Reduced influence of nitric oxide on arteriolar tone in hypertensive Dahl rats.

The aim of this study was to evaluate the influence of endogenous nitric oxide on resting microvascular tone in the Dahl salt-sensitive (DS) rat and to determine how this influence is altered in salt-induced hypertension. Intravital microscopy was used to examine the arteriolar network in the spinotrapezius muscle of DS rats maintained on low (0.45% NaCl) or high (4% NaCl) salt diets for 6-7 weeks. Mean arterial pressure for DS rats on high salt (163 +/- 3 mm Hg) was significantly greater than that for DS rats on low salt (128 +/- 4 mm Hg). Inhibition of microvascular nitric oxide synthesis with NG-nitro-L-arginine-methyl ester caused arteriolar constriction in normotensive DS but not in hypertensive DS rats. Application of L-arginine consistently caused arteriolar dilation in normotensive DS but not hypertensive DS rats. In contrast, arteriolar responses to iontophoretically applied acetylcholine and sodium nitroprusside were similar in both groups. These results indicate that basal release of nitric oxide, presumably from the endothelium, normally influences arteriolar tone in skeletal muscle of DS rats and that this influence is suppressed in established salt-induced hypertension. However, the normal arteriolar response to acetylcholine in hypertensive DS rats suggests that a generalized impairment of endothelial function may not occur in the microcirculation of these animals. Unaltered arteriolar responsiveness to sodium nitroprusside in hypertensive DS rats also suggests that salt-induced hypertension is not accompanied by a change in the responsiveness of arteriolar smooth muscle to nitric oxide.

Arteriolar diameter and tissue oxygen tension during muscle contraction in hypertensive rats.

This study evaluated the possibility that in hypertension, mechanisms that maintain near normal arteriolar diameters at elevated arteriolar pressures limit the ability of skeletal muscle arterioles to dilate in response to an increase in tissue metabolism. The spinotrapezius muscles of 16- to 20-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were contracted at frequencies of 1, 2, 4, and 8 Hz. The inner diameters of first-order through third-order arterioles were measured at rest and following 3 minutes of contractions. Tissue oxygen tension (PO2) at the venous end of capillaries was monitored during 8-Hz contractions. At rest, following contractions, and after maximum dilation with adenosine, the inner diameters of arterioles of equivalent branch order were not significantly different in SHR and WKY. Opening of closed arterioles during muscle contraction and adenosine application occurred in less than 5% of the observations in both groups. The resting tissue PO2 was 25.5 +/- 1.3 mm Hg in normal rats and 26.1 +/- 2.1 mm Hg in SHR. At nearly maximum vasodilation during 8-Hz stimulation, tissue PO2 recovered to 81.9 +/- 12.7% of control in WKY but only to 41.2 +/- 13.0% of control in SHR. These observations indicate that the expression of local regulatory mechanisms related to tissue metabolism is virtually normal in the spinotrapezius muscle vasculature of SHR in the context of arteriolar dilation. However, at near maximum performance, factors other than absolute arteriolar diameter preclude the normal preservation of tissue PO2 in the spinotrapezius muscle of SHR.

Effect of dietary salt on the skeletal muscle microvasculature in Dahl rats.

The purpose of this study was to identify microvascular alterations that could contribute to increased peripheral vascular resistance in the Dahl salt-sensitive rat with salt-induced hypertension. Intravital microscopy was used to study the spinotrapezius muscle arteriolar network in anesthetized salt-sensitive rats fed either a high salt (7% sodium chloride) or low-normal salt (0.45% sodium chloride) diet for 4 weeks. Age-matched Dahl salt-resistant rats on high and low-normal salt diets served as controls. The high salt diet had no effect on arterial pressure in salt-resistant rats but increased arterial pressure in salt-sensitive rats. Mean resting diameter of arcade arterioles in salt-sensitive rats on high salt diet was reduced by 25% compared with salt-sensitive rats on low salt or salt-resistant rats on either diet. After abolition of vascular tone with 10(-3) M adenosine, arcade diameters were comparable in all groups. No difference among groups was found in either resting or passive diameter of the more distal transverse arterioles. Measurement of vessel lengths and numbers in cleared muscle specimens revealed no differences among groups in the anatomic density of either arcade or transverse arterioles. These data suggest that a reduction in the resting diameter of arcade, but not transverse, arterioles may increase spinotrapezius muscle vascular resistance in hypertensive salt-sensitive rats. The similarity in vascular densities among groups indicates that structural rarefaction of arterioles does not contribute to any increase in spinotrapezius muscle resistance at this stage of salt-induced hypertension.

Relative contributions of dietary Na+ and Cl- to salt-sensitive hypertension.

The effect of dietary NaCl on blood pressure has generally been attributed to the sodium ion. However, recent evidence indicates that the anion accompanying sodium plays an important role in determining the magnitude of the blood pressure increase in response to a high dietary intake of NaCl. The purpose of this review is to describe studies of blood pressure responses in several experimental models of salt-sensitive hypertension and in hypertensive humans to selective dietary sodium loading (without chloride) and to selective dietary chloride loading (without sodium). The full expression of salt sensitivity depends on high dietary intakes of both sodium and chloride. This observation has implications for understanding mechanisms contributing to NaCl-induced hypertension in the susceptible host.

Importance of dietary chloride for salt sensitivity of blood pressure.

Recent evidence indicates that the anion accompanying sodium plays an important role in determining the magnitude of the blood pressure increase in response to a high dietary intake of NaCl. The purpose of this review is to describe studies of blood pressure responses to selective dietary sodium loading (without chloride) and to selective dietary chloride loading (without sodium) in several experimental models of salt-sensitive hypertension and in hypertensive humans. The full expression of salt sensitivity depends on high dietary intakes of both sodium and chloride. This observation has implications for understanding mechanisms contributing to NaCl-induced hypertension.

Nutrition and hypertension prevention.

Hypertension has been related to both obesity and a high salt intake. Evidence for the associations of blood pressure with body weight and dietary salt intake is summarized. In both adolescents and adults correlations between blood pressure and weight are highly significant, and in longitudinal studies change in blood pressure over time is correlated with change in weight. Correlations between salt intake and blood pressure are less striking, and the results of trials of modest salt restriction demonstrate a small but significant effect on blood pressure. Individuals vary in their susceptibility to salt, and hypertensive individuals are more responsive than normotensive individuals. Dietary deficiencies of potassium and calcium may amplify the effect of a high salt intake on blood pressure. Animal models provide compelling evidence for a genetic component to salt sensitivity of blood pressure. In two hypertension prevention trials, change in blood pressure was more convincingly related to change in weight than to change in dietary salt. Avoidance of obesity, or weight reduction in overweight individuals, should be key strategies for hypertension prevention. Avoidance of salt excess is also appropriate, although currently available trial data do not justify a recommendation of rigorous salt restriction for the entire population.

Effect of salt-induced hypertension on microvascular pressures in skeletal muscle of Dahl rats.

The purpose of this study was to determine the effect of salt-induced hypertension on the microvascular pressure profile in skeletal muscle. Measurements were made in the spinotrapezius muscle of anesthetized Dahl salt-sensitive (DS) rats fed either a high- (7%) or low-normal (0.45%)-NaCl diet for 4 wk. Age-matched Dahl salt-resistant (DR) rats on high- or low-normal salt diets were also studied. The high-salt diet had no effect on either arterial or microvascular pressures in DR rats. DS rats maintained on high salt developed arterial hypertension accompanied by a pressure increase of 49% in the feed arterioles and 31% in the transverse arterioles. Hypertensive DS rats exhibited a greater pressure drop across the small arteries, arcade arterioles, and distal arterioles, indicating that each of these segments contributes to increased whole organ resistance. Pressures in the collecting and draining venules were not elevated in DS on 7% NaCl, suggesting that increased precapillary resistance in salt-induced hypertension effectively shields the skeletal muscle capillary and venular networks from high hydrostatic pressures.

Effect of dietary salt on arteriolar nitric oxide in striated muscle of normotensive rats.

This study evaluated the influence of high dietary salt intake on nitric oxide (NO) activity in the arteriolar network of rats resistant to salt-induced hypertension. The spinotrapezius muscle microvasculature was studied in inbred Dahl salt-resistant (SR/Jr) rats fed low (0.45%)- or high (7%)-salt diets for 4-5 wk. Arterial pressures were not different between groups at any time during the study. NO synthesis inhibition with NG-nitro-L-arginine-methyl ester (L-NAME) constricted arcade arterioles in low-salt SR/Jr and dilated arcade arterioles in high-salt SR/Jr. Arcade arteriole dilation to acetylcholine (ACh), but not sodium nitroprusside (SNP), was impaired in high-salt SR/Jr. In contrast, transverse and distal arteriole responses to L-NAME, ACh, and SNP were identical in high- and low-salt SR/Jr. These findings indicate that high salt intake, in the absence of increased arterial pressure, suppresses the influence of basal and evoked NO on vascular tone in arcading arterioles, but not in smaller transverse and distal arterioles. Unaltered SNP responses in high-salt SR/Jr suggest that this effect does not involve a change in arteriolar smooth muscle responsiveness to NO.

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.

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.

Microvascular changes associated with high salt intake and hypertension in Dahl rats.

This study was undertaken to determine if an active reduction in small arteriole diameters or functional closure of these vessels contributes to increased vascular resistance in inbred hypertensive Dahl salt-sensitive (SS/Jr) rats. Fluorescence microscopy was used to study the spinotrapezius muscle arteriolar network in SS/Jr fed high (7%) or low (0.45%) salt diets for 4 weeks. Dahl salt-resistant (SR/Jr) rats fed high or low salt diets were also studied to evaluate the exclusive effect of dietary salt on the microvasculature. High salt intake did not alter arterial pressure in SR/Jr, but caused marked hypertension (mean arterial pressure = 168 mm Hg) in SS/Jr. The diameter of large (arcade bridge) arterioles was significantly smaller in SS/Jr than SR/Jr, regardless of diet. The establishment of hypertension in SS/Jr was accompanied by an active diameter reduction in the intermediate (arcade) arterioles. However, there were no differences among groups in the density of perfused small arterioles or their diameters, and arteriolar density did not change with abolition of vascular tone in any group. In both strains, high salt intake reduced the passive diameter of large arterioles by 20%. Therefore, the establishment of hypertension in SS/Jr rats is associated with increased tone of proximal, but not distal, arterioles. In addition, distal arteriole rarefaction (functional or structural) does not contribute to increased spinotrapezius muscle resistance at this stage of salt-induced hypertension. The salt-induced reduction in passive arteriolar diameters suggests that high salt intake alone can alter the structural and/or mechanical properties of the arteriolar wall.

Enhanced arteriolar vasomotion in rats with chronic salt-induced hypertension.

The aims of this study were (1) to determine if the establishment of hypertension in the Dahl salt-sensitive (DS) rat is accompanied by alterations in arteriolar vasomotion and (2) to explore the influence of endogenous nitric oxide on vasomotion in normotensive and hypertensive DS rats. Rhythmic diameter changes of arcading arterioles were studied in the superfused spinotrapezius muscle of DS fed high (4%) or low (0.45%) salt diets for 6 weeks. Mean arterial pressure for DS on high salt (166 +/- 7 mm Hg) was significantly greater than that for DS on low salt (131 +/- 9 mm Hg). There was no difference between hypertensive and normotensive DS in time-averaged arteriolar diameter, vasomotion frequency, or vasomotion cycle length. However, average vasomotion amplitude was 93% greater in hypertensive DS than in normotensive DS. Inhibition of nitric oxide synthesis with NG-Nitro-L-arginine methyl ester did not alter vasomotion in hypertensive DS, but increased vasomotion amplitude in normotensive DS to a level not different from that in hypertensive DS. L-Arginine had no effect on vasomotion in either group. Therefore, cyclic variations in arcade arteriole diameter are normally limited by basal nitric oxide, and the enhancement of these variations in animals with salt-induced hypertension may be attributable to the loss of this nitric oxide influence. The increased vasomotion amplitude and unchanged average diameter in hypertensive DS suggests a reduced hydraulic resistance within this particular segment of the arteriolar network.

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.

Response of arteriolar network of skeletal muscle to sympathetic nerve stimulation.

The influence of vessel location on arteriolar responses to sympathetic nerve stimulation was systematically studied in a skeletal muscle arteriolar network under normal and altered tissue O2 levels. The exteriorized cat sartorius muscle was exposed to 0, 5, and 10% ambient O2 during sympathetic chain stimulation at 8 pulses/s. Under 0% O2, stimulation initially caused a 25-45% constriction that was faster and more pronounced in distal arterioles. Ninety-one percent of vessels showed a secondary dilation (sympathetic escape), which was largest in distal arterioles. Escape had little effect on calculated volume flow, which, after a large initial fall, showed a modest secondary increase. Under 5 and 10% O2, resting arteriolar diameter was reduced by 12 and 17%, respectively, and escape was reduced by 60 and 73%. Escape was not attenuated in proximal arterioles preconstricted with vasopressin, suggesting that O2 did not attenuate escape through increased vascular tone. Therefore, the arteriolar response to sympathetic stimulation depends largely on location within the network and is modulated to varying degrees by metabolic influences.

Periarteriolar and tissue PO2 during sympathetic escape in skeletal muscle.

The purpose of this study was to determine whether vascular escape from sympathetic nerve stimulation in skeletal muscle is caused by a fall in the tissue O2 level. O2 microelectrodes were used to measure PO2 at the wall of arterioles (periarteriolar PO2) and near the venous end of capillary networks (parenchymal tissue PO2) in the exteriorized cat sartorius muscle during sympathetic nerve stimulation. Measurements were made under a low O2 suffusate (equilibrated with 5% CO2-95% N2) and under a high O2 suffusate (10% O2-5% CO2-85% N2). During sympathetic stimulation under low O2, mean diameter of proximal (second-order) arterioles decreased from 55 to 32 micron before returning to 37 micron (sympathetic escape). Mean periarteriolar PO2 fell from 50 to 25 mmHg with no secondary increase. Distal (fifth-order) arterioles initially constricted from 7 to 4 micron before relaxing to 6 micron. Periarteriolar PO2 of these vessels fell from 40 to 13 mmHg with no secondary increase. During stimulation under high O2, periarteriolar PO2 levels of both proximal and distal arterioles were similar to those under low O2, yet escape was substantially reduced. The lack of relationship between periarteriolar PO2 and vascular escape argues against a role of vascular wall PO2 in this behavior. Parenchymal tissue PO2 fell to 9 mmHg during stimulation under low O2 but did not fall below 22 mmHg during stimulation under high O2. The attenuation of escape under conditions where tissue PO2 did not fall is consistent with the hypothesis that sympathetic escape in skeletal muscle is mediated through a fall in parenchymal cell PO2.

Microvascular responses to oxygen and muscle contraction in hypertensive Dahl rats.

We evaluated the possibility that increased arteriolar tone in salt-sensitive hypertension could be partially due to an altered vascular responsiveness to oxygen and/or tissue metabolites. The microvasculature of the superfused spinotrapezius muscle was studied with fluorescence microscopy in Dahl salt-sensitive rats fed low (0.45%) or high (7%) salt diets for 4 weeks. High salt intake produced hypertension (mean arterial pressure = 162 +/- 5 mm Hg vs 130 +/- 4 mm Hg for low salt rats, p < 0.05), and increased vascular tone at the level of the arcade arterioles but not in the smaller transverse or distal arterioles. Arcade arteriole constriction induced by increasing superfusate oxygen content (from 0% to 10% O2) was 60% greater in hypertensive rats than in normotensive rats, whereas oxygen-dependent constriction of transverse and distal arterioles was similar in the two groups. The oxygen-induced reduction in capillary perfusion (the fraction of time during which flow was observed in each vessel) was also greater in hypertensive than in normotensive rats. Arcade arteriole dilation during 2 Hz muscle contraction was significantly greater in hypertensive than in normotensive rats, but there were no differences between groups in the dilation of any other vessel type or in the capillary flow increases accompanying 2 or 8 Hz contraction. These results suggest that a hyperresponsiveness to the actions of blood-borne or tissue oxygen could contribute to increased arcade arteriolar tone in the spinotrapezius muscle of Dahl rats with salt-induced hypertension. The enhanced dilation of arcade arterioles during muscle contraction also suggests a localized hyperresponsiveness to tissue metabolites in this form of hypertension.

Arteriolar network morphology in gracilis muscle of rats with salt-induced hypertension.

The purpose of this study was to determine if structural rarefaction of arterioles occurs in the gracilis muscle of Dahl salt-sensitive (DS) rats with salt-induced hypertension. Arteriolar network architecture was studied in cleared muscles removed from DS fed either a high (7% NaCl) or low-normal (0.45% NaCl) salt diet for 4 weeks. Muscles removed from Dahl salt-resistant (DR) rats on high and low-normal salt diets served as controls. The 7% NaCl diet had no effect on arterial pressure in DR, but caused marked hypertension in DS. The density of arcade arterioles was significantly lower in DS than in DR (0.77 vs 1.26 segments/mg tissue, respectively) and was unrelated to either dietary salt content or mean arterial pressure in both strains. The number of transverse arterioles/mm parent vessel was 19% lower in DS on 7% NaCl than in DS on 0.45% NaCl and DR on either diet. These data indicate that compared to normotensive DR, the DS rat with salt-induced hypertension exhibits a lower vascular density within both the arcading and the transverse portions of the gracilis muscle arteriolar network. The lower arcade vessel density reflects an inherent characteristic of the DS strain, whereas the lower transverse arteriole density reflects a true structural rarefaction associated with salt-induced hypertension.

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.

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.