The nitric oxide pathway is amplified in venular vs arteriolar cultured rat mesenteric endothelial cells.

To determine if there are differences in nitric oxide activity between pre- and postcapillary microvessels, we studied cultured rat mesenteric arteriolar and venular endothelial cells (RMAEC, RMVEC). We measured expression of endothelial nitric oxide synthase (eNOS), the activity of eNOS, and L-arginine transport in live RMAEC and RMVEC and the L-arginine content of RMAEC and RMVEC lysates. The abundance of eNOS was significantly greater in RMVEC vs RMAEC; this was also true for freshly harvested, pooled microvessels. Baseline NOS activity was higher in RMVEC than in RMAEC. NG-monomethyl-L-arginine (L-NMA; 5 mM) inhibited NOS activity by approximately 70-80% in both RMAEC and RMVEC, indicating that metabolism of l-arginine is largely via NOS. Intracellular L-arginine levels were higher in RMVEC vs RMAEC and well above the eNOS Km in both cell types. L-arginine levels increased with L-NMA in both RMAEC and RMVEC, presumably due to reduced substrate utilization. Since L-arginine transport was not higher in RMVEC vs RMAEC, this may reflect higher intracellular arginine synthesis. A higher intrinsic level of baseline NO production in the postcapillary microvascular endothelium may reflect both the contribution of venular derived NO to control of arteriolar tone and a key role of venular-derived NO in local thrombosis control.

Adenosine linking reduced O2 to arteriolar NO release in intestine is not formed from extracellular ATP.

We have previously reported that adenosine formed in response to reduced arteriolar and/or tissue PO(2) preserves endothelial nitric oxide (NO) synthesis during sympathetic vasoconstriction in the rat intestine. To more precisely identify the site and mechanism of adenosine formation under these conditions, we tested the hypothesis that ATP released in response to reduced O(2) levels serves as a source of adenosine. Direct application of ATP to the wall of first-order arterioles elicited dose-dependent dilations of 15-33% above resting diameter that were reduced by 71-80% by the 5'-ectonucleotidase inhibitor alpha,beta-methyleneadenosine 5'-diphosphate (AOPCP, 4.5 x 10(-5) M) and completely abolished by N(G)-monomethyl-L-arginine (L-NMMA, 10(-4) M). Under control conditions, sympathetic nerve stimulation at 3 and 8 Hz induced arteriolar constrictions of 11 +/- 1 and 19 +/- 1 microm, respectively. These responses were enhanced by 58-69% in the presence of L-NMMA or when local PO(2) was maintained at resting levels. However, in the presence of AOPCP, the enhancing effects of L-NMMA and the high O(2) superfusate on sympathetic constriction were preserved. These results suggest that, although exogenously applied ATP can stimulate arteriolar NO release in the intestine largely through its sequential extracellular hydrolysis to adenosine, this process does not contribute to adenosine formation and sustained NO release during sympathetic constriction in this vascular bed.

Reduced PO(2) and adenosine formation preserve arteriolar nitric oxide synthesis during sympathetic constriction in the rat intestine.

Previous reports by this laboratory have indicated that a flow-dependent fall in arteriolar wall PO(2 )may be a stimulus for the sustained release of endothelial nitric oxide (NO) during sympathetic vasoconstriction in the superfused rat intestine. In this study, we tested the hypothesis that locally formed adenosine serves as the link between the fall in local PO(2) and NO synthesis under these conditions. Adenosine applied via pressurized micropipettes directly onto the wall or at a distance of 25 microm from the wall of first-order arterioles (resting diameter = 54 +/- 1 microm) elicited dose-dependent dilations of 15-46% that were significantly reduced by the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA, 10(-4) M). Arteriolar responses to sympathetic nerve stimulation were enhanced by 57-66% in the presence of L-NMMA or when tissue PO(2) was prevented from falling under a high O(2) superfusate. Adenosine deaminase (2.0 U/ml) or the selective A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (4 x 10(-4) M) completely blocked the enhancing effect of L-NMMA on sympathetic constriction. These results are consistent with the hypothesis that the fall in arteriolar wall and/or tissue PO(2) that accompanies sympathetic arteriolar constriction in the rat intestine can lead to local adenosine production, which in turn preserves endothelial NO release.

Arteriolar wall PO(2) and nitric oxide release during sympathetic vasoconstriction in the rat intestine.

Endothelium-derived nitric oxide (NO) attenuates arteriolar constriction in the rat small intestine during periods of increased sympathetic nerve activity. This study was undertaken to test the hypothesis that a flow-dependent fall in arteriolar wall PO(2) serves as the stimulus for endothelial NO release under these conditions. Sympathetic nerve stimulation at 3-16 Hz induced frequency-dependent arteriolar constriction, with arteriolar wall O(2) tension (PO(2)) falling from 67 +/- 3 mmHg to as low as 41 +/- 6 mmHg. Arteriolar responses to nerve stimulation were enhanced after inhibition of NO synthase with N(G)-monomethyl-L-arginine (L-NMMA). Under a high-O(2) (20%) superfusate, the fall in wall PO(2) was significantly attenuated, arteriolar constrictions were increased by 57 +/- 9 to 66 +/- 12%, and these responses were no longer sensitive to L-NMMA. The high-O(2) superfusate had no effect on vascular smooth muscle responsiveness to NO (as judged by arteriolar responses to sodium nitroprusside) or on arteriolar wall oxidant activity (as determined by the reduction of tetranitroblue tetrazolium dye). These results indicate that a flow-dependent fall in arteriolar wall PO(2) may serve as a stimulus for the release of endothelium-derived NO during periods of increased sympathetic nerve activity.

Reactive oxygen species may contribute to reduced endothelium-dependent dilation in rats fed high salt.

In normotensive rats, an increase in dietary salt leads to decreased arteriolar responsiveness to acetylcholine (ACh) because of suppressed local nitric oxide (NO) activity. We evaluated the possibility that generation of reactive oxygen species in the arteriolar wall is responsible for this loss of NO activity. Arteriolar responses to iontophoretically applied ACh were examined in the superfused spinotrapezius muscle of Sprague-Dawley rats fed a low-salt (LS; 0.45%) or high-salt diet (HS; 7%) for 4-5 wk. Responses to ACh were significantly depressed in HS rats but returned to normal in the presence of the oxidant scavengers superoxide dismutase + catalase or 2,2,6, 6-tetamethylpiperidine-N-oxyl (TEMPO) + catalase. Arteriolar responses to the NO donor sodium nitroprusside were similar in HS and LS rats. Arteriolar and venular wall oxidant activity, as determined by reduction of tetranitroblue tetrazolium, was significantly greater in HS rats than in LS rats. Exposure to TEMPO + catalase reduced microvascular oxidant levels to normal in HS rats. These data suggest that a high-salt diet leads to increased generation of reactive oxygen species in striated muscle microvessels, and this increased oxidative state may be responsible for decreased endothelium-dependent responses associated with high salt intake.

Reinforcement of arteriolar myogenic activity by endogenous ANG II: susceptibility to dietary salt.

The purpose of this study was to determine whether endogenous ANG II augments arteriolar myogenic behavior in striated muscle. Because circulating ANG II is decreased during high salt intake, we also investigated whether dietary salt could alter any influence of ANG II on myogenic behavior. Normotensive rats fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated box with the spinotrapezius muscle exteriorized for intravital microscopy. Dietary salt did not affect resting arteriolar diameters. Microvascular pressure elevation by box pressurization caused greater arteriolar constriction in LS rats (up to 12 microm) than in HS rats (up to 4 microm). The ANG II-receptor antagonists saralasin and losartan attenuated myogenic responsiveness in LS rats but not HS rats. The bradykinin-receptor antagonist HOE-140 had no effect on myogenic responsiveness in LS rats but augmented myogenic responsiveness in HS rats. HOE-140 with the angiotensin-converting enzyme inhibitor captopril attenuated myogenic responsiveness to a greater extent in LS rats than in HS rats. We conclude that endogenous ANG II normally reinforces arteriolar myogenic behavior in striated muscle and that attenuated myogenic behavior associated with high salt intake is due to decreased circulating ANG II and increased local kinin levels.

Limitation of arteriolar myogenic activity by local nitric oxide: segment-specific effect of dietary salt.

The purpose of this study was to determine if local nitric oxide (NO) activity attenuates the arteriolar myogenic response in rat spinotrapezius muscle. We also investigated the possibility that hypertension, dietary salt, or their combination can alter any influence of local NO on the myogenic response. Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated airtight box with the spinotrapezius muscle exteriorized for intravital microscopy. Mean arterial pressure was unaffected by dietary salt in WKY but was significantly higher and augmented by dietary salt in SHR. In all experiments, elevation of microvascular pressure by box pressurization caused a 0-30% decrease in the diameter of large (arcade bridge) arterioles and a 21-27% decrease in the diameter of intermediate (arcade) arterioles. Inhibition of NO synthase with N(G)-monomethyl-L-arginine (L-NMMA) significantly enhanced myogenic responsiveness of arcade bridge arterioles in WKY-LS and SHR-LS but not in WKY-HS and SHR-HS. L-NMMA significantly enhanced the myogenic responsiveness of arcade arterioles in all four groups. Excess L-arginine reversed this effect of L-NMMA in all cases, and arteriolar responsiveness to the NO donor sodium nitroprusside was not different among the four groups. High-salt intake had no effect on the passive distension of arterioles in either strain during box pressurization. We conclude that 1) local NO normally attenuates arteriolar myogenic responsiveness in WKY and SHR, 2) dietary salt impairs local NO activity in arcade bridge arterioles of both strains, and 3) passive arteriolar distensibility is not altered by a high-salt diet in either strain.

Growth-related changes in the influence of nitric oxide on arteriolar tone.

This study was designed to determine whether juvenile growth is accompanied by changes in the local influence of nitric oxide (NO) or prostaglandins on arteriolar tone. In vivo microscopy was used to study proximal arterioles in the spinotrapezius muscle of rats 4-5 wk (weanling), 7-8 wk (juvenile), and 11-12 wk (mature) of age. From 4 to 12 wk of age, arterioles underwent an increase in resting diameter (from 31 +/- 2 to 49 +/- 2 micrometer) and volume flow (from 7 +/- 1 to 10 +/- 1 nl/s) but a decrease in resting wall shear rate (from 1,901 +/- 150 to 748 +/- 50 s(-1)). NO synthase inhibition with N(G)-monomethyl-L-arginine (L-NMMA) had no effect on arteriolar diameters in weanling rats but reduced diameters by 14 +/- 4% in juvenile rats and by 13 +/- 4% in mature rats. Cyclooxygenase inhibition with meclofenamate reduced arteriolar diameters by a similar amount (13 +/- 4 to 18 +/- 3%) in all age groups. There were no age-related differences in arteriolar responsiveness to locally applied sodium nitroprusside or prostaglandin E(2). Arteriolar responsiveness to ACh was also similar in all groups, but the L-NMMA-sensitive portion of this response was smaller in mature rats than in weanling rats. Elevation of flow-related shear stress caused arteriolar dilation in juvenile rats but not in weanling rats. These findings suggest that arteriolar smooth muscle responsiveness to NO or prostaglandins does not change during juvenile growth and that basally released vasodilator prostaglandins exert a constant influence on arteriolar tone throughout this period. Basal NO activity also modulates arteriolar tone in juvenile and mature rats but not in weanling rats. In contrast, agonist-stimulated NO release is prominent in weanling and juvenile rats but somewhat decreased in mature rats, where cyclooxygenase products also contribute to ACh induced dilation.

High dietary salt alters arteriolar myogenic responsiveness in normotensive and hypertensive rats.

We evaluated arteriolar myogenic responsiveness in normotensive, salt-loaded and hypertensive rats and investigated the potential influence of luminal blood flow or shear stress on myogenic responses under each of these conditions. Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated airtight box with the spinotrapezius muscle exteriorized for intravital microscopy. Dietary salt did not affect mean arterial pressure (MAP) in WKY, whereas MAP in SHR was significantly higher and augmented by dietary salt. In all groups, box pressurization caused similar increases in MAP that were completely transmitted to the arterioles. After these pressure increases, large arteriole diameters decreased by 0-30% and intermediate arteriole diameters decreased by 21-27%. Arteriolar myogenic responsiveness was not different between WKY-LS and SHR-LS. Large arterioles in WKY-HS displayed an attenuated pressure-diameter relationship compared with that in WKY-LS. Large arterioles in SHR-HS displayed an augmented pressure-diameter relationship compared with that in SHR-LS. There were no correlations between resting flow or wall shear rate and the magnitude of initial myogenic constriction in any group or vessel type. The capacity for sustained myogenic constriction was unrelated to secondary decreases in flow (14-41%) or increases in wall shear rate (21-88%) in each group. We conclude that 1) dietary salt impairs the myogenic responsiveness of large arterioles in normotensive rats and augments the myogenic responsiveness of large arterioles in hypertensive rats, 2) hypertension does not alter arteriolar myogenic responsiveness in this vascular bed, and 3) flow- or shear-dependent mechanisms do not attenuate myogenic responses in the intact arteriolar network of normal, salt-loaded, or hypertensive rats.

Modulation of arteriolar sympathetic constriction by local nitric oxide: onset during rapid juvenile growth.

The goal of this study was to determine if the endogenous activity of nitric oxide (NO) and/or prostanoids can limit arteriolar responses to increased sympathetic nerve activity in striated muscle, and to explore possible changes in these influences during rapid juvenile growth. Using intravital microscopy, arteriolar responses to 2-16 Hz sympathetic nerve stimulation were studied in the superfused spinotrapezius muscle of weanling (4-5 weeks old) and juvenile (7-8 weeks old) rats. Nerve stimulation elicited frequency-dependent arteriolar constrictions that were abolished in both age groups by the fast Na+-channel blocker tetrodotoxin or the alpha-antagonist phentolamine. Diameter and flow responses to 2-8 Hz stimulation were greater in juvenile rats than in weanling rats. In juvenile rats but not in weanling rats, the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) reduced arteriolar diameters and blood flow at rest and enhanced the arteriolar diameter and flow responses to sympathetic nerve stimulation. The cyclooxygenase inhibitor meclofenamate reduced resting arteriolar diameters in both age groups, but had no effect on responses to sympathetic nerve stimulation in either group. These results suggest that juvenile growth is accompanied by an overall increase in arteriolar responsiveness to sympathetic nerve activity, and by the onset of local NO activity that limits this increased responsiveness.

Postjunctional alpha 2-adrenoceptors are not present in proximal arterioles of rat intestine.

The purpose of this study was to evaluate two potential stimuli for nitric oxide (NO) release in rat intestinal arterioles during sympathetic nerve activation. To determine whether these vessels contain endothelial alpha 2-adrenoceptors linked to the L-arginine-NO pathway, intravital microscopy was used to study the response of first-order arterioles (1As, 20-40 microns ID) to direct application of 1) the selective alpha 2-agonist BHT-933 and 2) norepinephrine (NE) or sympathetic nerve stimulation before and after alpha 1- or alpha 2-receptor blockade. The effect of sympathetic nerve stimulation on 1A wall shear rate (WSR) was also determined to evaluate the possibility of hemodynamic shear stress as a stimulus for NO release. BHT-933 had no effect on 1A diameter, whereas NE produced dose-dependent constrictions of 5 +/- 3 to 15 +/- 3 microns, which were usually abolished by the alpha 1-antagonist prazosin but unaffected by the alpha 2-antagonist idazoxan. Sympathetic nerve stimulation at 3, 8, and 16 Hz induced constrictions of 4 +/- 1, 8 +/- 2, and 17 +/- 4 microns, respectively, and these constrictions were also usually abolished by prazosin but unaffected by idazoxan. Resting WSR averaged 1,997 +/- 163 s-1 and decreased to 1,587 +/- 209, 1,087 +/- 195, and 537 +/- 99 s-1 during 3-, 8-, and 16-Hz nerve stimulation. These results suggest that alpha 2-adrenoceptor-dependent pathways do not influence either resting tone or sympathetic constriction of proximal arterioles in the intestinal submucosa and that luminal shear stress in these vessels significantly decreases with sympathetic constriction. It therefore appears unlikely that either alpha 2-receptor activation or changes in hemodynamic shear serve as stimuli for arteriolar NO release during periods of increased sympathetic nerve activity.

Heterogeneity of endothelial function within the circulation.

As our understanding of endothelial function continues to evolve, it has become increasingly clear that the peripheral vasculature exhibits striking regional and segmental heterogeneity in the influence of the endothelial cell layer on vascular tone. This heterogeneity encompasses not only the normal interactions between endothelium-derived factors and vascular smooth muscle, but also the way in which these interactions can change during juvenile growth or in disease states such as hypertension. The underlying causes of this heterogeneity are multifactorial and include intrinsic differences among endothelial cell populations and differences in the endothelial cell microenvironment.

Endothelium-derived nitric oxide limits sympathetic neurogenic constriction in intestinal microcirculation.

We have recently shown that endogenous nitric oxide (NO) activity can attenuate the sympathetic neurogenic constriction of intestinal arterioles. The purpose of this study was to determine whether the microvascular endothelium is an important site of NO production under these conditions. In the superfused small intestine of the rat, intravital microscopy was used to study the responses of first-order arterioles (1A) to perivascular sympathetic nerve stimulation and directly applied norepinephrine before and then after passage of a CO2 embolus through the 1A lumen to inhibit endothelial function. CO2 embolization did not significantly alter resting arteriolar diameter (50 +/- 4 microns before vs. 51 +/- 4 microns after embolization) but abolished the dilator response to acetylcholine without altering the dilator response to sodium nitroprusside. Stimulation at 3, 8, and 16 Hz caused respective constrictions of 4 +/- 1, 11 +/- 1, and 18 +/- 2 microns, and after CO2 these responses were significantly increased to 9 +/- 1, 18 +/- 1, and 29 +/- 3 microns, respectively. Exposure to the nitric oxide synthase inhibitor NG-monomethyl-L-arginine(10(-4) M in superfusate) after CO2 embolization had no further effect on the magnitude of neurogenic constriction. Similar results were seen when embolization was achieved with N2, and CO2 embolization had the same effect on norepinephrine-induced constriction as it did on neurogenic constriction. These results suggest that nitric oxide of endothelial origin can attenuate sympathetic neurogenic constriction in the intestinal microvasculature.

Modulation of sympathetic constriction by the arteriolar endothelium does not involve the cyclooxygenase pathway.

We have recently shown that the responsiveness of rat intestinal arterioles to increased sympathetic nerve activity is modulated by the actions of endothelial-derived nitric oxide. Because the microvascular endothelium can also produce vasodilator prostaglandins, the purpose of this study was to determine if endogenous cyclooxygenase products also limit sympathetic arteriolar constriction in this vascular bed. Intravital microscopy was used to study the responses of small feed arteries, first-order arterioles and second-order arterioles to perivascular sympathetic nerve stimulation in the superfused rat small intestine. Stimulation at 3, 8 and 16 Hz caused frequency-dependent constrictions of each vessel type that are abolished by the alpha-adrenoceptor antagonist phentolamine (10(-6) M superfusate concentration). The cyclooxygenase inhibitor meclofenamate (3 x 10(-5) M superfusate concentration) completely abolished the dilator responses to topically applied arachidonic acid, but had no effect on the magnitude or rate of sympathetic constriction in any vessel type. These results suggest that endogenous cyclooxygenase activity does not influence sympathetic tone in the intestinal microvasculature.

Nitric oxide modulates arteriolar responses to increased sympathetic nerve activity.

The purpose of this study was to determine whether arteriolar responses to increased sympathetic nerve activity are limited by the actions of endogenous nitric oxide. Intravital microscopy was used to examine diameter responses of small feed arteries (SFA), first-order arterioles (1A) and second-order arterioles (2A) to perivascular sympathetic nerve stimulation in the superfused rat small intestine. Stimulation induced a frequency-dependent constriction in all vessel types that was completely abolished by the alpha-adrenoceptor antagonist phentolamine (10(-6) M). In SFA and 1A, the magnitude of sympathetic constriction was increased significantly in the presence of the nitric oxide synthase inhibitor NG-monomethyl-L-arginine(L-NMMA, 10(-4) M). In SFA (n = 7), stimulation at 3, 8, and 16 Hz induced constrictions of 11 +/- 1, 28 +/- 4, and 42 +/- 3%, respectively, under the normal superfusate vs. 28 +/- 3, 46 +/- 5, and 76 +/- 3% in the presence of L-NMMA. For 1A (n = 7), stimulation induced constrictions of 10 +/- 1, 27 +/- 4, and 37 +/- 3% under the normal superfusate vs. 24 +/- 2, 47 +/- 3, and 72 +/- 4% in the presence of L-NMMA. The effect of L-NMMA on sympathetic constriction in SFA (n = 7) was completely reversed by the additional presence of 5 x 10(-3) M L-arginine in the superfusate. These results suggest that endogenous nitric oxide activity can attenuate sympathetic neurogenic constriction in the intestinal microvasculature.

Arteriolar network growth in rat striated muscle during juvenile maturation.

To better understand normal microvascular network growth, we conducted a longitudinal study on the spinotrapezius muscle of Sprague-Dawley rats of 3 ages: weanling (3-4 weeks), juvenile (7-8 weeks) and mature (11-12 weeks). Morphometric analysis revealed that despite dramatic growth of the muscle (from 117 +/- 60 mg in weanling to 417 +/- 112 mg in mature rats), there is no significant change in the total number of arcade arteriole segments per network over this period (from 140 +/- 35 to 181 +/- 51). The mean arcade segment length increased over this period (from 0.96 +/- 0.17 to 1.45 +/- 0.32 mm), but not in proportion to tissue growth. Consequently, the total arcade segment length per unit muscle volume significantly decreased (from 1.36 +/- 0.48 to 0.66 +/- 0.12 mm/mm3). The estimated number of transverse arteriolar trees per muscle (approximately 600) did not appreciably change with growth, leading to a decrease in the number of trees per millimeter arcade arteriole (from 4.4 +/- 1.2 to 2.3 +/- 0.6). Transverse arteriolar trees underwent age-dependent increases in the number of segments within each branch order and in mean segment length. These observations suggest that arteriolar network growth during juvenile maturation occurs by elongation of pre-existing arcade and transverse vessels with angiogenesis occurring in the distal segments of transverse arteriolar trees.

Shear-dependent release of venular nitric oxide: effect on arteriolar tone in rat striated muscle.

This study was designed to determine whether shear-dependent changes in venular nitric oxide (NO) production can influence nearby arteriolar tone and whether this mechanism contributes to functional arteriolar dilation in contracting muscle. In resting spinotrapezius muscle of anesthetized rats, occlusion of one branch of an arcade venular bifurcation with divergent flow caused flow and wall shear rate in the parallel branch to increase by an average of 99 and 72%, respectively. After 10-30 s, the paired arcade arteriole dilated by an average of 30%, with a correlation between the increase in venular shear rate and the magnitude of arteriolar dilation. During muscle contraction, arcade arterioles dilated by 73-97% and arcade venular shear rate increased by 48-83%. The NO synthase inhibitor NG-monomethyl-L-arginine greatly attenuated arteriolar dilation to increased venular shear rate in resting muscle but did not affect arteriolar dilation in contracting muscle. These findings suggest that a shear-dependent increase in venular NO release can dilate nearby arterioles, but this mechanism is not important for the sustained dilation of these arterioles during functional hyperemia.

Increased nitric oxide activity in early renovascular hypertension.

A decreased influence of nitric oxide (NO) in the peripheral vasculature is associated with the pathophysiology of established hypertension, and some studies suggest that increased blood pressure positively correlates with decreased NO production. If so, then the increased arterial pressure in one-kidney, one-clip (1K1C) hypertensive rats should be associated with decreased circulating levels of nitrite/nitrate (NO2/NO3; stable metabolites of NO) and guanosine 3',5'-cyclic monophosphate (cGMP; mediator of NO action). We measured serum NO2/NO3 and cGMP levels in early hypertensive 1K1C (2 wk after clipping) and shamoperated one-kidney (1K) normotensive rats, treated orally with or without the NO-synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 2 wk). Compared with those in 1K rats, NO2/NO3 and cGMP levels were increased in 1K1C hypertensive rats but not in 1K1C rats treated with L-NAME. NO2/NO3 and cGMP levels in L-NAME-treated 1K and 1K1C rats were similar. Compared with that in 1K rats, systolic blood pressure (SBP) was increased in 1K1C rats and in L-NAME-treated 1K and 1K1C rats. The SBP increase in L-NAME-treated 1K1C rats was more rapid than in untreated 1K1C rats. In early hypertension, increases in SBP positively correlated with increases in serum NO2/NO3 and cGMP. After 2 wk of hypertension, circulating NO2/NO3 levels gradually declined and reached prehypertension levels by the fifth week of hypertension. These results provide evidence for increased NO synthesis in early hypertensive 1K1C rats, and this increased NO could be a compensatory mechanism to slow the development of hypertension in these animals.

Flow-dependent arteriolar dilation in normotensive rats fed low- or high-salt diets.

Ingestion of a high-salt diet has previously been shown to suppress the endogenous influence of nitric oxide (NO) on arteriolar tone in hypertension-resistant, salt-resistant Dahl (SR/Jr) rats. Because luminal blood flow can be an important stimulus for endothelial NO release, this study was undertaken to determine whether high salt intake can also lead to a deficit in the direct flow-dependent regulation of arteriolar diameter. The spinotrapezius muscle microvasculature was studied by in vivo microscopy in SR/Jr rats fed low (0.45%)- or high (7%)-salt diets for 2 wk, and arcade arteriole responses to increased luminal flow (via parallel vessel occlusion) were studied in both dietary groups. There was no significant difference between groups in arterial pressure or in resting arteriolar diameters, volume flows, or wall shear rates. In low-salt SR/Jr, a 36% increase in luminal flow produced an average arteriolar dilation of 38% that was significantly reduced by the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). In high-salt SR/Jr, a similar flow increase produced an average dilation of only 16% (P < 0.05 vs. low-salt SR/Jr), and this response was unaffected by L-NMMA. Inhibition of cyclooxygenase activity with meclofenamate had no effect on this response in either group. These findings suggest that NO release mediates a portion of flow-dependent arteriolar dilation in rat spinotrapezius muscle and that high salt intake, in the absence of hypertension, can attenuate this response via a suppression of NO activity.