Sensitivity of caudal arteries and the mesenteric vascular bed to norepinephrine in DOCA-salt hypertension

Contractile force is enhanced in Aortas from pendrin null mice due to stimulation of angiotensin II-dependent signaling

Pendrin is a Cl⁻/HCO₃⁻ exchanger expressed in the apical regions of renal intercalated cells. Following pendrin gene ablation, blood pressure falls, in part, from reduced renal NaCl absorption. We asked if pendrin is expressed in vascular tissue and if the lower blood pressure observed in pendrin null mice is accompanied by reduced vascular reactivity. Thus, the contractile responses to KCl and phenylephrine (PE) were examined in isometrically mounted thoracic aortas from wild-type and pendrin null mice. Although pendrin expression was not detected in the aorta, pendrin gene ablation changed contractile protein abundance and increased the maximal contractile response to PE when normalized to cross sectional area (CSA). However, the contractile sensitivity to this agent was unchanged. The increase in contractile force/cross sectional area observed in pendrin null mice was due to reduced cross sectional area of the aorta and not from increased contractile force per vessel. The pendrin-dependent increase in maximal contractile response was endothelium- and nitric oxide-independent and did not occur from changes in Ca²⁺ sensitivity or chronic changes in catecholamine production. However, application of 100 nM angiotensin II increased force/CSA more in aortas from pendrin null than from wild type mice. Moreover, angiotensin type 1 receptor inhibitor (candesartan) treatment in vivo eliminated the pendrin-dependent changes contractile protein abundance and changes in the contractile force/cross sectional area in response to PE. In conclusion, pendrin gene ablation increases aorta contractile force per cross sectional area in response to angiotensin II and PE due to stimulation of angiotensin type 1 receptor-dependent signaling. The angiotensin type 1 receptor-dependent increase in vascular reactivity may mitigate the fall in blood pressure observed with pendrin gene ablation.

Splanchnic sympathetic nerves in the development of mild DOCA-salt hypertension

We previously reported that mild deoxycorticosterone acetate (DOCA)-salt hypertension develops in the absence of generalized sympathoexcitation. However, sympathetic nervous system activity (SNA) is regionally heterogeneous, so we began to investigate the role of sympathetic nerves to specific regions. Our first study on that possibility revealed no contribution of renal nerves to hypertension development. The splanchnic sympathetic nerves are implicated in blood pressure (BP) regulation because splanchnic denervation effectively lowers BP in human hypertension. Here we tested the hypothesis that splanchnic SNA contributes to the development of mild DOCA-salt hypertension. Splanchnic denervation was achieved by celiac ganglionectomy (CGX) in one group of rats while another group underwent sham surgery (SHAM-GX). After DOCA treatment (50 mg/kg) in rats with both kidneys intact, CGX rats exhibited a significantly attenuated increase in BP compared with SHAM-GX rats (15.6 ± 2.2 vs. 25.6 ± 2.2 mmHg, day 28 after DOCA treatment). In other rats, whole body norepinephrine (NE) spillover, measured to determine if CGX attenuated hypertension development by reducing global SNA, was not found to be different between SHAM-GX and CGX rats. In a third group, nonhepatic splanchnic NE spillover was measured as an index of splanchnic SNA, but this was not different between SHAM (non-DOCA-treated) and DOCA rats during hypertension development. In a final group, CGX effectively abolished nonhepatic splanchnic NE spillover. These data suggest that an intact splanchnic innervation is necessary for mild DOCA-salt hypertension development but not increased splanchnic SNA or NE release. Increased splanchnic vascular reactivity to NE during DOCA-salt treatment is one possible explanation.

Differences in functional and structural properties of segments of the rat tail artery

The investigation of resistance vessels is generally costly and difficult to execute. The present study investigated the diameters and the vascular reactivity of different segments of the rat tail artery (base, middle, and tail end) of 30 male Wister rats (EPM strain) to characterize a conductance or resistance vessel, using a low-cost simple technique. The diameters (mean +/- SEM) of the base and middle segments were 471 +/- 4.97 and 540 +/- 8.39 microm, respectively, the tail end was 253 +/- 2.58 microm. To test reactivity, the whole tail arteries or segments were perfused under constant flow and the reactivity to phenylephrine (PHE; 0.01-300 microg) was evaluated before and after removal of the endothelium or drug administration. The maximal response (Emax) and sensitivity (pED50) to PHE of the whole tail and the base segment increased after endothelium removal or treatment with 100 microM L-NAME, which suggests modulation by nitric oxide. Indomethacin (10 microM) and tetraethylammonium (5 mM) did not change the Emax or pED50 of these segments. PHE and L-NAME increased the pED50 of the middle and the tail end only and indomethacin did not change pED50 or Emax. Tetraethylammonium increased the sensitivity only at the tail end, which suggests a blockade of vasodilator release. Results indicate that the proximal segment of the tail artery possesses a diameter compatible with a conductance vessel, while the tail end has the diameter of a resistance vessel. In addition, the vascular reactivity to PHE in the proximal segment is nitric oxide-dependent, while the tail end is dependent on endothelium-derived hyperpolarizing factor.

Differential contributions of alpha-1 and alpha-2 adrenoceptors to vasoconstriction in mesenteric arteries and veins of normal and hypertensive mice

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenergic receptor agonists. In the present study, we tested the hypothesis that alpha(2)-adrenergic receptors (alpha(2)-ARs) contribute to in vitro agonist-induced constriction in veins but not arteries and that alpha(2)-AR function is down-regulated in mesenteric arteries and veins in deoxycorticosterone acetate-salt (DOCA-salt) hypertension. Norepinephrine (NE) concentration-response curves were similar in SHAM and DOCA-salt arteries and veins indicating that adrenergic reactivity of mesenteric blood vessels is not altered in murine DOCA-salt hypertension in vitro. Veins were 30-fold more sensitive to NE than arteries. The alpha(1)-AR antagonist, prazosin (0.003-0.3 microM), produced concentration-dependent rightward shifts of the NE concentration-response curves in arteries but not veins. The alpha(2)-AR agonists, clonidine and UK-14,304, did not constrict arteries or veins in the absence or presence of indomethacin (10 microM) and nitro-L-arginine (NLA; 100 microM). The alpha(2)-AR antagonists, yohimbine (0.003-0.3 microM) and rauwolscine (0.1 microM) did not affect NE responses in SHAM or DOCA-salt arteries but antagonized NE responses in veins. These data indicate that there are different alpha-AR contractile mechanisms in murine mesenteric arteries and veins. Alpha(1)-ARs, but not alpha(2)-ARs, mediate direct contractile responses in arteries and veins while alpha(2)-ARs contribute indirectly to NE-induced constrictions in veins but not arteries in vitro. There may be direct protein-protein interactions between alpha(1)- and alpha(2)-ARs or between their signaling pathways in veins. This contribution of alpha(2)-ARs may account for the greater sensitivity of veins compared to arteries to the contractile effects of NE.

Disturbed homeostasis in sodium-restricted mice heterozygous and homozygous for aldosterone synthase gene disruption

We have determined that differences in expression of aldosterone synthase (AS) affect responses to a low-salt diet. In AS-null mice (AS(-/-)), but not in wild-type, low salt significantly decreased plasma sodium and increased potassium. The increased urine volume (1.5xwild-type) and decreased urine osmolality (0.7xwild-type), present in AS(-/-) mice on normal salt, became more severe (2.3xwild-type and 0.5xwild-type) on low salt, but neither changed in wild-type. In both genotypes, plasma vasopressin was similar on normal and low salt, and desmopressin injection significantly increased urine osmolality. Renal mRNA levels for aquaporin 1 and 3 were unchanged by genotype or diet and epithelial sodium channel and Na(+)-K(+)-2Cl(-)-cotransporter by genotype. In AS(-/-) mice, aquaporin 2 mRNA increased on normal salt, whereas Na(+)Cl(-)-cotransporter and cortex K(+) channel mRNAs decreased on both diets. The low blood pressure of AS(-/-) mice was decreased further by low salt, despite additional increases in renin, intrarenal arterial wall thickness, and macula densa cyclogenase-2 mRNA. In AS(+/-) mice on normal salt, adrenal AS mRNA was slightly decreased (0.7xwild-type), but blood pressure was normal. On low salt, their blood pressure was less than wild-type (101+/-2 mm Hg versus 106+/-2 mm Hg), even though renin mRNA increased to 2xwild-type. We conclude that aldosterone is critical for urine concentration and maintenance of blood pressure and even a mild reduction of AS expression makes blood pressure sensitive to low salt, suggesting that genetic differences of AS levels in humans may influence how blood pressure responds to dietary salt.

Reduced NOS3 phosphorylation mediates reduced NO/cGMP signaling in mesenteric arteries of deoxycorticosterone acetate-salt hypertensive rats

Salt-sensitive hypertension is associated with impaired NO/cGMP signaling. We hypothesized that increased superoxide production by NADPH oxidase and altered endothelial NO synthase (NOS3) phosphorylation determine endothelial dysfunction in hypertension. Experiments tested if NO/cGMP signaling and NOS3 serine phosphorylation are decreased and NADPH oxidase activity is increased in mesenteric arteries from deoxycorticosterone acetate (DOCA)-salt rats compared with arteries from placebo rats. Concentration response curves to phenylephrine were performed in mesenteric arteries in the presence and absence of Nomega-nitro-L-arginine (LNA) and antioxidants to determine the influence of basal NO and superoxide production on vascular tone. LNA increased phenylephrine sensitivity in arteries from placebo, but not DOCA-salt rats, regardless of antioxidant treatment. To determine basal cGMP production, mesenteric arteries were incubated with 3-isobutyl-1-methylxanthine in the presence or absence of LNA, sodium nitroprusside (SNP), antioxidants, or tetrahydrobiopterin. NOS-dependent cGMP production was reduced in arteries from DOCA-salt rats compared with arteries from placebo rats and was not restored by acute treatment with antioxidants or tetrahydrobiopterin. SNP-induced cGMP production was similar between groups as was NADPH oxidase activity, measured by lucigenin chemiluminescence, in mesenteric arteries. Expression and phosphorylation of NOS3 were examined by Western blotting. Phosphorylation of NOS3 was decreased in arteries from DOCA-salt rats compared with placebo at serine residues 1179 and 635. These findings indicate that diminished NO/cGMP signaling in mesenteric arteries from DOCA-salt rats is caused by reduced phosphorylation of NOS3 at serine 1179 and serine 635, rather than NO scavenging by superoxide.

Increased reactivity of murine mesenteric veins to adrenergic agonists: functional evidence supporting increased alpha1-adrenoceptor reserve in veins compared with arteries

These studies examined adrenergic reactivity of mesenteric arteries and veins from deoxycorticosterone acetate-salt (DOCA-salt) hypertensive and sham control mice. We measured constrictions in unpressurized arteries and veins by monitoring vessel diameter using computer-assisted video micros-copy in vitro. Veins were more sensitive than arteries to the constricting effects of norepinephrine (NE) and phenylephrine (PE), but the alpha2-agonists clonidine and UK 14,304 [5-bromo-6-(2-imidazolin-2-yl-amino)-quinoxaline] did not constrict arteries or veins. Reactivity was not altered in arteries or veins from DOCA-salt mice. We next investigated the mechanism of increased venous reactivity to NE and PE by studying desensitization to maximum concentrations of NE and PE. Sham arteries desensitized to NE and PE more than DOCA-salt arteries, whereas DOCA-salt and sham veins maintained 80% of the initial NE and PE constriction. To determine whether the increased reactivity and resistance to desensitization in veins was due to a greater alpha-adrenoceptor reserve, vessels were incubated with the alkylating agent phenoxybenzamine (PBZ; 0.3, 3, 10, and 30 nM). The NE-elicited initial constriction was reduced by PBZ (3, 10, and 30 nM) in sham but only by PBZ (30 nM) in DOCA-salt veins. All doses of PBZ blocked NE responses in sham and DOCA-salt arteries. These data suggest that mesenteric veins express more alpha1-adrenoceptors than arteries, accounting for greater reactivity and resistance to desensitization compared with arteries.

African Americans with LVH demonstrate depressed sensitivity of the coronary microcirculation to stimulated relaxation

Excess coronary heart disease morbidity and mortality among African Americans remains an important yet unexplained public health problem. We hypothesized that adverse outcome is in part due to intrinsic or acquired abnormalities in coronary endothelial function and vasoreactivity. We compared dose-response curves relating changes in coronary blood flow and epicardial diameter to graded infusions of acetylcholine in 50 African American and 65 white subjects with hypertensive left ventricular hypertrophy (LVH) and normal coronary arteries. These groups were similar for age, body mass index, mean arterial pressure, and indexed left ventricular mass. The same protocol was conducted in 24 normotensive African American and 56 similar white subjects. We found significant depression in the coronary blood flow dose-response curve relation among African Americans when compared with white subjects with similar LVH (P<0.03). Racial differences were observed at all doses of acetylcholine but were less precisely estimated at the highest dose. The same testing among normotensive subjects revealed similar dose-response curves with no significant effect of race. Qualitatively similar results were found with respect to coronary diameter. Adenosine responses, a measure of endothelium-independent function, were similar after partitioning by LVH. Our study demonstrates that there are racial differences in sensitivity of coronary arteries to acetylcholine-stimulated relaxation among those with LVH. These results provide a mechanism whereby racial differences in coronary vasoreactivity might contribute to adverse coronary heart disease outcome among African Americans, a group in whom LVH is prevalent.

Aldosterone regulates the Na-K-2Cl cotransporter in vascular smooth muscle

Aldosterone increases cation transport and contractility of vascular smooth muscle, but the specific transporter involved and how it is linked to smooth muscle tone is unknown. Because the Na-K-2Cl cotransporter (NKCC1) contributes to vascular smooth muscle contraction and is regulated by vasoactive compounds, we sought to determine whether this transporter is a target of aldosterone in rat aorta. Treatment of adrenalectomized rats with aldosterone for 7 days resulted in a 63% increase in NKCC1 activity as measured by bumetanide-sensitive efflux of 86Rb+. Treatment of normal aortas in culture with aldosterone for 3 and 7 days resulted in 29% and 47% increases in NKCC1 activity, respectively. Aldosterone had no acute effect on 86Rb+ efflux. Stimulation of NKCC1 was blocked by spironolactone, a mineralocorticoid receptor antagonist, but not by RU38486, a glucocorticoid receptor antagonist. Aldosterone did not augment the stimulation of NKCC1 by phenylephrine and did not increase NKCC1 mRNA as determined by real-time polymerase chain reaction. We conclude that aldosterone regulates the Na-K-2Cl cotransporter in vascular smooth muscle through classic mineralocorticoid receptors but not through changes in the abundance of NKCC1 mRNA. This could account for the increase in Na+, K+, and Cl- fluxes previously observed in vascular smooth muscle from mineralocorticoid-treated animals and may contribute to increased vascular tone.

Differential alterations in sympathetic neurotransmission in mesenteric arteries and veins in DOCA-salt hypertensive rats

Sympathetic control of arteries and veins may be altered in hypertension. To test this hypothesis, constrictions of mesenteric arteries and veins caused by nerve stimulation and by norepinephrine (NE) and ATP were studied in vitro in tissues from deoxycorticosterone acetate (DOCA)-salt hypertensive and sham normotensive rats. In DOCA-salt arteries, the maximum neurogenic response was greater than that in sham arteries. The P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 10 microM), greatly reduced neurogenic responses in sham but not DOCA-salt arteries. The alpha1-adrenergic receptor antagonist, prazosin (0.1 microM), inhibited responses in DOCA-salt but not sham arteries. Concentration-response curves for norepinephrine and ATP were similar in sham and DOCA-salt arteries, indicating that reactivity to sympathetic vasoconstrictor transmitters was not changed in DOCA-salt arteries. Neurogenic constrictions in sham and DOCA-salt veins were similar in amplitude, and they were completely blocked by prazosin. However, concentration-response curves for norepinephrine in DOCA-salt veins were right-shifted compared to those in sham veins. Cocaine (10 microM) and corticosterone (10 microM) caused a leftward shift in norepinephrine concentration-response curves in DOCA-salt but not sham veins. Norepinephrine content was decreased in DOCA-salt arteries and veins, and there was an increased norepinephrine transporter (NET) level in DOCA-salt veins. These data indicate that, in DOCA-salt hypertension, there is an increased norepinephrine release from sympathetic nerves associated with mesenteric arteries and veins. In arteries, this results in an increase in the amplitude of neurogenic constrictions. In veins, increased norepinephrine release maintains neurogenic constrictions in the presence of increased NET levels.

Impaired relaxation to acetylcholine in 2K-1C hypertensive rat aortas involves changes in membrane hyperpolarization instead of an abnormal contribution of endothelial factors

The contribution of endothelial factors and mechanisms underlying decreased acetylcholine-induced relaxation and endothelial inhibitory action on phenylephrine-induced contraction were evaluated in aortas of two-kidney, one-clip hypertensive (2K-1C) and normotensive (2K) rats. Relaxation induced by acetylcholine in 2K-1C precontracted by phenylephrine was lower [Maximum Effect (ME): 71.33+/-3.36%; pD(2): 7.050+/-0.03] than in 2K (ME: 95.26+/-1.59%; pD(2): 7.31+/-0.07). This response was abolished by N(G)-nitro-L-arginine (L-NNA) in 2K-1C, but was only reduced in 2K (ME: 29.21+/-9.28%). Indomethacin had no effect in 2K-1C, and slightly attenuated acetylcholine-induced relaxation in 2K. The combination of L-NNA and indomethacin almost abolished acetylcholine-induced relaxation in 2K-1C, while in 2K, the inhibition (ME: 56.61+/-8.95%) was lower than the effect of L-NNA alone. During the KCl-induced precontraction, 2K and 2K-1C aortas showed similar acetylcholine-induced relaxation (43.50+/-5.64% vs. 41.60+/-4.36%), which was abolished by L-NNA. The levels of cGMP produced in response to acetylcholine were not different between 2K and 2K-1C. The sensitivity to sodium nitroprusside was lower in phenylephrine-precontracted aortas from 2K-1C than 2K, as showed by the pD(2) values (7.72+/-0.20 vs. 8.59+/-0.17), and this difference was abolished in aortas precontracted by KCl. The membrane potential was less negative in 2K-1C than in 2K (-41.57+/-1.19 vs. -51.00+/-1.13 mV) and hyperpolarization induced by acetylcholine was lower in 2K-1C than in 2K aortas (6.00+/-0.66 vs. 13.27+/-1.61 mV). Phenylephrine-induced contraction in aortas with endothelium was similar in both groups, and increased by the endothelium removal. This increase was lower in 2K-1C (from 1.32+/-0.06 to 1.90+/-0.21 g) than 2K (from 1.49+/-0.07 to 2.83+/-0.18 g). L-NNA and the endothelium removal had similar effect in 2K-1C (1.85+/-0.18 g) and were lower in 2K (2.18+/-0.20 g). Indomethacin decreased phenylephrine-induced contraction only in 2K. In conclusion, our major finding was a selective defect in smooth muscle membrane hyperpolarization, which could explain the decreased relaxation to acetylcholine and the attenuated inhibitory effect of endothelium on the contractile function in 2K-1C aortas.

Contribution of sarcoplasmic reticulum calcium uptake and L-type calcium channels to altered vascular responsiveness in the aorta of renal hypertensive rats

This study examined whether alterations in intracellular or extracellular Ca2+ mobilization were related to differences in caffeine and phenylephrine (PHE)-induced contractions between two-kidney. one-clip hypertensive (2K-1C) and normotensive (2K) rat aortas. After depletion and reloading of intracellular Ca2+ stores, caffeine and PHE-induced contractions in Ca2+-free solution were increased in 2K-1C. Thapsigargin reduced the contraction to caffeine in 2K-1C and 2K with similar sensitivity. PHE-induced contraction in 1.6-mM Ca2+ solution was decreased in 2K-1C, and nifedipine was less effective in lowering this response. The responsiveness to extracellular Ca2+ was decreased in 2K-1C hypertensive rat aortas. Our results indicate an increased intracellular Ca2+ stores that are not related to alteration in Ca2+-ATPase function and a lower contribution of L-type channels to the contraction of 2K-1C aortas.

Relations of vascular calcium channels with blood pressure and endothelium in hypertension and with aging

To investigate the relationships between the activity in potential operated Ca2+ channels (POC), blood pressure, and endothelium in hypertension, we tested the contractile responses to a Ca2+ channel agonist Bay K 8644 (BAY K) in aorta from deoxycorticosterone-acetate-saline (DOCA-S) and reduced renal mass-saline (RRM-S) hypertensive rats. The effects of mechanical rubbing, N omega-Nitro-L-Arginine Methyl Ester (l-NAME) and indomethacin were also examined. Sensitivity to BAY K increased in experimental rats before they became hypertensive and contractile responses were enhanced as hypertension developed. Force development to BAY K was correlated with blood pressure levels. Endothelium removal enhanced the contractile response to BAY K. L-NAME, but not indomethacin, potentiated the response to BAY K. Contractile response to BAY K was negatively correlated with relaxation to acetylcholine. An enhanced contractile response to BAY K was observed also in aged rats. Enhanced activation of vascular POC in hypertension results from elevated blood pressure and partly from diminished inhibitory action of endothelium. Senescence also enhances vascular POC activity.

Endothelin-1 enhances pressor responses to norepinephrine: involvement of endothelin-B receptor

We investigated the effects of endothelin-1 (ET-1) on pressor responses to norepinephrine (NE) in perfused rat mesenteric arteries. Perfusion of the arteries with a subpressor dose of ET-1 (3 x 10(-10) M) for 15 min markedly enhanced the pressor responses to NE (10(-6) and 3 x 10(-6) M), and this effect was significantly prevented by BQ788 (10(-6) M) but not by FR139317 (10(-6) M). In DOCA-salt hypertensive rats, exogenous ET-1 had little effect on pressor responses to NE. Pressor responses to NE (10(-6) M) were significantly increased in DOCA-salt rats compared with those in normotensive rats. This increased response to NE was reduced to the level of normotensive rats by BQ788. FR139317 was without effect on the increased responses. These results suggest that ET-1 enhances contractile responses to NE through ETB receptors. Moreover, this phenomenon is stimulated tonically by endogenous ET-1 in DOCA-salt hypertensive rats and may contribute to the maintenance of hypertension in these rats.

Effects of endothelin-1 on norepinephrine-induced vasoconstriction in deoxycorticosterone acetate-salt hypertensive rats

We investigated the effects of endothelin-1 on pressor responses to norepinephrine in perfused mesenteric arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. The response to norepinephrine (10(-6) M) was significantly increased in DOCA-salt rats compared with that in uninephrectomized control rats. Perfusion of the arteries with subpressor dose of endothelin-1 (3 x 10(-10) M) for 15 min markedly enhanced the pressor responses to norepinephrine (10(-6) and 3 x 10(-6) M) in control rats and this effect was significantly prevented by BQ788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1-methoxycarbonyl-tryptophanyl-D-norleucine] (10(-6) M), but not by FR139317 ((R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl]amino-4-+ ++methylpentanoyl]amino-3-[3-(1-methyl-1H-indoyl)]propionyl]ami no-3-(2-pyridyl)propionic acid) (10(-6) M). In DOCA-salt hypertensive rats, increased pressor response to norepinephrine was declined to the level of control rats by BQ788, but not by FR139317. In contrast to the case seen with control rat, exogenous endothelin-1 had little effect on pressor responses to norepinephrine in the arteries of DOCA-salt hypertensive rats. Total immunoreactive endothelin content in the arteries of DOCA-salt hypertensive rats was significantly higher than that of uninephrectomized control rats. These results suggest that endothelin-1 enhances contractile responses to norepinephrine through endothelin ETB receptor. Moreover, this phenomenon is stimulated tonically by endogenous endothelin-1 in DOCA-salt hypertensive rats and may contribute to the maintenance of hypertension in DOCA-salt rats.

Reactivity of the isolated perfused rat tail vascular bed

Isolated segments of the perfused rat tail artery display a high basal tone when compared to other isolated arteries such as the mesenteric and are suitable for the assay of vasopressor agents. However, the perfusion of this artery in the entire tail has not yet been used for functional studies. The main purpose of the present study was to identify some aspects of the vascular reactivity of the rat tail vascular bed and validate this method to measure vascular reactivity. The tail severed from the body was perfused with Krebs solution containing different Ca2+ concentrations at different flow rates. Rats were anesthetized with sodium pentobarbital (65 mg/kg) and heparinized (500 U). The tail artery was dissected near the tail insertion, cannulated and perfused with Krebs solution plus 30 microM EDTA at 36 degrees C and 2.5 ml/min and the procedures were started after equilibration of the perfusion pressure. In the first group a dose-response curve to phenylephrine (PE) (0.5, 1, 2 and 5 micrograms, bolus injection) was obtained at different flow rates (1.5, 2.5 and 3.5 ml/min). The mean perfusion pressure increased with flow as well as PE vasopressor responses. In a second group the flow was changed (1.5, 2, 2.5, 3 and 3.5 ml/min) at different Ca2+ concentrations (0.62, 1.25, 2.5 and 3.75 mM) in the Krebs solution. Increasing Ca2+ concentrations did not alter the flow-pressure relationship. In the third group a similar protocol was performed but the rat tail vascular bed was perfused with Krebs solution containing PE (0.1 microgram/ml). There was an enhancement of the effect of PE with increasing external Ca2+ and flow. PE vasopressor responses increased after endothelial damage with air and CHAPS, suggesting an endothelial modulation of the tone of the rat tail vascular bed. These experiments validate the perfusion of the rat tail vascular bed as a method to investigate vascular reactivity.

Calcium handling by vascular myocytes in hypertension

Calcium ions (Ca2+) trigger the contraction of vascular myocytes and the level of free intracellular Ca2+ within the myocyte is precisely regulated by sequestration and extrusion mechanisms. Extensive evidence indicates that a defect in the regulation of intracellular Ca2+ plays a role in the augmented vascular reactivity characteristic of clinical and experimental hypertension. For example, arteries from spontaneously hypertensive rats (SHR) have an increased contractile sensitivity to extracellular Ca2+ and intracellular Ca2+ levels are elevated in aortic smooth muscle cells of SHR. We hypothesize that these changes are due to an increase in membrane Ca2+ channel density and possibly function in vascular myocytes from hypertensive animals. Several observations using various experimental approaches support this hypothesis: 1) the contractile activity in response to depolarizing stimuli is increased in arteries from hypertensive animals demonstrating increased voltage-dependent Ca2+ channel activity in hypertension; 2) Ca2+ channel agonists such as Bay K 8644 produce contractions in isolated arterial segments from hypertensive rats and minimal contraction in those from normotensive rats; 3) intracellular Ca2+ concentration is abnormally increased in vascular myocytes from hypertensive animals following treatment with Ca2+ channel agonists and depolarizing interventions, and 4) using the voltage-clamp technique, the inward Ca2+ current in arterial myocytes from hypertensive rats is nearly twice as large as that from myocytes of normotensive rats. We suggest that an alteration in Ca2+ channel function and/or an increase in Ca2+ channel density, resulting from increased channel synthesis or reduced turnover, underlies the increased vascular reactivity characteristic of hypertension.

The effects of cyclopiazonic acid on intracellular Ca2+ in aortic smooth muscle cells from DOCA-hypertensive rats

We tested the hypothesis that cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, increases intracellular Ca2+ concentration ([Ca2+]) in aortic myocytes and that the increase in [Ca2+]i is higher in aortic cells from deoxycorticosterone acetate (DOCA)-hypertensive rats. Male Sprague-Dawley rats, 250-300 g, underwent uninephrectomy, received a silastic implant containing DOCA (200 mg/kg) and had free access to water supplemented with 1.0% NaCl and 0.2% KCl. Control rats were also uninephrectomized, received normal tap water, but no implant. Intracellular Ca2+ measurements were performed in aortic myocytes isolated from normotensive (Systolic blood pressure = 120 +/- 3 mmHg; body weight = 478 +/- 7 g, N = 7) and DOCA-hypertensive rats (195 +/- 10 mmHg; 358 +/- 16 g, N = 7). The effects of CPA on resting [Ca2+]i and on caffeine-induced increase in [Ca2+]i after [Ca2+]i depletion and reloading were compared in aortic cells from DOCA and normotensive rats. The phasic increase in [Ca2+]i induced by 20 mM caffeine in Ca(2+)-free buffer was significantly higher in DOCA aortic cells (329 +/- 36 nM, N = 5) compared to that in normotensive cells (249 +/- 16 nM, N = 7, P < 0.05). CPA (3 microM) inhibited caffeine-induced increases in [Ca2+]i in both groups. When the cells were placed in normal buffer (1.6 mM Ca2+, loading period), after treatment with Ca(2+)-free buffer (depletion period), an increase in [Ca2+]i was observed in DOCA aortic cells (45 +/- 11 nM, N = 5) while no changes were observed in normotensive cells. CPA (3 microM) potentiated the increase in [Ca2+]i (122 +/- 30 nM, N = 5) observed in DOCA cells during the loading period while only a modest increase in [Ca2+]i (23 +/- 10 nM, N = 5) was observed in normotensive cells. CPA-induced increase in [Ca2+]i did not occur in the absence of extracellular Ca2+ or in the presence of nifedipine. These data show that CPA induces Ca2+ influx in aorta from both normotensive and DOCA-hypertensive rats. However, the increase in [Ca2+]i is higher in DOCA aortic cells possibly due to an impairment in the mechanisms that control [Ca2+]i. The large increase in [Ca2+]i in response to caffeine in DOCA cells probably reflects a greater storage of Ca2+ in the SR.

Differential contribution of endothelial function to vascular reactivity in conduit and resistance arteries from deoxycorticosterone-salt hypertensive rats

The purpose of these studies was to compare changes in conduit and resistance artery function in deoxycorticosterone-salt hypertensive rats. We hypothesized that if there was a common mechanism producing changes in vascular function in hypertension, then there would be similar alterations in reactivity of conduit and resistance arteries. Helically cut strips of common carotid artery were prepared for measurement of isometric force generation, and segments of small mesenteric arteries were pressurized for video dimension analysis. Sensitivity of arteries to phenylephrine and acetylcholine was determined. Carotid arteries from deoxycorticosterone-salt hypertensive rats were more sensitive to phenylephrine than arteries from control rats, whereas mesenteric resistance arteries from hypertensive rats were less sensitive to phenylephrine. In carotid arteries, endothelial denudation or incubation with N psi-nitro-L-arginine increased phenylephrine sensitivity in control rats to the level seen in deoxycorticosterone-salt rats. These manipulations had no effect on phenylephrine sensitivity in arteries from deoxycorticosterone-salt rats. In mesenteric resistance arteries, endothelium denudation normalized the depressed phenylephrine sensitivity in arteries from hypertensive rats but had no effect on arteries from normotensive rats. This depressed phenylephrine sensitivity in deoxycorticosterone-salt mesenteric arteries was not reversed by incubation with Npsi-nitro-L-arginine. Acetylcholine-induced relaxation was depressed in carotid arteries from deoxycorticosterone-salt hypertensive rats, and Npsi-nitro-L-arginine blocked these relaxations. In contrast, acetylcholine relaxation in the mesenteric arteries from normotensive and hypertensive rats did not differ. N psi-nitro-L-arginine slightly but significantly attenuated acetylcholine dilation only in mesenteric resistance arteries from the hypertensive rats. We conclude that qualitatively different changes in vasoconstrictor sensitivity to phenylephrine occur in carotid arteries and mesenteric resistance arteries of deoxycorticosterone-salt hypertensive rats. The increased phenylephrine sensitivity in carotid arteries in this model of hypertension is due to the loss of endothelium-derived nitric oxide production. In contrast, the decreased phenylephrine sensitivity in mesenteric resistance arteries from deoxy-corticosterone-salt rats is due to a non-nitric oxide-mediated influence of the endothelium that is absent in arteries from normotensive rats.

Sarcoplasmic reticulum Ca2+ uptake is not decreased in aorta from deoxycorticosterone acetate hypertensive rats: functional assessment with cyclopiazonic acid

Ca2+ plays a major role in vascular contraction, and a defect in intracellular Ca2+ regulation has been associated with increased vascular reactivity in hypertension. To test the hypothesis that the sarcoplasmic reticulum does not adequately buffer Ca2+ in deoxycorticosterone acetate (DOCA) hypertension, contractile experiments were performed with a specific inhibitor of the sarcoplasmic reticulum Ca2+ ATPase, cyclopiazonic acid (CPA). Contractile force in aortic strips from DOCA and control rats was measured, using standard muscle bath procedures, to evaluate (i) Ca2+ handling, assessing caffeine and serotonin (5HT) induced contractions in Ca(2+)-free buffer and (ii) relaxation rate after 5HT washout. Contractile responses elicited with 5HT (3 x 10(-6) mol/L) and caffeine (20 mmol/L) were greater in DOCA than in control arteries. CPA (1 x 10(-7) to 3 x 10(-5) mol/L) reduced phasic contractions to 5HT and caffeine in DOCA and control aorta, and no differences in the IC50 values were observed. Aortae from DOCA rats contracted when placed in normal buffer, subsequent to treatment with Ca(2+)-free buffer, but control aortae did not. CPA potentiated these responses in DOCA aorta and only caused a modest contraction in control aorta. CPA-induced contraction did not occur in Ca(2+)-free buffer, and it was inhibited by nifedipine (IC50 = 4 x 10(-9) mol/L). The relaxation rate, after 5HT washout (3 x 10(-6) mol/L), was increased in DOCA aorta (2.6 +/- 0.3 min) compared with control (1.7 +/- 0.2 min), and CPA (10(-5) mol/L) increased the relaxation rate in both groups. The results support the hypothesis of defective Ca2+ handling in DOCA hypertension. However, an increased Ca2+ influx, and not a decreased buffering ability of the sarcoplasmic reticulum, contributes to the enhanced vascular reactivity observed in DOCA hypertension.

Dietary L-arginine attenuates blood pressure in mineralocorticoid-salt hypertensive rats

The present study was designed to investigate the influence of dietary L-arginine supplementation on blood pressure and on ex vivo vascular reactivity in mineralocorticoid-salt (DOCA-salt) hypertensive rats. Systolic blood pressure and heart rate were determined throughout the experimental period in unanaesthetized rats. Plasma and urine electrolyte levels were measured. Vasoconstrictor response to noradrenaline and vasodilator responses to acetylcholine and sodium nitroprusside were evaluated in the isolated perfused mesenteric vascular bed. DOCA-salt hypertensive rats were divided into 2 groups: a control group and a treated group receiving 0.8% L-arginine supplementation in drinking water. Dietary L-arginine supplementation attenuated systolic blood pressure in conscious DOCA-salt hypertensive rats, but did not modify heart rate. Plasma calcium and sodium concentrations and urinary magnesium excretion were decreased by L-arginine supplementation. Noradrenaline-induced vasoconstriction decreased and acetylcholine-induced vasodilatation increased, whereas sodium nitroprusside-induced vasodilatation was not modified, in the L-arginine-supplemented rats. It is concluded that dietary L-arginine supplementation in the diet lowers systolic blood pressure in DOCA-salt hypertensive rats, probably through vascular action.

Corticosterone metabolism and effects on angiotensin II receptors in vascular smooth muscle

It has been postulated that mineralocorticoids can bind to corticosteroid receptors in the kidney, because glucocorticoids are metabolized to inactive compounds. The present study was performed to delineate glucocorticoid metabolism by rat vascular tissue and to determine the activity of these metabolites. Vascular segments converted 25% to 30% of corticosterone (compound B), the major glucocorticoid in the rat, to 11-dehydrocorticosterone (compound A) but not to aldosterone or 6 beta-hydroxycorticosterone. In cultured vascular smooth muscle cells, 10% of compound B was converted to compound A, whereas > 60% of compound A was converted to compound B. The 11 beta-hydroxysteroid dehydrogenase inhibitor carbenoxolone (1 mumol/L) completely blocked conversion in both directions. Whereas 6 beta-hydroxycorticosterone did not upregulate angiotensin II receptor binding (a marker for corticosteroid action in vascular smooth muscle), compound A caused concentration-dependent upregulation. Compound A was almost (75%) as effective and as potent as compound B in upregulating angiotensin II binding. Upregulation elicited by exposure to compound A persisted in the presence of 1 mumol/L carbenoxolone, which completely prevented the conversion of compound A to compound B. Compound A, even in the presence of carbenoxolone, effected other glucocorticoid actions by inhibiting cell growth and potentiating angiotensin II-stimulated inositol phosphate formation. In summary, compound B and compound A are interconverted in vascular tissue, and the latter displays significant glucocorticoid action. The concentration excess of compound B in the circulation and the activity of its metabolite compound A will make it difficult for mineralocorticoids to gain access to corticosteroid receptors in the vasculature.

Sustained hypertension in Dahl rats. Negative correlation of agonist response to blood pressure

The perfused mesenteric vasculature of Dahl salt-sensitive rats on a high salt diet for 5 days (prehypertensive or early hypertensive) is selectively supersensitive to norepinephrine. The present goal was to determine whether that supersensitivity was maintained as hypertension developed. Littermates of salt-sensitive and salt-resistant rats (Dahl Brookhaven strain) were followed on low or high salt for up to 6 weeks. Systolic blood pressure was elevated in the salt-sensitive, high salt rats after 3 or 6 weeks but not after 5 days of the diet. The perfused mesenteric vascular beds from salt-sensitive rats were supersensitive to norepinephrine and nerve stimulation but not to potassium chloride when the rats had been maintained for 5 days or 3 weeks on the high salt diet. However, responses to norepinephrine declined after 6 weeks of the high salt diet. To determine whether sustained high blood pressure has a negative effect on mesenteric vascular responses, we conducted additional experiments with perfused mesenteric vascular beds from salt-sensitive Brookhaven (high salt, 5 weeks) and Rapp (high salt, 6 weeks) animals. Both groups exhibited significant negative correlations between in vivo systolic pressure and maximal responses of mesenteric vessels to norepinephrine and potassium chloride. We suggest that sustained hypertension in Dahl rats has a negative effect on the contractility of the mesenteric arterial system that, by 5 to 6 weeks, masks the initial supersensitivity to norepinephrine. No effects of any diet on the dilating responses of the mesenteric vessels to acetylcholine were observed in any group.

The effect of high calcium intake on intracellular free [Ca2+] and Na(+)-H+ exchange in DOC-NaCl-hypertensive rats

The effects of calcium supplementation on blood pressure, intracellular free calcium concentration ([Ca2+]i) and rate of Na(+)-H+ exchange were studied in DOC-NaCl-hypertensive rats. All the animals were uninephrectomized and divided into two main groups: the first group received deoxycorticosterone (DOC) (25 mg/kg, s.c.) once a week and had 0.7% NaCl as drinking fluid while the other received equal volumes of saline and tap water to drink. The animals were further divided according to dietary calcium intake: in the Control and DOC groups the chow contained 1.1% calcium, in the Calcium and DOC+Calcium groups, 2.5%. After 6 and 8 weeks, blood pressure in the DOC group was higher than in the Control group; on the other hand, the development of hypertension was attenuated in the DOC+Calcium compared with the DOC group. The Control and Calcium groups did not differ from each other. Platelets and lymphocytes were used as experimental models to study changes in the regulation of [Ca2+]i, evaluated by fluorescent indicators indo-1 and quin-2. In lymphocytes, basal [Ca2+]i was highest in the DOC group, but similar in DOC+Calcium and Control groups. In platelets, both basal and thrombin-stimulated [Ca2+]i were higher in the DOC and DOC+Calcium groups than in the Control group. In both cell types [Ca2+]i was similar in Control and Calcium groups. In addition, platelets were used to study the ability of the cells to recover from intracellular acidification by first blocking the Na(+)-H+ exchange in a Na(+)-free medium and then restarting the exchange mechanism by increasing the extracellular Na+ concentration at constant speed.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular responsiveness in rats resistant to aldosterone-salt hypertension

Wistar-Furth rats have been shown to be resistant to mineralocorticoid-salt hypertension, but the mechanism for this resistance is unknown. In the current experiments, adult male Wistar and Wistar-Furth rats were given a subcutaneous aldosterone infusion (0.15 microgram/hr) for 4 weeks, and changes in blood pressure and vascular reactivity were studied. Rats received a 1% NaCl, 0.2% KCl solution to drink. After 4 weeks of aldosterone infusion, systolic blood pressure measured using a tail-cuff technique had increased by 60 mm Hg in Wistar rats but was unchanged in Wistar-Furth rats. Hypokalemia occurred in both strains in response to the aldosterone infusion. Isolated, helically cut strips of common carotid artery and aorta were prepared for isometric force recording. Cumulative concentration-response curves to norepinephrine, serotonin, KCl, calcium, nitroprusside, and acetylcholine were performed in carotid artery strips, and concentration-response curves to ouabain were performed in aortic strips. Increased vascular contractile sensitivity to KCl, ouabain, norepinephrine, and serotonin was observed in vessels from Wistar rats treated with aldosterone and salt. The same treatment in Wistar-Furth rats produced only increased vascular sensitivity to ouabain and serotonin, and these changes were of smaller magnitude than those seen in Wistar rats. Aldosterone-salt treatment produced decreased vascular sensitivity to acetylcholine and nitroprusside in both Wistar and Wistar-Furth rats. These results support the hypothesis that resistance of Wistar-Furth rats to aldosterone-salt hypertension is due to resistance to the effects of aldosterone-salt treatment that normally result in increased vasoconstrictor sensitivity.

The pathogenesis of DOCA-salt hypertension

Deoxycorticosterone acetate (DOCA) is an agent commonly used to induce hypertension in experimental animals. This form of hypertension is dependent on altered regulation of central pressor mechanisms including the brain renin-angiotensin system. Additionally, there are characteristic changes involving the cardiovascular system and baroreflex responses. This review will discuss aspects of the pathogenesis of DOCA hypertension and the effect of various antihypertensive agents on the development of this form of hypertension.

Chlorthalidone reduces vascular hyperresponsiveness in DOCA-salt hypertensive rats

The mechanisms of anti-hypertensive effect of diuretics remain unknown. The purpose of this study was to test the hypothesis that long-term treatment with chlorthalidone decreases the responsiveness of resistance vessels to neurohormones. The study was performed in deoxycorticosterone acetate (DOCA)-salt hypertensive rats with and without treatment with chlorthalidone (Chlor. 8 mg/day, for 20 days). Resting mean arterial pressure in freely moving state was significantly reduced in DOCA-salt-Chlor rats when compared to DOCA-salt rats (116 +/- 3 vs 147 +/- 7 mmHg, respectively). Chlorthalidone treatment reduced the high plasma sodium content observed in DOCA-salt rats to the same levels observed in normotensive control groups. Results obtained in isolated perfused mesenteric arteries showed: a) the increase in perfusion pressure elicited by norepinephrine (NE), serotonin (SE) and vasopressin (VP) was significantly greater in DOCA-salt than in DOCA-salt + Chlor rats or control normotensive rats; b) the endothelium removal increased the pressor responses to NE, SE and VP in a similar way in all groups. These data provide evidence that long-term chlorthalidone treatment reduces vascular hyperresponsiveness to these neurohormones. In addition, these results indicate that this reduction in vascular hyperresponsiveness, associated with a decrease in extracellular sodium level, could be a possible mechanism by which the diuretics reduce the high blood pressure.

Isolation and purification of a vascular hyperreactivity factor from rabbit kidney cortex

A compound capable of amplifying the threshold pressor response to norepinephrine (NE) was obtained from rabbit kidney cortex. This compound was purified and characterized using a series of techniques including gel filtration, ion exchange chromatography, preparative electrofocusing, HPLC, FAB mass spectrometry (FAB-MS), and Fourier transform infrared spectrometry. From this, an acid/heat stable (6N HC1, 160 degrees C, 24 hours), low molecular weight (ca 147) compound with a strong (+) charge density (Pi greater than 10) was identified. When injected into assay rats (i.v.), this compound amplified the pressor response to fixed doses of NE. Taken together, this compound exhibits nearly identical characteristics (i.e. acid/heat stability, structure, charge and biologic activity to the naturally occurring polyamine spermidine (SPD-145.6 daltons). Moreover, bolus injections of SPD (10 micrograms, i.v.) amplified the pressor response to NE over a range of doses from 5-25 ng.

Specific supersensitivity of the mesenteric vascular bed of Dahl salt-sensitive rats

Dahl salt-sensitive (DS) and salt-resistant (DR) rats were maintained on a diet containing normal (0.45%) or high (7%) salt for 5 days. The DS rats had slightly higher systolic blood pressures than DR rats, although a high salt diet failed to significantly elevate pressure in either group when compared with their appropriate (low salt diet) controls. The sensitivity of the isolated, perfused mesenteric vasculature from DS rats fed a high salt diet to nerve stimulation was greater when compared with all other groups in the presence or absence of cocaine (1 microM). A similar difference in sensitivity between high salt DS rats and high salt DR rats to bolus injections of norepinephrine was observed only in the presence of cocaine. The change in sensitivity was characterized by a leftward shift of the dose-response curve without a change in maximum response. No difference in sensitivity between the high salt DS group and any other treatment group was observed in response to the pressor agents KCl, angiotensin II, 5-hydroxytryptamine or the depressor agent acetylcholine. These data indicate that DS rats on a short-term, high salt diet possess a significant and specific elevation in sensitivity to nerve stimulation and norepinephrine in the absence of an increase in blood pressure. Differences in the effectiveness of cocaine among the groups suggest that differences may exist in neuronal uptake (uptake 1).(ABSTRACT TRUNCATED AT 250 WORDS)