Vascular reactivity in the spontaneously hypertensive rat. Effect of high pressure stress and extracellular calcium

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

Impaired vascular β-adrenergic relaxation in spontaneously hypertensive rats: The differences between conduit and resistance arteries

It was suggested that impaired β-adrenergic relaxation in spontaneously hypertensive rats (SHR) might contribute to their high blood pressure (BP). Our study was focused on isoprenaline-induced dilatation of conduit femoral or resistance mesenteric arteries and on isoprenaline-induced BP reduction in SHR and Wistar-Kyoto rats (WKY). We confirmed decreased β-adrenergic relaxation of SHR femoral arteries due to the absence of its endothelium-independent component, whereas endothelium-dependent component of β-adrenergic smooth muscle relaxation was similar in both strains. Conversely, isoprenaline-induced relaxation of resistance mesenteric arteries was similar in both strains and this was true for endothelium-dependent and endothelium-independent components. We observed moderately reduced sensitivity of SHR mesenteric arteries to salmeterol (β2-adrenergic agonist) and this strain difference disappeared after endothelium removal. However, there was no difference in mesenteric arteries relaxation by dobutamine (β1-adrenergic agonist) which was independent of endothelium. The increasing isoprenaline doses elicited similar BP decrease in both rat strains, although BP sensitivity to isoprenaline was slightly decreased in SHR. The blockade of cyclooxygenase (indomethacin) and NO synthase (L-NAME) further reduced BP sensitivity to isoprenaline in SHR. On the other hand, salmeterol elicited similar BP decrease in both strains and the blockade of cyclooxygenase and NO synthase increased BP sensitivity to salmeterol in SHR as compared to WKY. In conclusion, attenuated β-adrenergic vasodilatation of conduit arteries of SHR but similar β-adrenergic relaxation of resistance mesenteric arteries from WKY and SHR and their similar BP response to β-adrenergic agonists do not support major role of altered β-adrenergic vasodilatation for high BP in genetic hypertension.

Calcimimetic R-568 vasodilatory effect on mesenteric vascular beds from normotensive (WKY) and spontaneously hypertensive (SHR) rats. Potential involvement of vascular smooth muscle cells (vSMCs)

The potential role of calcimimetics as vasculotropic agents has been suggested since the discovery that calcium sensing receptors (CaSRs) are expressed in cardiovascular tissues. However, whether this effect is CaSR-dependent or -independent is still unclear. In the present study the vascular activity of calcimimetic R-568 was investigated in mesenteric vascular beds (MVBs) isolated from Spontaneously Hypertensive rats (SHR) and the relative age-matched Wistar-Kyoto (WKY) control rats. Pre-constricted MBVs were perfused with increasing concentrations of R-568 (10 nM– 30 μM) resulting in a rapid dose-dependent vasodilatation. However, in MVBs from SHR this was preceded by a small but significant vasoconstriction at lowest nanomolar concentrations used (10–300 nM). Pre-treatment with pharmacological inhibitors of nitric oxide (NO) synthase (NOS, L-NAME), K_Ca channels (CTX), cyclo-oxygenase (INDO) and CaSR (Calhex) or the endothelium removal suggest that NO, CaSR and the endothelium itself contribute to the R-568 vasodilatory/vasoconstrictor effects observed respectively in WKY/SHR MVBs. Conversely, the vasodilatory effects resulted by highest R-568 concentration were independent of these factors. Then, the ability of lower R-568 doses (0.1–1 μM) to activate endothelial-NOS (eNOS) pathway in MVBs homogenates was evaluated. The Akt and eNOS phosphorylation levels resulted increased in WKY homogenates and Calhex significantly blocked this effect. Notably, this did not occur in the SHR. Similarly, vascular smooth muscle cells (vSMCs) stimulation with lower R-568 doses resulted in Akt activation and increased NO production in WKY but not in SHR cells. Interestingly, in these cells this was associated with the absence of the biologically active dimeric form of the CaSR thus potentially contributing to explain the impaired vasorelaxant effect observed in response to R-568 in MVB from SHR compared to WKY. Overall, these findings provide new insight on the mechanisms of action of the calcimimetic R-568 in modulating vascular tone both in physiological and pathological conditions such as hypertension.

Age-related alterations in endothelial function of femoral artery in young SHR and WKY rats

The present study was designed to evaluate the effects of vascular aging in juvenescence on endothelial function in femoral arteries and to assess differences between normotensive and hypertensive rats. The aim of the study was to determine if age affected nitric oxide- (NO-) mediated relaxations in normotensive and hypertensive rats. Juvenile (7-week-old) and young adult (22-week-old) male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were used in this study. Femoral artery (FA) reactivity was determined by wire myograph and NO synthase activity by conversion of [³H]-L-arginine. During juvenescence systolic blood pressure (tail-cuff) increased significantly only in SHR, while NO synthesis decreased significantly in both strains. Endothelium-dependent relaxations to acetylcholine were reduced in the FA of SHR compared to age-matched WKY at both ages, yet these parameters were unchanged in adult rats compared with juvenile animals. The NO-dependent component of vasorelaxation was markedly reduced, whereas the NO-independent component was increased in adult compared to juvenile rats in both strains. The endothelial dysfunction in SHR at both ages was associated with reduction of NO-independent mechanisms. In conclusion, aging in early periods of life was associated with reduction of vascular NO production and bioavailability in both strains investigated. This reduction was however fully compensated by accentuation of NO-independent mechanisms.

Exposure to stress

Stress-induced vascular adaptive response in SHR was investigated, focusing on the endothelium. Noradrenaline responses were studied in intact and denuded aortas from 6-week-old (prehypertensive) and 14-week-old (hypertensive) SHR and age-matched Wistar rats submitted or not to acute stress (20-min swimming and 1-h immobilization 25 min apart), preceded or not by chronic stress (2 sessions 2 days apart of 1-h day immobilization for 5-consecutive days). Stress did not alter the reactivity of denuded aorta. Moreover, no alteration in the EC50 values was observed after stress exposure. In intact aortas, acute stress-induced hyporeactivity to noradrenaline similar between strains at both age. Chronic stress potentiated this adaptive response in 6- and 14-week-old Wistar but not in 6-week-old SHR, and did not alter the reactivity of 14-week-old SHR. Maximum response (g) in intact aortas [6-week-old: Wistar 3.25+/-0.12, Wistar/acute 1.95+/-0.12*, Wistar/chronic 1.36+/-0.21*(+), SHR 1.75+/-0.11, SHR/acute 0.88+/-0.08*, SHR/chronic 0.85+/-0.05*; 14-week-old: Wistar 3.83+/-0.13, Wistar/acute 2.72+/-0.13*, Wistar/chronic 1.91+/-0.19*(+), SHR 4.03+/-0.17, SHR/acute 2.26+/-0.12*, SHR/chronic 4.10+/-0.23; inside the same strain: *P < 0.05 relate to non-stressed rat, +P < 0.05 related to acute stressed rat; n = 6-18]. Independent of age and strain, L-NAME and endothelium removal abolished the stress-induced aorta hyporeactivity.

CONCLUSION

The vascular adaptive response to stress is impaired in SHR, independently of the hypertensive state. Moreover, this vascular adaptive response is characterized by endothelial nitric oxide-system hyperactivity in both strains.

Upregulation of L-type Ca2+ channels in mesenteric and skeletal arteries of SHR

An increased Ca2+ influx attributed to dihydropyridine-sensitive L-type Ca2+ channels has been demonstrated in mesenteric vascular smooth muscle cells of spontaneously hypertensive rats (SHR). This study examined whether an upregulation of the pore-forming alpha1C subunit of the L-type Ca2+ channel underlies this ionic defect. With the use of mesenteric arcade arteries from 12- to 16-week-old SHR and normotensive Wistar Kyoto (WKY) rats, reverse transcriptase-polymerase chain reaction demonstrated an increased level of amplified cDNA corresponding to the alpha1C subunit mRNA in the SHR arteries. Western blots confirmed that the increased mRNA expression was associated with a 3.4-fold increase in the immunoreactive signal of the alpha1C subunit protein in SHR compared with WKY mesenteric arteries, and immunocytochemistry confirmed this abnormality at the single-cell level. Finally, isolated mesenteric arteries from SHR were highly reactive to Bay K8644 and developed anomalous Ca2+-dependent tone, suggesting a functional role for alpha1C subunit upregulation in vascular hyperreactivity. To determine if these Ca2+ channel abnormalities extended to the SHR skeletal muscle bed, we repeated a similar series of studies in WKY and SHR hind limb arteries. Skeletal muscle arteries from SHR also expressed higher levels of alpha1C subunit mRNA and protein than WKY arteries and developed anomalous Ca2+-dependent tone attributed to L-type Ca2+ channels. Our data provide the first evidence that the alpha1C subunit mRNA and protein are upregulated in SHR arteries and that the increased numbers of L-type Ca2+ channel pores are associated with the generation of abnormal vascular tone.

Selective effect of high arterial pressure in hypertension upon inhibition of cGMP versus cAMP mediated vascular relaxation

We tested the hypothesis as to whether elevated arterial pressure in hypertension alters cGMP, or cAMP, mediated vasorelaxation. Relaxation to nitroglycerin and isoproterenol was determined in isolated aortic rings from one-kidney, one clip hypertensive (1K1C), coarctation hypertensive (CH) and normotensive control (C) rats. Thoracic aortas from 1K1C and CH rats, as well as abdominal aortas from 1K1C rats, but not abdominal aortas from CH rats were exposed chronically (4-6 weeks) to elevated arterial pressure. Sensitivity of rings with and without endothelium to nitroglycerin was suppressed significantly only in vessels exposed chronically to high arterial pressure. Impaired sensitivity to nitroglycerin in abdominal rings from 1K1C rats could not be abolished by exposure to 100 uM L-arginine, the substrate for production of NO by endothelial nitric oxide synthase, or 100 uM L-cysteine, the source of thiol groups required for the production of nitric oxide from nitroglycerin. Maximum relaxation to isoproterenol was impaired significantly in thoracic and abdominal rings, with and without endothelium, from 1K1C and CH rats. Relaxation to 8-bromo-cGMP and dibutyryl cAMP was similar in abdominal rings from all groups. We conclude that impaired vasorelaxation to nitroglycerin and isoproterenol in hypertension involves mechanisms prior to activation of vascular smooth muscle cGMP-dependent and cAMP-dependent protein kinase, respectively. Impaired cGMP, but not cAMP, mediated relaxation of aortas appears to result from their exposure to high arterial pressure per se. This effect does not appear to involve the vascular endothelium or vascular sources of thiols, but rather may reflect an effect of high arterial pressure to impair the ability of the artery to respond to nitric oxide derived from nitroglycerin.

Spontaneous and agonist-induced contractions and endothelium-dependent relaxation in aortae from SHRSP and WKY rats under various levels of passive force

1. The influence of the passive force on the contraction and endothelium-dependent relaxation in aortae of normotensive Wistar Kyoto (WKY) rats and stroke-prone spontaneously hypertensive rats (SHRSP) were compared. 2. Force changes of endothelium-intact and -removed preparations were measured isometrically by a force-displacement transducer. Endothelium-dependent relaxation was observed by applying acetylcholine to the preparation precontracted in the presence of 5 x 10(-7) mol/L noradrenaline. 3. The preparations showed spontaneously developed tension (tone) that increased with the increase in the passive force. The effect of passive force was greater in preparations from SHRSP. Contraction initiated by noradrenaline was also increased by passive force up to 30 mN, then showed a tendency to decrease. 4. Endothelium-dependent relaxation was depressed as the passive force was increased. Preparations from SHRSP showed impaired endothelium-dependent relaxation and were influenced by passive force to a lesser degree when compared with preparations from WKY rats. 5. Relaxation by sodium nitroprusside was influenced by passive force to a much lesser extent than that by acetylcholine. 6. Indomethacin potentiated endothelium-dependent relaxation and blocked the effect of passive force in both preparations. 7. The difference in relaxation and the effect of passive force is primarily caused by the difference in the release of endothelium-derived contracting factor, which is thought to be a product of the cyclo-oxygenase pathway of the arachidonic acid cascade.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Effects of ethanol on brain monoamine content of spontaneously hypertensive rats (SHR)

Spontaneously hypertensive rats (SHR) were administered either 2.4 g/kg ethanol or an isocaloric glucose daily for 4 weeks and the levels of norepinephrine (NE), epinephrine (EP), dopamine (DA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in different brain regions were determined. Results indicated a 3-fold increase in NE level in brain stem and hypothalamus and more than 2-fold increase in DA in corpus striatum in alcohol-treated rats as compared to controls. There was a significant increase in the level of DA in the corpus striatum but the levels in cerebral cortex, brain stem and hippocampus were decreased instead. Decreases in 5-HT levels were found in hypothalamus, brain stem, cortex and cerebellum of alcohol-treated brain as compared to untreated controls. These results indicate alterations of the biogenic amine contents in different regions of the SHR brain after chronic ethanol ingestion. Since stimulated release of biogenic amines in the SHR brain has been implicated in the regulation of blood pressure, changes due to ethanol ingestion may be a risk factor in hypertensive patients.

Contractions induced by potassium-free solution and potassium relaxation in vascular smooth muscle of hypertensive and normotensive rats

1. Vascular contractions induced by K(+)-free solution and relaxation responses following the return of K+ to the organ bath were studied in mesenteric arterial rings from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) with particular focus on the role of vascular adrenergic nerve-endings and endothelium. 2. In endothelium-denuded rings the omission of K+ from the incubation medium resulted in gradual contractions, the rate of which was slower in SHR than WKY. Nifedipine (1 microM) inhibited the contractions more effectively in SHR than WKY. 3. Adrenergic denervation in vitro with 6-hydroxydopamine reduced the contractions induced by the K(+)-free medium in endothelium-denuded rings. The remaining contractions after denervation were markedly greater in SHR than WKY. 4. The presence of intact vascular endothelium attenuated the K(+)-free contractions in both strains, the attenuation being smaller in SHR than WKY. NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) and methylene blue (10 microM), but not indomethacin (10 microM), abolished the attenuating effect of endothelium on the K(+)-free contractions. L-Arginine (1 mM) reversed the effect of L-NAME in WKY but not in SHR. 5. The re-addition of K+ after full K(+)-free contractions dose-dependently relaxed the rings. The rate of this K(+)-induced relaxation was significantly slower in SHR than WKY at all K+ concentrations (0.1-5.9 mM) studied, whether the endothelium or functioning adrenergic nerve-endings were present or not. Ouabain (1 mM) totally inhibited the K+ relaxation in SHR but only partially in WKY.6. Vascular smooth muscle contractions induced by high concentrations of potassium were comparable between the strains. The EC50 for noradrenaline-induced contractions was lower in SHR than WKY, but the maximal forces did not differ significantly.7. In conclusion, the contractile response in K+-free solution more clearly differentiates vascular rings from SHR and WKY than the responses induced by the classical contractile agents noradrenaline and high concentrations of potassium. The depressant effect of the presence of intact endothelium on the K+-free contractions, which was smaller in SHR than WKY, is mediated via the endothelium-derived relaxing factor. Neurotransmitter release from vascular adrenergic nerve-endings participates less in the K+-free contractile response in SHR than WKY. Moreover, the contractile response is more dependent on calcium entry through nifedipine-sensitive calcium channels in SHR than WKY. The greater K+-free contractions of denervated endothelium-denuded rings and the reduced K+ relaxation rate in SHR when compared to WKY suggest increased cell membrane permeability and decreased activity of vascular Na+, K+-ATPase, respectively, in this type of genetic hypertension.

Effects of protection from pressure on resistance artery morphology and reactivity in spontaneously hypertensive and Wistar-Kyoto rats

The effects of regional hypotension on femoral resistance artery reactivity and morphology were investigated in spontaneously hypertensive rats (SHR) and control Wistar-Kyoto (WKY) rats. A partially constricting ligature (0.4 mm i.d.) was placed around the left external iliac artery at 5 weeks, which resulted in significantly reduced femoral mean arterial pressures distal to the ligature at 12 and 24 weeks. The femoral mean arterial pressure distal to the ligature in SHR was similar to that in WKY unprotected hind limbs. Resistance arteries (approximately 200 microns i.d.) were taken from unligatured and protected hind limbs and mounted in a myograph for reactivity and morphological measurements. Each experiment therefore utilized one artery distal to a ligature and one from the control hind limb. Histological examination revealed that nuclear density differed neither between strains nor between arteries from protected and unprotected femoral beds. Media thickness, media cross-sectional area, and media/lumen ratios were reduced in arteries from the hypotensive hind limb in SHR and WKY rats at 12 and 24 weeks. Arteries from the protected hind limbs of SHR were structurally indistinguishable from those from the normally perfused WKY vasculature. It is concluded that the medial content and maximal contractile responses of femoral resistance arteries from SHR and WKY rats are mainly determined by the local perfusion pressure and that normalization of perfusion pressure in SHR normalizes resistance artery structure.

In vitro vasoconstriction induced by calcium in renovascular hypertensive or old rats

We studied the changes in calcium-induced vasoconstriction in isolated tail arteries from young (2 months) and old (12 months) normotensive, and young renovascular hypertensive rats (3 months old, with unilateral renal artery clipping at 6 weeks), pretreated with reserpine. The tail artery was removed and perfused/superfused with either a high potassium Krebs depolarizing solution or Krebs solution plus phenylephrine. Concentration-response curves to calcium were produced. Old rats had a low plasma renin activity and their depolarized tail arteries showed a weak vasoconstrictor response to calcium. Renovascular hypertensive rats had a high mean blood pressure and plasma renin activity. Responses of their depolarized tail arteries to calcium were greater. Responses to calcium in tail arteries perfused with phenylephrine were similar in all groups. We conclude that age and renovascular hypertension produce opposite changes in vasoconstriction induced by calcium in depolarized tail arteries.

Exaggerated depressor response to 6-iodoamiloride in NaCl-sensitive spontaneously hypertensive rats

The current study tested the hypothesis that NaCl-sensitive hypertension may result from increased membrane sodium channel activity. The effect of 6-iodoamiloride, and analog of the sodium channel blocker amiloride, on mean arterial pressure (MAP) was examined in conscious, freely moving NaCl-sensitive spontaneously hypertensive rats (SHR-S) fed high (8%) or normal (1%) NaCl diets. SHR-S and age-matched NaCl-resistant SHR (SHR-R) and normotensive Wistar-Kyoto (WKY) control rats were studied at 9 weeks of age after 2 weeks on either high (8%) NaCl or control (1%) NaCl diets. 6-iodoamiloride was infused intravenously in doses of 0.38 or 0.76 mg/100 g body weight, and MAP and heart rate (HR) were monitored from a femoral arterial cannula for 2 hours. The 8% NaCl diet caused a significant elevation in MAP in SHR-S but not in SHR-R or WKY. Administration of 6-iodoamiloride (both doses) produced a significant, sustained decrease in MAP in both SHR-S and SHR-R. Maximal depressor responses to high dose 6-iodoamiloride were significantly enhanced in SHR-S fed 8% NaCl (31.2 +/- 3.7 mm Hg) compared to SHR-S fed 1% NaCl (14.8 +/- 2.4 mm Hg) or SHR-R fed either 8% or 1% NaCl diets (15.6 +/- 4.2 and 10.2 +/- 3.0 mm Hg, respectively). In contrast, feeding an 8% NaCl diet had no significant effect on the depressor responses to 6-iodoamiloride in either SHR-R or WKY rats. In WKY, these doses of 6-iodoamiloride had no significant effect on MAP in either diet group. 6-iodoamiloride had no significant effect on heart rate in any group. These results support the hypothesis that the exacerbation of hypertension in SHR-S fed a high NaCl diet may result from increased membrane sodium channel activity.

Altered pressor response to norepinephrine in calcium- and vitamin D-deficient rats

Norepinephrine (NE: 100-1000 ng/kg) pressor response studies in calcium- (0.005, 0.17, 1.4, and 2.8% in diet) and vitamin D-supplemented (0, 95, 190, and 950 U vitamin D3/day) normotensive conscious male rats were performed prior to and after administration of propranolol (100 ng/kg). Eight weeks of the above dietary treatments (beginning at 4 weeks of age) increased blood pressure (B.P.) in the 0.005% Ca group but reduced it in the 2.8% Ca group compared with the 1.4% Ca (control) group, whereas infusion of NE produced increased and decreased pressor responses, respectively. In the vitamin D groups, B.P. was increased only in the 0 U group, but NE pressor response was decreased in all groups compared with the 190 U (control) group. Plasma and bone calcemic parameters reflected disturbed Ca metabolism due to Ca and vitamin D deficiencies and excesses. These data suggest that Ca and vitamin D-induced changes in B.P. regulation in rats may in part be due to an altered adrenergic system.

Acute hypertension selectively potentiates constrictor responses of large coronary arteries to serotonin by altering endothelial function in vivo

We tested the hypothesis that acute coronary artery hypertension may damage vascular endothelium and alter vasomotor responses to humoral agents. We examined effects of intracoronary infusion of the endothelium-dependent agent serotonin and two endothelium-independent agents, angiotensin II and methoxamine, on large coronary artery diameter in the blood perfused dog heart. Responses were examined before and 30 minutes after brief periods of coronary hypertension (200 mm Hg for 10 seconds to 15 minutes). In open-chest anesthetized dogs, the left anterior descending coronary artery was perfused at constant pressure. Coronary diameter (D) was measured with piezoelectric crystals. At a control perfusion pressure of 80 mm Hg, serotonin produced dose-dependent constriction of the large coronary artery (mean +/- SEM; delta D = -22 +/- 10 microns at 5 micrograms/min; -108 +/- 50 microns at 50 micrograms/min). Increasing perfusion pressure to 200 mm Hg increased flow 515 +/- 79% and coronary diameter 509 +/- 9 microns. After 15 minutes of hypertension, when coronary diameter had returned to baseline values, the constriction of the large artery to serotonin was potentiated (delta D = -89 +/- 33 microns at 5 micrograms/min; -207 +/- 45 microns at 50 micrograms/min; p less than 0.05). Hypertension for 1-5 minutes potentiated constrictor responses of large coronary arteries for at least 2 1/2 hours. Removal of endothelium prevented effects of hypertension on constrictor responses of large arteries to serotonin. Hypertension did not alter constrictor responses to angiotension II (1 and 2.5 micrograms/min) or methoxamine (50 and 100 micrograms/min) or the dilator response to acetylcholine (40 micrograms/min). Acute hypertension altered endothelial morphology. There were small endothelial craters following 10 seconds of hypertension, and disruption of endothelial junctions with leukocyte adherence following 1-15 minutes of hypertension. We conclude that acute hypertension alters constrictor responses of large coronary arteries to serotonin by impairing endothelial function and not by directly affecting vascular smooth muscle. These effects of acute hypertension on vascular reactivity are selective in that they do not involve non-endothelium-dependent agents or the endothelium-dependent agent, acetylcholine. The effect of hypertension also persists long after pressure is restored to normotensive levels.

Vascular reactivity in the spontaneously hypertensive stroke-prone rat. Effect of antihypertensive treatment

This study investigated vascular responsiveness in stroke-prone spontaneously hypertensive rats (SHRSP) and the effect of antihypertensive treatment on this responsiveness. Weanling (4-week-old) male and female SHRSP and Wistar-Kyoto rats (WKY) received either the antihypertensive combination treatment of hydralazine plus hydrochlorothiazide in drinking water or tap water alone (controls) for 15 weeks. Whereas the antihypertensive combination prevented the development of hypertension in treated SHRSP (SHRSP-T), blood pressure remained unchanged in treated WKY (WKY-T). Femoral arterial smooth muscle responsiveness to KCl, norepinephrine, and calcium (in the presence of either 40 mM KCl or 1 microM norepinephrine) was not altered in SHRSP when compared with WKY. A significant increase in the sensitivity of femoral arteries to KCl and calcium (in the presence of 40 mM KCl) was seen, however, in SHRSP-T and WKY-T. An increased sensitivity to norepinephrine and calcium (in the presence of 1 microM norepinephrine) was seen only in SHRSP-T. Isoproterenol-induced relaxation was significantly attenuated in both SHRSP and SHRSP-T. Relaxation induced by sodium nitroprusside and calcium (membrane stabilization) was not different between the four groups. These results show that femoral arterial smooth muscle responsiveness to vasoconstrictor stimuli is not altered in SHRSP but that beta-adrenergic-mediated relaxation is attenuated. Antihypertensive treatment resulted in an enhanced responsiveness to these vasoconstrictor stimuli but had no effect on the relaxation properties of femoral arterial smooth muscle.

Diminished beta-adrenoceptor-mediated relaxation of arteries from spontaneously hypertensive rats before and during development of hypertension

beta-Adrenoceptor agonists and other drugs were studied for their relaxant effects on femoral and mesenteric arterial strips from spontaneously hypertensive rats (SHR). The potency and efficacy of isoproterenol (ISO) in these arteries were decreased in SHR before and during the development of hypertension as compared with age-matched Wistar Kyoto rats (WKY). Reserpine and 6-hydroxydopamine inhibited the development of hypertension but did not alter the reduced ISO-induced relaxation of the arteries. These arteries from prehypertensive SHR (PHSHR) were less sensitive to salbutamol and cyclic AMP and cyclic GMP derivatives than arteries from age-matched WKY. The relaxation response to nitroprusside was less in the femoral but not in the mesenteric arteries from PHSHR than in arteries from age-matched WKY. The relaxation response to papaverine was not diminished in the PHSHR arteries. It was found that the SHR arteries had a reduced responsiveness to the beta-adrenoceptor agonists before the initiation of hypertension and that the diminished relaxation was not specific to the beta-agonists, although there was no generalized defect in vasorelaxation in PHSHR.

Chronic salt loading and adrenergic mechanisms in the Sprague-Dawley rat

The effect of chronic salt loading on adrenergic mechanisms was evaluated in Sprague-Dawley rats (and NMRI mice) maintained on a high sodium (8%) or normal sodium (0.3%) regime for 4 weeks. The basal blood pressure (carotid artery) was not influenced by the high salt diet but the heart rate and blood pressure increases to mental stress (jet air) were larger in the salt loaded rats. There were indications of an increased sympathetic tone in rats on the high salt diet since in these rats sympathoinhibitory treatment with ganglionic blockade or clonidine induced larger falls in blood pressure and heart rate than in the controls. Central catecholamines (brain stem, striatum, hemispheres) were determined spectrofluorimetrically after cation exchange chromatography. The high salt diet influenced neither the endogenous levels of noradrenaline nor central noradrenaline turnover (disappearance of noradrenaline after synthesis inhibition by alpha-methyltyrosine and accumulation of dihydroxyphenylalanine after decarboxylase inhibition by 3-hydroxybenzylhydrazine). There were no changes in central alpha 2-adrenoceptor responsiveness when assessed as clonidine-induced deceleration of noradrenaline turnover in the brain and in central alpha 1-adrenoceptor responsiveness (clonidine-induced increase of flexor reflex in spinalized rats and clonidine-induced increase of motor activity in reserpinized mice). Peripheral sympathetic function was assessed in pithed rats. The pressor responses to intravenously administered noradrenaline (0.01-10 micrograms/kg) and electrical stimulation of the spinal sympathetic nerves (SNS, 0.25-2 Hz) were similar in the two groups, suggesting that salt did not influence vascular alpha-adrenoceptor responsiveness or transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular calcium and altered vascular responsiveness in the deoxycorticosterone acetate-salt rat

This study investigated the effects of altered extracellular Ca2+ on in vitro femoral arterial smooth muscle responsiveness in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Compared with controls, femoral arteries from DOCA-salt rats showed a significant increase in sensitivity to KCl and norepinephrine in normal Ca2+ (2.5 mM). Although no difference in maximal contractile response to KCl was observed between groups, there was a significant difference in maximal response to norepinephrine. Dose-response curves in low Ca2+ (0.25 mM) resulted in a significant decrease in the sensitivity of femoral arteries from DOCA-salt rats to KCl and NE so that the responses were similar to those of controls. Relaxation of femoral arteries from DOCA-salt rats after washout of the KCl contraction was significantly slower than that of controls in both low and normal Ca2+. Isoproterenol-induced relaxation of femoral arteries from DOCA-salt rats was significantly attenuated in normal Ca2+. Sensitivity of femoral arteries from DOCA-salt rats to isoproterenol increased in low Ca2+, but maximal relaxation was unaltered. Whereas no difference in maximal relaxation to NaNO2 was seen in femoral arteries from either group in normal Ca2+, a significant decrease in sensitivity to NaNO2 was observed in femoral arteries from DOCA-salt rats. In low Ca2+ the response of femoral arteries from DOCA-salt rats to NaNO2 was similar to that of controls. These results suggest that the increased vascular smooth muscle responsiveness to KCl and norepinephrine seen in DOCA-salt hypertension is due to increased sensitivity of the vascular smooth muscle to Ca2+. Extracellular Ca2+, however, plays only a minor role in the decreased vasodilator responsiveness seen in this form of hypertension.