Endothelium-derived relaxing factor released by 5-HT: distinct from nitric oxide in basilar arteries of normotensive and hypertensive rats.

1. The role of the endothelium in cerebrovascular responses to 5-hydroxytryptamine (5-HT) was investigated in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in vitro. 2. Cumulative addition of 5-HT caused concentration-dependent contractions in ring preparations of SHR basilar arteries; the contractile response was smaller in WKY basilar arteries. 3. Removal of the endothelium enhanced markedly the contractile responses to 5-HT in WKY arteries but had only a slight effect in SHR arteries. The responsiveness to 5-HT in WKY arteries after removal of endothelium was comparable to that in SHR arteries. 4. The endothelium-dependent relaxation induced by acetylcholine in WKY basilar arteries was almost abolished by treatment with 10 microM methylene blue or 10 microM NG-nitro-L-arginine (L-NOARG). However, the response to 5-HT was not affected by treatment with methylene blue, L-NOARG or indomethacin. 5. Application of 10-20 mM K+ or 3.2 mM tetraethylammonium (TEA) did not change significantly, or only increased slightly, the resting tension, but markedly enhanced the contractile response to 5-HT in WKY arteries with endothelium. In contrast, the submaximal response to 5-HT in SHR arteries with endothelium was significantly enhanced by 0.3 mM TEA. 6. In the presence of 1 mM TEA, the application of 10 microM L-NOARG further enhanced the responses of 5-HT in WKY arteries with endothelium. In SHR arteries with endothelium, 10 microM L-NOARG per se enhanced slightly but significantly the responses to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of H-7 (protein kinase inhibitor) and phorbol ester on aortic strips from spontaneously hypertensive rats

We investigated the vascular responsiveness to vasoactive agents and the inhibition by H-7 (1-(5-isoquinoline-sulfonyl)-2-methylpiperazine), which inhibits cyclic nucleotide-dependent protein kinases and protein kinase C(PKC) equally potently in helically cut strips of thoracic aortas from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The susceptibility of norepinephrine (NE)- and angiotensin II (Ang II)-induced contractions to H-7 was significantly higher in the aortas from SHR than in those from WKY. H-7 decreased the contractile responses to KCl to a similar extent in both strains without affecting the high K(+)-stimulated Ca2+ influx. H-7 produced a shift to the right of the dose-response curve for the PKC activator, 12-o-tetradecanoylphorbol-13-acetate (TPA) in the case of SHR aortas, while no such shift was noted in tissues from WKY. Functional alterations in the PKC branch of the Ca2+ messenger system in vascular smooth muscle may play an important role in SHR during the sustained contraction.     

Nitric oxide in mesenteric vascular reactivity: a comparison between rats with normotension and hypertension

1. Nitric oxide (NO) plays an important role in various physiological functions. The continuous formation of endogenous NO from endothelial cells maintains a vasodilator tone and regulates blood flow and pressure. However, the role of NO in hypertension remains controversial. 2. In the present study, we used an in situ mesenteric perfusion system. The primary objectives of the study were to examine whether or not mesenteric vasoreactivity is changed by alterations in perfusion pressure and to assess the role of NO in changes of vascular reactivity in hypertension. 3. Spontaneously hypertensive rats (SHR; 12-15 weeks of age) and age-matched normotensive Wistar-Kyoto (WKY) rats were used as the experimental and control groups, respectively. Endothelium-dependent and -independent vasodilation was detected by acetylcholine (ACh) or NO donors (sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP)). Dose-dependent reactivity to these agents (10(-6) to 10(-4) mol/L) was detected by bolus intra-arterial injections of 10 microL of the test agents at 5 min intervals. Dose-dependent responses to vasoconstrictor drugs, such as noradrenaline (NA) and phenylephrine (PE; 10(-6) to 10(-4) mol/L) were also observed. The NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg) was given to examine the contribution of NO to the vasoreactivity of the mesenteric bed. 4. Acetylcholine, SNP and SNAP produced dose-dependent vasodilation in both WKY rats and SHR. The magnitude of the vasodilation was significantly greater in SHR than in WKY rats. It was also greater at high than low flow rates in SHR. The increase in mesenteric perfusion pressure following L-NAME was significantly higher in SHR than in WKY rats. However, there were no differences in responses to L-NAME between low and high flow rates in SHR. Endothelium-independent vasoconstriction (NA and PE) was dose dependent in both SHR and WKY rats. The magnitude of the endothelium-independent vasoconstriction was greater in SHR than in WKY rats. 5. The results suggest that endothelium-dependent or -independent mesenteric vasoconstriction and vasodilation is enhanced in SHR compared with WKY rats, supporting the concept of enhancement of NO function in the hypertensive state. Flow-induced shear stress is also a key factor in the regulation of peripheral resistance depending on NO formation in hypertension.     

Vascular protein kinase C in Wistar-Kyoto and spontaneously hypertensive rats.

Phorbol esters which activate protein kinase C (PKC) produced concentration-related force development in aorta from spontaneously hypertensive rat (SHR) and Wistar-Kyoto rat (WKY); all were 2-7 x more potent in SHR. However, total PKC activity in aortas, as well as carotid, caudal and renal arteries, was not different, when SHR was compared with WKY. Binding of phorbol dibutyrate to particulate aortic PKC was similar in SHR and WKY (same apparent Kd and Bmax values), as was potency for displacement of phorbol dibutyrate by phorbol myristate acetate. Furthermore, there was no difference in potency with staurosporine, H-7, and calmidazolium in inhibiting SHR and WKY aortic PKC. These data demonstrate enhanced contractile sensitivity to PKC-activating phorbol esters in SHR aortic smooth muscle that is not related to activity, phorbol ester binding, or sensitivity to inhibitors when SHR PKC is compared with WKY PKC. Thus, signal transduction events distal to PKC activation may be responsible for enhanced vascular contractile sensitivity to phorbol esters in SHR.     

Potentiation of pressor responses to serotonin by ketamine in isolated perfused rat mesentery

The effects of ketamine on vasoconstrictor responses to periarterial sympathetic nerve stimulation (PNS), norepinephrine (NE), and 5-hydroxytryptamine (5-HT) were studied in normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The isolated mesenteric arteries were perfused at a constant rate (5 ml/min), and the perfusion pressure was recorded. The pressor responses to PNS (8 Hz, 2 ms, 30 s) were augmented by ketamine (2 X 10(-5) and 10(-4) M) in WKY and SHR. Those to intraarterially infused NE (3 X 10(-10) M) were statistically unaltered. However, in three of seven arterial preparations from WKY and in six of nine preparations from SHR, ketamine (2 X 10(-5) and 10(-4) M) decreased the pressor responses to NE. In contrast, the responses to intraarterial 5-HT (1.3 X 10(-9) mol) were potentiated by ketamine (2 X 10(-5) and 10(-4) M) in SHR and WKY--to a much greater extent in SHR. Fractional release of tritium by PNS from isolated mesenteric arteries previously labeled with 1-[7,8-(3)H]NE (10(-7) M) was unaltered by ketamine (10(-4) M) in SHR and WKY. Cocaine (10(-5) M) prevented the ketamine-induced potentiation of PNS and 5-HT responses. Ketamine as well as cocaine inhibited the accumulations of [3H]5-HT and [3H]NE in intact mesenteric arteries from SHR and WKY to a comparable extent. In tissues denervated by 6-hydroxydopamine, the accumulation of 5-HT was about 70% (WKY) and 60% (SHR) of those in intact tissues, whereas that of NE was about 11% (WKY) and 9% (SHR).(ABSTRACT TRUNCATED AT 250 WORDS)     

Alterations of cyclo-oxygenase products and NO in responses to angiotensin II of resistance arteries from the spontaneously hypertensive rat.

1. The involvement of cyclo-oxygenase (COX) products and nitric oxide (NO) in contractile responses of resistance arteries to angiotensin II (AII) were investigated in small mesenteric arteries from spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats. 2. In endothelium intact vessels, AII induced concentration-dependent responses without any significant difference between the two strains. However, removal of functional endothelium resulted in enhanced sensitivity to AII, the pD2 value increasing from 8.4 +/- 0.2 to 8.9 +/- 0.2 (P < 0.05) in WKY and from 8.2 +/- 0.1 to 8.6 +/- 0.1 (P < 0.05) in SHR (not significantly different between strains, n = 9 - 12). In addition, endothelium removal enhanced maximal contractions elicited by AII in SHR (1.4 +/- 0.1 to 2.1 +/- 0.2 mN mm-1, n = 5; P < 0.05) but not in WKY (1.0 +/- 0.1 to 1.2 +/- 0.1 mN mm-1, n = 5) vessels. 3. In the absence of functional endothelium, the COX inhibitor indomethacin (10(-5) M) reduced contractile responses elicited by AII in SHR arteries, resulting in 33 +/- 5% (n = 5) decrease in maximal contraction. However, it produced minimal if any, effect on responses of WKY vessels. In both strains, the TP receptor antagonist GR32191 B (3 x 10(-6) M) did not modify contractions elicited by AII in these conditions. 4. In the presence of functional endothelium, indomethacin (10(-5) M) almost abolished the responses to AII in both strains. GR32191 B (3 x 10(-6) M) reduced the sensitivity of WKY arteries to AII (pD2 = 8.1 +/- 0.1, P < 0.01) without any effect on maximal contraction. In SHR arteries, it markedly reduced maximal contraction (47 +/- 3.5%). 5. In both strains, the NO synthase inhibitor NG-nitro-L-arginine methy lester (L-NAME; 10(-4) M) had no effect in the absence of functional endothelium but it markedly reduced the inhibitory influence of endothelium on contractile responses to AII. Furthermore, in arteries with endothelium, it reduced the effect of both indomethacin and GR32191 B to the same level as observed in vessels without functional endothelium. 6. The results suggest that enhanced contraction caused by COX products was counteracted by enhanced relaxation caused by endothelium-derived NO in resistance mesenteric arteries of the SHR exposed to AII, compared to WKY arteries. The COX products involved in alterations of SHR responses comprised an endothelium-derived prostaglandin activating TP receptors and another nonendothelial unidentified vasoconstrictor compound which did not activate these receptors.     

Nitric oxide-mediated changes in vascular reactivity in pregnancy in spontaneously hypertensive rats.

1. To examine the mechanisms which may account for pregnancy-induced vasodilatation in spontaneously hypertensive rats (SHR), we have investigated the changes in vascular reactivity and the effects of endothelial nitric oxide (NO) inhibition in the in situ blood-perfused, mesenteric resistance vessels of 18-20 day pregnant SHR. The effects of NG-nitro-L-arginine (L-NOARG) were compared in pregnant and nonpregnant SHR and gestation matched normotensive Wistar-Kyoto (WKY) rats. 2. Intra-arterial mean blood pressures (MBP) were similar in pregnant and nonpregnant SHR. Basal perfusion pressures (BPP) were decreased in pregnant compared with nonpregnant SHR. Pregnant WKY had lower MBP and BPP than either pregnant or nonpregnant SHR. 3. Vasoconstrictor responses to electrical stimulation (ES) and intra-arterial noradrenaline (NA) were decreased in pregnant compared with nonpregnant SHR. These responses were still greater in pregnant SHR when compared with pregnant WKY. Vascular reactivity to angiotensin II (AII) in pregnant SHR was reduced to a similar level to that in pregnant WKY. 4. L-NOARG (5 mg kg-1, i.v.), an inhibitor of nitric oxide synthase, increased MBP and BPP in all groups. After L-NOARG, BPP were equalized between pregnant and nonpregnant SHR. Pregnant WKY still showed lower MBP and BPP than SHR groups. 5. L-NOARG potentiated vascular responses to ES, NA and AII in all groups. The blunted vascular responses to NA and ES were normalized and the reactivity to AII was only partially reversed in pregnant SHR compared with nonpregnant SHR. Pregnant WKY still had much lower vascular responses to ES and NA than either pregnant or nonpregnant SHR.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tachyphylaxis to 5-hydroxytryptamine in perfused kidneys from spontaneously hypertensive and normotensive rats

Isolated perfused kidneys from 4- to 6-month-old spontaneously hypertensive rats (SHR, Japanese strain) exhibit increased "vascular reactivity" to 5-hydroxytryptamine (5-HT) and a slower rate of development of tachyphylaxis to this substance when compared with kidneys from normotensive Wistar-Kyoto (WKY) rats. We investigated the possibility that the reduced rate of development of tachyphylaxis could be related to a interaction of 5-HT with adrenergic mechanisms or with endogenous 5-HT. Tachyphylaxis was induced by repeated administration of 5-HT to kidneys from SHR and WKY rats. This procedure did not affect vasoconstrictor responses evoked by norepinephrine. The development of tachypylaxis to 5-HT in kidneys from SHR and WKY rats was not changed by chemical sympathectomy with 6-hydroxydopamine. Treatment of SHR with para-chlorophenylalanine did not affect their blood pressure or the development of tachyphylaxis to 5-HT. These results indicate that delayed tachyphylaxis to 5-HT in kidneys of SHR is not due to an interference with adrenergic mechanisms and does not depend on endogenous 5-HT levels. The phenomenon represents an unusual modification of vascular smooth muscle exposed to chronic high pressure, but it is unlikely that the vasoconstrictor effects of 5-HT contribute to the maintenance of hypertension in the SHR.     

Hydrogen peroxide induces a greater contraction in mesenteric arteries of spontaneously hypertensive rats through thromboxane A(2) production.

1. Hydrogen peroxide (H(2)O(2)) caused a transient contraction in endothelium-intact (E+) and -denuded (E-) mesenteric arteries (MA) from 8 - 10-month-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in a concentration-dependent manner (10(-5) M to 10(-3) M). 2. The contraction to H(2)O(2) in MA (E+ or E-) was greater in SHR than in WKY. Removal of endothelium potentiated the contraction to H(2)O(2) in WKY but not in SHR. Tachyphylaxis to H(2)O(2) was less prominent in SHR than in WKY. 3. The contraction of aorta to H(2)O(2) (5 x 10(-4) M), expressed as a percentage of 80 mM KCl-induced contraction, was approximately half of that found in the MA. A greater contraction was found in E+ but not E- SHR aortic rings. 4. The contraction of MA to H(2)O(2) (5 x 10(-4) M) was greatly inhibited by SQ 29548 and ICI 192605 (thromboxane A(2) (TXA(2))/prostaglandin H(2) receptor antagonists), quinacrine (a phospholipase A(2) (PLA(2)) inhibitor), indomethacin and diclofenac (cyclooxygenase (COX) inhibitors), and furegrelate (a TXA(2) synthase inhibitor). 5. Production of thromboxane B(2) induced by H(2)O(2) (5 x 10(-4) M) was greater in SHR MA than in WKY, and was inhibited by quinacrine, indomethacin and diclofenac, and furegrelate, but not by SQ 29584 and ICI 192605. 6. These results suggested (1) that SHR MA exhibits a higher contraction involving an increased smooth muscle reactivity and less tachyphylaxis to H(2)O(2) than WKY; (2) that a greater production of TXA(2) through activation of PLA(2)-COX-TXA(2) synthase pathway appeared to be responsible for the enhanced contraction in SHR MA. The enhanced vascular response to H(2)O(2) may be related to hypertension in SHR.     

Direct effects of quercetin on impaired reactivity of spontaneously hypertensive rat aortae: comparative study with ascorbic acid

1. There is a growing interest in the anti-oxidant characteristics and use of flavonoids in the management of cardiovascular diseases. The cardiovascular mechanism of action of these plant derivatives remains controversial. This study compared the effects of the flavonoid quercetin with those of the anti-oxidant vitamin ascorbic acid (vitamin C) on the reactivity of aortic rings from spontaneously hypertensive rats (SHR). 2. The phenylephrine (PE)-induced contractile and the endothelium-dependent and independent relaxant responses of aortic rings from 21 to 22 week old SHR and age-matched normotensive Wistar (WKY) rats were observed in the presence of quercetin or ascorbic acid. All the experiments were performed in the presence of the cyclooxygenase inhibitor, indomethacin (10 micromol/L). 3. The endothelium-dependent and independent relaxations to acetylcholine (ACh) and sodium nitroprusside (SNP), respectively, were significantly lesser in the SHR compared to the WKY tissues whereas the contractile responses to PE were similar in both tissues. Pretreatment of WKY rings with quercetin or ascorbic acid had no effect on the responses to ACh or PE. In the SHR tissues, however, quercetin or ascorbic acid significantly improved the relaxation responses to ACh and reduced the contractions to PE with greater potency for quercetin. Both compounds lacked any effects on the responses to SNP in either aortic ring types. N(omega)-nitro-L-arginine methyl ester (l-NAME, 10 micromol/L) significantly attenuated the vasodepressor effects of quercetin and ascorbic acid, raising the responses to PE to a level similar to that observed in the control SHR tissues. In l-NAME pretreated aortic rings, quercetin and ascorbic acid inhibited the contractile responses to PE with the same magnitude in WKY and SHR tissues. 4. The present results suggest that acute exposure to quercetin improves endothelium-dependent relaxation and reduces the contractile responses of hypertensive aortae with a greater potency than ascorbic acid. This suggests a better vascular protection with this flavonoid than ascorbic acid in the SHR model of hypertension and possibly in human cardiovascular diseases.     

Hyperreactivity of alpha 1-adrenoceptors, but not of P2X-purinoceptors, in vas deferens of spontaneously hypertensive rats.

We evaluated the contractile reactivity to various stimuli, and the content and release of noradrenaline (NA) from a non-vascular tissue, the vas deferens, isolated from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The concentration-contraction curves for NA in tissue from animals of two ages (10-25 weeks and 30-45 weeks) were shifted to the left in SHR as compared with in age-matched WKY, with significant differences at 1.0 and/or 10 microM of NA. Similarly, the amplitude of contraction produced by electrical stimulation at 4, 8 and 16 Hz in the tissue was much larger in SHR than in WKY. However, ATP (10-100 microM) evoked contractions of the tissue to a similar extent in both SHR and WKY. The electrically evoked contractions of vas deferens from both strains were inhibited by isoprenaline in an approximate dose-dependent and equipotent manner. The tissue NA content, determined by HPLC-ECD, was nearly same in both SHR and WKY. In addition, the same amount of NA was released from the vas deferens of both strains by electrical stimulation in the presence of 4-aminopyridine. The present findings indicate that the contractile response of vas deferens to stimulation of alpha 1-adrenoceptors, but not of beta-adrenoceptors or P2X-purinoceptors, is more pronounced in SHR than in WKY and that a response indicative of hypertension may also occur in non-vascular tissue as it does in vascular tissue.     

Characterization of 5-hydroxytryptamine receptors mediating contractions in basilar arteries from stroke-prone spontaneously hypertensive rats.

1. The 5-hydroxytryptamine (5-HT) induced-contraction in ring preparations of basilar arteries from Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) was pharmacologically characterized in vitro. 2. Contractile responses to 5-HT (1 nM-100 nM) and their pD2 values in arteries from SHRSP at 6 months of age were significantly greater than those in age-matched WKY, although the maximum response did not differ between the two groups. 3. There were no significant differences in contractile responses to U-44619, endothelin-1, neuropeptide Y, and angiotensin II between WKY and SHRSP arteries. 4. Spiperone (1 nM-1 microM, a 5-HT2 receptor antagonist), produced biphasic displacement of the 5-HT curves in WKY and SHRSP arteries. The response to high concentrations of 5-HT was concentration-dependently antagonized by spiperone, while the response to low concentrations of 5-HT was resistant to blockade by spiperone, and the spiperone-resistant contractile responses induced by 5-HT were greater in SHRSP than in WKY. Ketanserin (1-100 nM, 5-HT2) also produced a biphasic shift of the 5-HT curves for both arteries. 5. Methiothepin (10 and 100 nM, 5-HT1 and 5-HT2) potently inhibited 5-HT-induced contractions in both groups. In addition, methiothepin (100 nM) produced a parallel shift to the right of the component of 5-HT-induced contractile responses that was resistant to blockade by spiperone in both groups. 6. The contractile effects of 5-HT in WKY and SHRSP arteries were not affected by MDL 72222 (1 microM, 5-HT3) and SDZ 205-557 (1 microM, 5-HT4). In addition, cocaine (10 microM), pargyline (50 microM), prazosin (10 microM), indomethacin (3 microM) and SQ 29,548 (1 microM) did not affect the contractile effects of 5-HT in either artery. 7. Contractile responses to 5-carboxamidotryptamine, CGS 12066B, pindolol and propranolol were greater in SHRSP arteries than in WKY arteries, whereas contractions in response to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), alpha-methyl-5-HT and 2-methyl-5-HT did not differ between the two groups. Cisapride failed to contract basilar arteries in both groups. Furthermore, a correlation analysis showed a highly significant correlation between the pD2 values of 5-HT agonists in WKY and SHRSP arteries and their published binding affinities at the 5-HT1B subtype. 8. These findings suggest that 5-HT elicits vasoconstriction in rat basilar arteries by stimulation of a mixed receptor population of 5-HT2 and 5-HT1-like receptors (similar to the 5-HT1B receptor subtype), and that the contraction mediated by 5-HT1-like receptors is enhanced in the basilar artery from SHRSP.     

Effects of aging and hypertension on the participation of endothelium-derived constricting factor (EDCF) in norepinephrine-induced contraction of rat femoral artery

Endothelium-dependent contraction elicited by high concentrations of acetylcholine was described in hypertensive as well as in aged normotensive rats. The contribution of endothelium-derived constricting factor (EDCF) to norepinephrine-induced contraction is still unknown. We aimed to compare EDCF participation to norepinephrine-induced arterial contraction in spontaneously hypertensive rats (SHR) and aged normotensive Wistar-Kyoto (WKY) rats. Femoral arteries from either adult (7-months-old) or aged (14-months-old) animals were placed in myograph and norepinephrine-induced concentration-response curves were recorded under control conditions and in the presence of indomethacin (cyclooxygenase inhibitor, 10(-5) mol/l) or L-NNA (NO synthase inhibitor, 10(-4) mol/l) or both. Norepinephrine-induced concentration-response curve was enhanced in SHR compared to WKY rats, but concentration-response curve of aged WKY rats was similar to those of adult SHR. Cyclooxygenase inhibition largely attenuated concentration-response curves in all groups. However, this effect was greater in aged WKY rats and adult SHR compared to adult WKY rats. NO synthase inhibition augmented norepinephrine-induced contraction in arteries of adult WKY rats, but not in arteries from aged WKY rats or adult SHR. The combined administration of L-NNA and indomethacin had no additive effects on concentration-response curves. EDCF contribution to norepinephrine-induced contractions of arteries was considerably greater in adult SHR (80±3%) and aged WKY rats (86±2%) compared to adult WKY rats (35±10%). The inhibition of NO synthase augmented EDCF contribution to norepinephrine-induced contraction only in arteries from adult WKY rats (76±9%). We conclude that EDCF contribution to norepinephrine-induced contraction of conduit arteries is similarly enhanced in adult hypertensive and aged normotensive rats.     

Evidence for the presence of A(1) adenosine receptors in the aorta of spontaneously hypertensive rats.

1. Isolated aortic rings (endothelium-intact and -denuded) from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats were used in this study to examine the vasoactive effects of various adenosine analogues. 2. In phenylephrine contracted aortic rings, concentration-response curves were constructed by cumulative additions (10(-11) - 10(-5) M) of (2S)-N(6)-[2-endo-Norbornyl] adenosine (ENBA), N(6)-cyclopentyladenosine (CPA), R-N(6)-(2-phenylisopropyl) adenosine (R-PIA), 2-p-(-2-carboxyethyl) phenethylamino-5'-N-thylcarboxamido adenosine (CGS-21680). 3. A non-specific adenosine receptor agonist 2-chloroadenosine (CAD) resulted in biphasic response with a small contraction at lower concentrations (10(-9) - 10(-8) M) followed by a significant relaxation at higher concentration in endothelium-intact SHR tissues, suggesting presence of both A(1) and A(2) adenosine receptors in SHR aorta. However, only relaxation was observed in WKY. 4. Contractile response in SHR had the following rank order of potency: ENBA>CPA>R-PIA>CAD. The relaxation response in SHR and WKY had the following rank order of potency: CGS 21680>CAD>R-PIA>CPA>ENBA. 5. Removal of endothelium abolished the adenosine analogue induced contractions in SHR aorta and attenuated the vasorelaxation responses in the WKY and SHR. 6. The contractile response in SHR was abolished by A(1) adenosine receptor antagonist N(6)-endonorbornan-2-yl-9-methyladenine (N-0861). A(2) adenosine receptor antagonist, 3,7-dimethyl-1-proparglyxanthine (DMPX) did not affect the contraction response of adenosine analogues. 7. Endothelium-dependent contractions elicited by A(1) receptor agonists were blocked by indomethacin and by free radical scavengers. 8. These data suggest that the contractile response to adenosine analogues in SHR aorta is probably mediated by free radicals which are generated through the increased cyclo-oxygenase activity occurring in the vascular endothelium of SHR but not the WKY rats.     

Altered intra- and extraluminal effects of 5-hydroxytryptamine in hypertensive mesenteric resistance arteries: contribution of the endothelium and smooth muscle.

Vasoconstrictor responses to intraluminal and extraluminal 5-hydroxytryptamine (5-HT) were studied in isolated mesenteric resistance arteries of Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). Third-order branches of mesenteric arteries were dissected free and mounted on glass cannulae in organ chambers. Changes in intraluminal diameter of the perfused and pressurized vessels were measured. Extraluminal 5-HT (10(-8)-10(-4)M) evoked concentration-dependent contractions that were augmented after removal of the endothelium. The sensitivity of arteries without but not of those with endothelium to 5-HT was increased in SHRs compared to WKY rats. The pA2 value for ketanserin using 5-HT as an agonist was identical in WKY rats and SHRs. The slope of the Schild plots did not significantly differ from 1. Intraluminal 5-HT caused smaller contractions in arteries with endothelium than extraluminal 5-HT. After endothelial removal, the contractions to intraluminal 5-HT were increased. The contraction induced by intraluminal 5-HT in arteries with endothelium was greater in SHRs than WKY rats. Thus, contractile responses to 5-HT are mediated by homogeneous 5-HT2 receptors in rat mesenteric resistance arteries. In SHRs, the affinity of the receptor to 5-HT is not altered. The sensitivity of vascular smooth muscle to 5-HT is increased and the protective role of the endothelium against intraluminal 5-HT is decreased in SHRs.     

Arterial Smooth Muscle Responses in Adult and Moderately Aged Spontaneously Hypertensive Rats

In order to further clarify differences between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats as well as the effects of ageing, vascular smooth muscle responses of mesenteric arterial rings and intracellular free calcium concentration ([Ca²⁺]_i) in platelets and lymphocytes were studied in 20-week-old and 32-week-old animals. Arterial contractile responses induced by noradrenaline and potassium chloride were comparable in 20-week-old SHR and WKY rats, whereas at 32 weeks of age maximal contractile force generation to both of these agents was clearly lower in SHR. In both age groups the calcium entry blocker nifedipine was more effective in inhibiting potassium chloride-evoked responses in SHR than in WKY rats, and arterial relaxation responses by endothelium-dependent (acetylcholine) and endothelium-independent (nitroprusside, isoprenaline) mechanisms were more pronounced in WKY rats when compared with SHR. The ability of vascular smooth muscle cells to sequester calcium was evaluated by first depleting cellular calcium stores with maximal contractions to noradrenaline in calcium-free buffer, whereafter calcium was returned to the organ bath. After a 10 min. calcium loading period the arterial rings were rechallenged with noradrenaline. Both in 20-week-old and 32-week-old rats these reponses were less marked in SHR than in WKY rats, suggesting reduced ability of smooth muscle cells to sequester calcium. In addition, platelets and lymphocytes were used as cell models to examine [Ca²⁺]_i in the experimental groups by the fluorescent indicator quin-2. In these two cell types [Ca²⁺]_i was higher in SHR than in WKY rats in both of the age groups studied. In summary, differences between SHR and WKY rats in cellular calcium handling are expressed as impaired relaxation and reduced sequestration in SHR vascular smooth muscle and as higher [Ca²⁺]_i in blood cells. Ageing seems to have clear effect only on contractile force generation which is reduced in SHR mesenteric artery both in agonist-mediated and in depolarization-induced responses.     

Nitric oxide inhibition accelerates hypertension and induces perivascular inflammation in rats

1. Inflammatory changes in peripheral arteries have been reported in animal models of hypertension. Whether they occur in cerebral arteries (CA) with hypertension induced by deprivation of endogenous nitric oxide (NO) remains unknown. 2. In the present study, we compared the arteriolar injury score (AIS) and perivascular inflammation in CA between hypertensive and normotensive rats following NO deprivation with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Five-week-old male spontaneously hypertensive rats (SHR) and Wistar -Kyoto (WKY) rats were fed with L-NAME (1 mg/mL) for 4 weeks. 3. Nitric oxide deprivation resulted in time-dependent elevations in tail-cuff pressure (representing systolic blood pressure (SBP)) in both SHR and WKY rats. The magnitude of increase in SBP was larger in SHR (+81.0 +/- 3.2 vs+25.0 +/- 2.2 mmHg; P < 0.01). Arteriolar hyalinosis and AIS in various segments of the CA were assessed with periodic acid-Schiff staining and inflammatory cells were immunostained with the antibody against macrophage/monocyte marker (ED1). The ED1+ cells appeared in the middle CA of L-NAME-treated SHR as early as 2 weeks after treatment. These cells were not observed in L-NAME-treated WKY rats and untreated SHR. More ED1+ cells were found in L-NAME-treated SHR than L-NAME-treated WKY rats after 4 weeks treatment. 4. The AIS and number of ED1+ cells around the perivascular area of the internal carotid artery were significantly higher in L-NAME-treated compared with untreated rats (AIS: 137 +/- 28 vs 46 +/- 10 for WKY rats, respectively; 169 +/- 18 vs 53 +/- 6 for SHR, respectively (P < 0.01); ED1+ cells: 7.9 +/- 0.6 vs 1.3 +/- 0.9 for WKY rats, respectively; 13.6 +/- 2.7 vs 2.1 +/- 0.9 for SHR, respectively (P < 0.01)), although SBP was higher in untreated SHR than in L-NAME-treated WKY rats (170 +/- 4 vs 137 +/- 4 mmHg, respectively; P < 0.05). 5. These findings suggest that ED1+ cells appeared in the middle CA of L-NAME-SHR as early as 2 weeks after treatment. Chronic inhibition of NO accelerates hypertension and induces perivascular inflammation.     

Vessel reactivity and prejunctional modulatory changes in the portal vein of mature spontaneously hypertensive rats

The prejunctional effects of 2-chloroadenosine on the contractile responses to perivascular nerve stimulation were studied in conjunction with vessel reactivity in portal veins from mature (45-60 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. It was found that the contractility of the portal veins to exogenous noradrenaline was enhanced in SHR, while the sensitivity to noradrenaline, as seen from the EC50 values, was not altered. Responses to perivascular nerve stimulation (supramaximal voltage, 0.7 ms pulse duration for 10 s) were also enhanced in spontaneously hypertensive rats compared to WKY controls at all frequencies of stimulation tested (2-64 Hz), although the sensitivity to perivascular nerve stimulation was not changed since a response of 50% of the maximal obtainable was achieved at 8 Hz in both SHR and WKY. However, no evidence for change in the level of prejunctional modulation of nerve stimulation responses or postjunctional modulation of noradrenaline responses by 2-chloroadenosine (0.1-100 microM) was found. In conclusion, in mature SHR, responses to perivascular nerve stimulation and exogenous noradrenaline were found to be enhanced compared to responses in WKY, although there was no difference in the modulatory action of 2-chloroadenosine between SHR and WKY.     

Pulmonary vascular reactivity of spontaneously hypertensive rats is exacerbated in response to the central administration of exogenous nitric oxide

1. Centrally, nitric oxide (NO) is a sympathoinhibitory substance. Spontaneously hypertensive rats (SHR) have an impaired central nitroxidergic system and, consequently, NO-mediated decrease in sympathetic activity is exacerbated in SHR compared with Wistar-Kyoto (WKY) rats. We have demonstrated previously that acute hypoxic pulmonary vasoconstriction (HPV) is enhanced by central NO administration. Therefore, in the present study, we hypothesized that accentuation of the HPV by NO would be exacerbated in SHR compared with WKY rats. 2. Mean pulmonary arterial pressure, systemic mean arterial blood pressure, cardiac output and heart rate were measured in pentobarbitone-anaesthetized, artificially ventilated, male SHR and WKY rats. The brief, transient response to a bolus intracerebroventricular (i.c.v.) dose of N(G)-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) was recorded in all rats. Upon recovery, rats were exposed to acute hypoxia (10% O(2) for 4 min) before and after the i.c.v. administration of the NO donor 3-[4-morpholinyl]-sydnonimine-hydrochloride (SIN-1; 100 microg in 10 microL). 3. In WKY rats, central inhibition of NO synthesis by L-NAME caused a mild increase in tonic pulmonary vascular tone and induced a large systemic pressor response. These responses were not observed in SHR. In contrast, SIN-1 failed to alter tonic pulmonary vascular tone, although it enhanced the HPV in WKY rats and, significantly more so, in SHR. 4. These results confirm that accentuation of the HPV by NO is exacerbated in SHR compared with WKY rats. The mechanism(s) by which the HPV is accentuated by central NO remains to be fully elucidated, but is likely to be associated with the sympathoinhibitory effects of NO and, if so, supports the idea that the nitroxidergic system of the SHR is impaired. Further electrophysiological studies are essential to confirm these assumptions.