rate was unchanged (312 +/- 9 beats per minute). In contrast, nitric oxide synthesis inhibition increased blood pressure from 149 +/- 6 to 174 +/- 9 mm Hg (P < .001). Unclipping did not change blood pressure, which was 167 +/- 8 mm Hg after 60 minutes (P < .005 versus controls), and heart rate remained unchanged (282 +/- 13 versus 276 +/- 16 beats per minute). We determined the blood flow to the clipped kidneys using radioactive microspheres. Unclipping untreated hypertensive rats resulted in a 10-fold increase in renal blood flow (P < .001), concomitant with a decrease in blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)