Diminished Prostaglandin-Mediated Inhibition of Norepinephrine Release from the Sympathetic Nerve Endings in Spontaneously Hypertensive Rats

This study was performed to investigate prostaglandin(PG)-mediated regulation of norepinephrine release from the sympathetic nerve endings in spontaneously hypertensive rats(SHR). The effects of PGE₂ on the pressor responses and norepinephrine overflow during the periarterial nerve stimulation were examined in the perfused mesenteric vascular beds from SHR and age-matched Wistar Kyoto rats(WKY). In preliminary studies using normotensive Wistar rats, PGE₂ inhibited the overflow of norepinephrine in response to the electrical nerve stimulation. On the contrary, when indomethacin was infused into the preparations, the norepinephrine overflow induced by the nerve stimulation was increased. In SHR, the neurogenic vasoconstriction and norepinephrine overflow during the nerve stimulation were significantly greater than in WKY. PGE₂ in the medium inhibited the pressor responses and norepinephrine overflow during the nerve stimulation dose-dependently in WKY, while the inhibitory effects of PGE₂ in SHR were blunted. These results suggest that PGE₂ is an important hormone in regulating sympathetic vascular tone and that presynaptic inhibition of vascular adrenergic transmission by PGE₂ is attenuated in SHR.     

The Role of Calmodulin in Neurotransmitter Release and Vascular Responsiveness in Spontaneously Hypertensive Rats

ABSTRACT

This study was designed to investigate the role of calmodulin in adrenergic transmission in hypertension. In perfused mesenteric vasculature: from spontaneously hypertensive rats(SHR, 7-9 weeks of age) and age-matched Wistar Kyoto rats(WKY), the effects of a specific calmodulin antagonist(W-7) on norepinephrine overflow and vascular responsiveness to endogenous and exogenous norepinephrine were examined.

The vasoconstrictor responses to electrical nerve stimulation and exogenous norepinephrine as well as norepinephrine overflow during electrical nerve stimulation were significantly enhanced in SHR compared with those in age-matched WKY. The calmodulin antagonist, W-7, reduced not only vasoconstrictor responses but also norepinephrine overflow during nerve stimulation. These inhibitory effects of W-7 were significantly greater in SHR than in WKY.

The results demonstrate that norepinephrine overflow from the sympathetic nerve endings and vascular responsiveness were increased in SHR. The marked reduction in norepinephrine overflow and pressor responses by W-7 might suggest the greater calmodulin-dependent adrenergic transmission in this model of hypertension.     

Abnormalities in the Sodium Transport as the Causative Factor for Enhanced Norepinephrine Overflow in the Spontaneously Hypertensive Rat

ABSTRACT

These studies were designed to investigate whether alterations in the sodium transport could account for the enhanced transmitter release observed during sympathetic nerve stimulation in SHR. In the isolated in vitro perfused rat kidneys, norepinephrine (NE) storage sites were labelled with [³H]-NE and the transmitter overflow was evaluated at various frequencies during the periarterial nerve stimulation. Stimulus-induced transmitter overflow was consistently greater and the maximal overflow was 2-fold higher in the kidneys of SHR when compared to that of normotensive WKY. Addition of ouabain, a selective inhibitor of the sodium pump, (10^?3M in the medium) significantly enhanced stimulus induced overflow in both the groups. However, the magnitude of these changes was significantly greater in WKY than in SHR kidneys suggesting that the membrane Na⁺-pump was functionally less efficient in the SHR. Ouabain virtually eliminated the differences between the two groups in that the transmitter overflow was essentially identical in SHR and WKY in the presence of the Na⁺-pump inhibitor. These observations suggest that a genetic abnormality in the neuronal sodium pump could account for the enhanced sympathetic transmitter overflow and contribute to hypertension in the spontaneously hypertensive rats.     

Norepinephrine overflow in perfused mesenteric arteries of spontaneously hypertensive rats

We examined the overflow of endogenous norepinephrine with electrical stimulation, the associated pressor response, and rate of initial neuronal uptake of [3H]norepinephrine in perfused mesenteric arteries of 7- and 13-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. The tissues of two rats, a spontaneously hypertensive and a WKY control rat, were simultaneously processed and subjected to the same electrical stimulation. Both absolute and fractional overflow of endogenous norepinephrine during periarterial nerve stimulation (5 and 10 Hz for 1 minute) in the tissue of 7-week-old SHR was significantly greater whereas overflow of 13-week-old SHR was equivalent as compared with that of the age-matched WKY rats. The tissue content of norepinephrine was 20-25% higher in SHR of both ages. There was significantly enhanced [3H]norepinephrine uptake in the tissues of young SHR, but no difference was observed in the older SHR. The pressor response to periarterial nerve stimulation was significantly enhanced in 7-week-old SHR and much more so at the older age as compared with the WKY control rats. Exogenous norepinephrine dose-response curves in the tissues of 7-week-old SHR exhibited a parallel leftward shift, characteristic of a change in sensitivity, whereas that of 13-week-old SHR showed a much steeper slope as compared with the respective WKY control rats. This finding suggests that in addition to smooth muscle supersensitivity, structural alterations had occurred in vasculature of 13-week-old SHR. These data indicate that in SHR both the exocytotic release of norepinephrine and the responsiveness of the vascular smooth muscle cells are enhanced in the developmental stage of hypertension whereas smooth muscle supersensitivity to norepinephrine and nonspecific structural alterations primarily contribute to the maintenance of hypertension at 13 weeks of age.     

Altered insulin-like growth factor-1 and nitric oxide sensitivities in hypertension contribute to vascular hyperplasia

Vascular medial thickening, a hallmark of hypertension, is associated with vascular smooth muscle cell (VSMC) hypertrophy and hyperplasia. Although the precise mechanisms responsible are elusive, we have shown that strain induced regulation of autocrine insulin-like growth factor-1 (IGF-1) and nitric oxide (NO) reciprocally modulate VSMC proliferation. Therefore, we investigated potential IGF-1 and NO abnormalities in young (10-week-old) spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) and their respective VSMC ex vivo. The SHR had increased mean arterial pressure (173 ± 2 v 128 ± 3 mm Hg, n = 24, P < .05) but similar pulse pressures (31 ± 2 v 30 ± 3 mm Hg; P > .05) v WKY. The SHR exhibited increased aortic wall thickness in comparison with WKY (523 ± 16 v 355 ± 17μm; P < .05). No differences were seen in plasma combined NO₂ and NO₃ (NO_x) (0.48 ± 0.11 mmol/L for WKY v 0.58 ± 0.18 mmol/L for SHR) or plasma IGF-1 (1007 ± 28 ng/mL for WKY v 953 ± 26 ng/mL for SHR). Aortic VSMC from SHR displayed enhanced proliferation in comparison with WKY (P < .05). Underlying this enhanced proliferation was altered SHR VSMC sensitivity to the antiproliferative NO donor 2,2"[Hydroxynitrosohydrazono] bis-ethanimine (DETA-NO) (ID₅₀: 270 ± 20 mmol/L for SHR; 150 ± 11 mmol/L for WKY; P < .05). Basal cyclic guanosine monophosphate (cGMP) secretion from SHR VSMC was 65-fold greater than that seen from WKY (P < .001). In response to DETA-NO, cGMP secretion from SHR VSMC increased modestly (1.5-fold; P < .01), whereas treatment of WKY VSMC resulted in a 26-fold (P < .001) increase in cGMP. The SHR VSMC did not respond to exogenous IGF-1, whereas WKY VSMC exhibited a dose dependent increase in proliferation with IGF-1 (10⁻¹⁰ to 10⁻⁷ mol/L). These data suggest that VSMC hyperplasia in early hypertension is not reflected by imbalances in plasma IGF-1 or NO. Rather, altered SHR VSMC sensitivity to NO is likely responsible in part for the observed hyperproliferation seen in early stages of hypertension.     

The role of epinephrine as a neuromodulator in spontaneously hypertensive rats

The present study was carried out to elucidate the role of epinephrine as a neuromodulator in hypertension. The effects of epinephrine on norepinephrine release from the sympathetic nerve endings were examined in isolated perfused mesenteric arteries of spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). Norepinephrine overflow during electrical nerve stimulation (5, 15 Hz) was significantly greater in SHR than in WKY. Low concentration of exogenous epinephrine (5.5 X 10(-9) M) potentiated norepinephrine overflow during nerve stimulation in SHR, and this (at 15 Hz stimulation) was antagonized by propranolol (5.0 x 10(-7) M), whereas, the overflow in WKY was reduced by the same concentration of epinephrine. A higher concentration of epinephrine (1.4 x 10(-8) M) decreased norepinephrine overflow in both SHR and WKY, and this change (at 15 Hz stimulation) was antagonized by yohimbine (1.0 x 10(-7) M). Further, magnitudes of the suppressions were smaller in SHR than in WKY. These results suggest that altered modulations of norepinephrine release by epinephrine through presynaptic beta- and alpha 2-adrenoceptors might induce increased sympathetic nerve activity in SHR.     

Presynaptic alpha 2-adrenoceptor mediated regulation of norepinephrine release in perfused mesenteric vasculatures in young and adult spontaneously hypertensive rats

The present study was designed to evaluate the role of the presynaptic alpha 2-adrenoceptor in the pathogenesis of hypertension. Norepinephrine overflow during sympathetic nerve stimulation and its changes by presynaptic alpha 2-adrenoceptor inhibition were examined in the perfused mesenteric vasculatures of young and adult spontaneously hypertensive rats (SHR) compared with age-matched Wistar Kyoto rats (WKY). Electrical sympathetic nerve stimulation caused significantly greater overflow of endogenous norepinephrine from the adrenergic nerve terminals in young SHR than in age-matched WKY. Yohimbine, an alpha 2-adrenoceptor blocking agent, facilitated norepinephrine overflow from the adrenergic nerve terminals. The effects of yohimbine on norepinephrine overflow and pressor responses to electrical nerve stimulation were less in young SHR than in age-matched WKY. Norepinephrine overflow in adult SHR was similar to that in adult WKY, and differences in the effect of yohimbine on norepinephrine overflow between SHR and WKY were not marked at this chronic stage. These results suggest that enhanced norepinephrine overflow in the mesenteric vasculatures can be observed only in young SHR; this may be due in part to an impaired negative feed-back mechanism on the nerve terminals by presynaptic alpha 2-adrenoceptors.     

Converting enzyme inhibition with captopril abolishes sympathoexcitation to euglycemic hyperinsulinemia in rats

Converting enzyme inhibition and angiotensin II receptor antagonism attenuate elevations in heart rate and plasma norepinephrine in response to insulin, suggesting that integrity of the renin-angiotensin system is necessary for insulin-induced sympathoexcitation. To test this, we infused vehicle (saline) in control experiments, or insulin (40 mU/min) during euglycemic clamp in captopril-pretreated (intravenously 2.5 mg/kg, then 1 mg/kg/h) and in nonpretreated urethane-anesthetized Wistar rats while measuring mean arterial pressure, heart rate, and lumbar sympathetic nerve activity. Although euglycemic hyperinsulinemia produced similar blood pressure (BP) increases in insulin-infused rats (change in mean arterial pressure: +9 ± 3 mm Hg) compared with vehicle-controls (+6 ± 2 mm Hg), insulin decreased blood pressure (BP) in captopril-pretreated insulin-infused rats (−8 ± 3 mm Hg). Control rats developed mild heart rate increases (change in heart rate: +28 ± 15 beats/min), contrasting with a marked tachycardia in insulin-infused rats (+82 ± 13 beats/min) and a bradycardia in captopril-treated insulin-infused rats (−16 ± 18 beats/min). As with heart rate, insulin-infused rats experienced large increases in lumbar sympathetic nerve activity (+127 ± 29%), whereas small and equivalent elevations were observed in vehicle-treated rats (+39 ± 24%) and in captopril-pretreated insulin-infused rats (+61 ± 13%). These observations demonstrate an attenuation of insulin-induced sympathetic activation by renin-angiotensin blockade with captopril in Wistar rats, and suggest that the renin-angiotensin system is critical for insulin to exert its sympathoexcitatory effects.     

Elevated 12-Lipoxygenase Activity in the Spontaneously Hypertensive Rat

We have previously demonstrated that administration of inhibitors of the lipoxygenase (LO) pathway of arachidonic acid metabolism lowers blood pressure in hypertensive rats. In addition, we have shown that LO inhibition attenuates pressor agonist-induced vascular reactivity in vitro and calcium mobilization in cultured vascular smooth muscle cells (VSMC). To further elucidate the relationship between elevated LO activity and hypertension, 4, 8, and 12 week old hypertensive SHR were compared with age-matched Wistar-Kyoto (WKY) rats for plasma 12(S)-hydroxyeicosatetraenoic acid (12-HETE) concentration. 12-HETE levels were significantly elevated in the SHR compared to the WKY (SHR elevated by 154%, 159%, and 272% compared to WKY at 4, 8, and 12 weeks, respectively, P < .01 for all ages). There were no differences in plasma potassium levels between SHR and WKY at any of the ages tested. Plasma aldosterone levels and plasma renin activity were in the normal range at the three ages. At 12 weeks of age, both serum (4.72 ± 0.23 v 2.18 ± 0.33 μg/mL, P < .01), and aortic smooth muscle 12-HETE levels (0.94 ± 0.09 v 0.66 ± 0.08 μg/mg protein, P < .05) were elevated in SHR compared with WKY. The 12 week old SHR were given a bolus of the LO inhibitor 5,8,11-eicosatriynoic acid (ETI, 7 mg/kg, intravenously) and blood pressure measured after 20 min. ETI reduced mean systolic blood pressure from 175.8 ± 4.2 to 141.6 ± 5.9 mm Hg (P < .05). To investigate these effects of HETEs, cultured vascular smooth muscle cells were pretreated for 1 min with 12(S)HETE and then challenged with angiotensin II (AngII). The addition of 12(S)HETE increased AngII-induced intracellular calcium levels in normal cultured rat vascular smooth muscle cells by 78% compared to vehicle (P < .05). Thus, LO products, which are high in SHR, may contribute to vascular tone through alterations in the intracellular calcium signal by potentiating calcium responses to pressors such as Ang II.     

Inhibitory action on alpha-human atrial natriuretic polypeptide on vascular adrenergic neurotransmission is attenuated in spontaneously hypertensive rats

The purpose of the present study is twofold, firstly to investigate the effects of alpha-human atrial natriuretic polypeptide (alpha-hANP) on norepinephrine overflow from sympathetic nerve endings, and secondly to compare vascular responsiveness in perfused mesenteric preparations in spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-9 weeks old) and a cohort of Wistar Kyoto rats (WKY). In preliminary studies using normotensive Wistar rats, the pressor responses to electrical nerve stimulation or exogenous norepinephrine application were inhibited by alpha-hANP. Norepinephrine overflow was also suppressed by alpha-hANP, during nerve stimulation. The pressor responses and norepinephrine overflow during nerve stimulation were significantly greater in SHR than in WKY rats. The inhibitory effect of alpha-hANP on these responses was reduced in SHR. These results indicate that alpha-hANP could affect both pre- and post-synaptic sites of the resistance vessels. Further, the reduced inhibition of pressor responses and norepinephrine overflow by alpha-hANP in SHR suggests an insufficient regulation of adrenergic transmission by alpha-hANP in hypertension.     

Voluntary running improves glucose tolerance and insulin resistance in female spontaneously hypertensive rats

We evaluated the effects of voluntary exercise training on glucose metabolism and measures of insulin sensitivity in female spontaneously hypertensive rats (SHR). Age-matched Wistar-Kyoto rats (WKY) were used as normotensive controls. Exercising SHR were housed in running wheels for 8 weeks (SHRx8) or 16 weeks (SHRx16). At 22 weeks of age, we measured systolic blood pressure, performed oral glucose tolerance tests, and determined hexokinase activity and glucose transporter (GLUT) 4 content in skeletal muscle to assess intracellular glucose metabolism. Blood pressure was lower in WKY (139 ± 12 mm Hg) than untrained SHR (216 ± 13 mm Hg). Exercise training caused a reduction in blood pressure (−18 mm Hg) for SHRx8. After a brief (5-h) fast, serum glucose was lower in SHR that exercised compared with sedentary SHR, whereas insulin concentrations were identical between all SHR and WKY. Corresponding free fatty acids (FFA) were twofold higher in SHR than in WKY. In response to glucose, SHR demonstrated higher glucose and FFA responses, with exercise decreasing the glucose values in a dose-dependent manner. Although the insulin response was comparable in all groups, the glucose-to-insulin ratio was higher in SHR, indicating a relative insulin resistance for both glucose disposal and suppression of free fatty acids. Hexokinase activity and GLUT4 content were elevated 1.4- and 2.8-fold, respectively, in plantaris muscle of SHRx16, suggesting an improvement in the capacity for glucose transport and phosphorylation with exercise. These results provide evidence that voluntary running in female SHR lowers blood pressure and selectively increases glucose uptake and insulin action, but not suppression of FFA.     

Norepinephrine release and reuptake by hypothalamic synaptosomes of spontaneously hypertensive rats

We compared the overflow of endogenous norepinephrine during electrical field stimulation, the norepinephrine content, and the rate of initial neuronal uptake of [3H]norepinephrine in synaptosomes isolated from hypothalamus and brainstem of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 7 and 13 weeks of age. The synaptosomes of two rats, a SHR and a WKY rat control, were simultaneously processed and subjected to the same electrical field stimulation. The overflow of endogenous norepinephrine during electrical stimulation (2 Hz, 2 minutes) in the hypothalamic synaptosomes of 7-week-old SHR was significantly greater, whereas the overflow of 13-week-old SHR was equivalent to the age-matched WKY rat. The norepinephrine content of synaptosomes was about the same in SHR and age-matched controls. There was also significantly enhanced [3H]norepinephrine uptake in the hypothalamic synaptosomes of young SHR, but neither the hypothalamic nor the brainstem samples of 13-week-old SHR showed any significant difference in their rate of [3H]norepinephrine uptake. These data are similar to those we observed (unpublished observations) in perfused mesenteric artery system in which norepinephrine release was significantly elevated during periarterial nerve stimulation only in young SHR. Thus, these results suggest that a parallel enhancement of norepinephrine release in hypothalamus with that of peripheral nervous system may play an important role during development of hypertension in young SHR.     

Enhanced endogenous epinephrine release from the vascular adrenergic neurons in spontaneously hypertensive rats

This study was performed to investigate the role of endogenous epinephrine in the regulation of vascular tone in hypertension. The release of endogenous epinephrine and norepinephrine from the vascular adrenergic neurons by periarterial nerve stimulation was examined in spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). The isolated mesenteric vasculatures were prepared, and the epinephrine and norepinephrine release during electrical nerve stimulation was determined as the increase in epinephrine and norepinephrine contents in the vascular perfusate. Epinephrine and norepinephrine were measured by high performance liquid chromatography with an electrochemical detector. Vasoconstrictor responses and norepinephrine overflow during electrical nerve stimulation were significantly greater in SHR than in WKY. The amount of stimulation-evoked epinephrine overflow into the perfusate was also increased in SHR compared to that in WKY, especially at low frequency stimulation. These results suggest that epinephrine could be released from the vascular adrenergic neurons as a cotransmitter of norepinephrine and contribute to increased vascular tone in hypertension.     

Effects of ouabain on adrenergic neurotransmission in spontaneously hypertensive rats

The purpose of the present study is to determine the role of Na+,K+-ATPase in adrenergic neurotransmission of hypertension. Isolated perfused mesenteric vasculatures were prepared from spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY). The effects of ouabain, a Na+,K+-ATPase inhibitor, on the norepinephrine overflow from the sympathetic nerve endings were examined. Norepinephrine overflow from the nerve endings as well as pressor responses during electrical nerve stimulation were significantly greater in SHR than in WKY. Ouabain increased the norepinephrine overflow evoked by electrical nerve stimulation, even in the presence of an uptake-blocker of norepinephrine. Further, the facilitatory effect of ouabain on stimulation-induced norepinephrine overflow was more prominent in SHR than in WKY. These results suggested that ouabain-sensitive Na+,K+-ATPase on sympathetic nerve terminals could have an important role in the regulation of neurotransmitter release, and that its activity might be enhanced in SHR compared with WKY.     

Cardiovascular and Sympathetic Effects of Leukotriene D4 in Renovascular and Spontaneously Hypertensive Rats

Leukotriene D₄ (LTD₄), a constituent of slow-reacting substance of anaphylaxis (SRS-A), elicits a pressor response followed by prolonged hypotension in spontaneously hypertensive (SHR) but not in Wistar-Kyoto (WKY) rats. In order to investigate whether the depressor response to LTD₄ in SHR rats is related to hypertension itself, we have studied the cardiovascular and sympathetic effects of LTD₄ in 1-kidney, 1-clip hypertensive rats and 1-kidney, normotensive rats. In all groups of conscious rats, intra-arterial administration of LTD₄ (0.2 ? 20μg/kg) caused dose-dependent pressor responses of similar degree except in WKY rats, which responded less. Only SHR rats developed a significant and progressive hypotension (?58 ± 5 mm Hg) following pressor phase. In all but WKY rats, the response phase was attended by an increase in plasma levels of norepinephrine and epinephrine. Only SHR rats showed marked and persistent hemoconcentration following pressor effect.

Thus, depressor response of SHR rats to systemic administration of LTD₄ does not appear to be merely due to the magnitude of blood pressure elevation and may in part result from microcirculatory changes not present in other hypertensive rats.     

Effect of Short-Term Treatment of SHR With the Novel Calcium Channel Antagonist Mibefradil on Function of Small Arteries

Treatment of spontaneously hypertensive rats (SHR) and Wistar-Kyoto control rats (WKY) for at least 12 weeks with calcium channel antagonists is associated with regression of structural hypertensive changes in the heart and in conduit and small arteries. To establish whether structural or functional changes of small arteries could be corrected with shorter periods of specific antihypertensive treatment, SHR and WKY were treated for 4 weeks with the novel calcium channel blocker mibefradil. Blood pressure rise was significantly reduced by mibefradil treatment in SHR to 165 ± 1 mm Hg compared to a systolic blood pressure of 183 ± 2 mm Hg in untreated SHR (P < .01). Aortic hypertrophy in SHR was slightly reduced by treatment, but small artery hypertrophy in 4 vascular beds (mesenteric, renal, coronary, and femoral) was unaffected by administration of mibefradil for 4 weeks. Mibefradil treatment resulted in normalization of endothelium-dependent relaxation in mesenteric small arteries, with disappearance of acetylcholine-induced contractions, although hypertrophy and remodeling of these small arteries were not significantly affected by treatment. In WKY rats, treatment had no effect on either structure or function of small arteries. These results demonstrate that treatment with the calcium antagonist mibefradil may induce an improvement in altered endothelial function even before regression of cardiovascular hypertrophy and remodeling takes place under treatment, indicating that normalization of abnormal small artery endothelial function in SHR under antihypertensive therapy may be independent of correction of altered small artery structure.     

Quinapril Inhibits c-Myc Expression and Normalizes Smooth Muscle Cell Proliferation in Spontaneously Hypertensive Rats

A number of data suggest that angiotensin II-dependent activation of the protooncogene c-myc participates in the proliferative response of smooth muscle cells (SMC) of rats with spontaneous hypertension (SHR). We therefore investigated the effects of chronic treatment with the angiotensin converting enzyme (ACE) inhibitor quinapril on the oncoprotein c-Myc and the proliferating cell nuclear antigen cyclin A in SMC of small intramyocardial arteries from the left ventricle of SHR. The expression of c-Myc and cyclin A was assessed by immunocytochemical analysis. The number of smooth muscle cells was assessed by morphometrical analysis. As compared to normotensive Wistar-Kyoto (WKY) rats, untreated SHR exhibited an increased percentages of cells expressing c-Myc (33% ± 4% v 19% ± 2%, mean ± SEM, P < .005) and cyclin A (25 ± 2 v 11% ± 1%, P < .001). In quinapril-treated SHR compared with untreated SHR, we found decreased expression of c-Myc (22% ± 2%, P < .005) and cyclin A (13% ± 1%, P < .001). No significant differences were found between WKY rats and quinapril-treated SHR in the above parameters. Cyclin A was directly correlated with the number of SMCs in each group of rats. These results suggest that an enhanced expression of c-Myc may be involved in the increased proliferation seen in SMCs from small arteries of SHR. Quinapril administration normalizes proliferation in the SMCs of SHR, possibly by inhibiting the expression of the oncoprotein c-Myc and its effects on the cell cycle.     

α-tocopherol increased nitric oxide synthase activity in blood vessels of spontaneously hypertensive rats

Antioxidant protection provided by different doses of α-tocopherol was compared by determining nitric oxide synthase (NOS) activity in blood vessels of spontaneously hypertensive rats (SHR) treated with α-tocopherol.SHR were divided into four groups namely hypertensive control (C), treatment with 17 mg of α-tocopherol/kg diet (α1), 34 mg of α-tocopherol/kg diet (α2), and 170 mg of α-tocopherol/kg diet (α3). Wister Kyoto (WKY) rats were used as normal control (N). Blood pressure were recorded from the tail by physiography every other night for the duration of the study period of 3 months. At the end of the trial, animals were sacrificed. The NOS activity in blood vessels was measured by [³H]arginine radioactive assay and the nitrite concentration in plasma by spectrophotometry at wavelength 554 nm using Greiss reagent.Analysis of data was done using Student’s t test and Pearson’s correlation. The computer program Statistica was used for all analysis.Results of our study showed that for all the three α-tocopherol-treated groups, blood pressure was significantly (P < .001) reduced compared to the hypertensive control and maximum reduction of blood pressure was shown by the dosage of 34 mg of α-tocopherol/kg diet (C: 209.56 ± 8.47 mm Hg; α2: 128.83 ± 17.13 mm Hg). Also, NOS activity in blood vessels of SHR was significantly lower than WKY rats (N: 1.54 ± 0.26 pmol/mg protein, C: 0.87 ± 0.23 pmol/mg protein; P < .001). Although α-tocopherol in doses of α1, α2, and α3 increased the NOS activity in blood vessels, after treatment only that of α2 showed a statistical significance (P < .01). Plasma nitrite concentration was significantly reduced in SHR compared to normal WKY rats (N: 54.62 ± 2.96 mol/mL, C: 26.24 ± 2.14 mol/mL; P < .001) and accordingly all three groups showed significant improvement in their respective nitrite level (P < .001). For all groups, NOS activity and nitrite level showed negative correlation with blood pressure. It was significant for NOS activity in hypertensive control (r = −0.735, P = .038), α1 (r = −0.833, P = .001), and α2 (r = −0.899, P = .000) groups. For plasma nitrite, significant correlation was observed only in group α1 (r = −0.673, P = .016) and α2 (r = −0.643, P = .024). Only the α2 group showed significant positive correlation (r = 0.777, P = .003) between NOS activity and nitrite level.In conclusion it was found that compared to WKY rats, SHR have lower NOS activity in blood vessels, which upon treatment with antioxidant α-tocopherol increased the NOS activity and concomitantly reduced the blood pressure. There was correlation of lipid peroxide in blood vessels with NOS and nitric oxide, which implies that free radicals may be involved in the pathogenesis of hypertension.     

Differential effect of low-dose thiazides on the renin angiotensin system in genetically hypertensive and normotensive rats

Fifty years since thiazide diuretics were introduced, they are established as first-line antihypertensive therapy. Because the thiazide dosing profile lessened, the blood pressure lowering mechanism may lie outside their diuretic properties. We evaluated this mechanism in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) by examining the effects of low-dose hydrochlorothiazide (HCTZ) administration on renin-angiotensin system components. The 7-day, 1.5 mg/kg per day HCTZ did not change systolic pressure (SBP) in WKY, but decreased SBP by 41 ± 2 mm Hg (P < .0001) in SHR, independent of increased water intake, urine output, or alterations in electrolyte excretion. HCTZ significantly increased the plasma concentrations of angiotensin I (Ang I) and angiotensin II (Ang II) in both WKY and SHR while reducing angiotensin-converting enzyme (ACE) activity and the Ang II/Ang I ratio (17.1 ± 2.9 before vs. 10.3 ± 2.9 after, P < .05) only in SHR. HCTZ increased cardiac ACE2 mRNA and activity, and neprilysin mRNA in WKY. Conversely in SHR, ACE2 activity was decreased and aside from a 75% increase in AT₁ mRNA in the HCTZ-treated SHR, the other variables remained unaltered. Measures of cardiac mas receptor mRNA showed no changes in response to treatment in both strains, although it was significantly lower in untreated SHR. These data, which document for the first time the effect of low-dose thiazide on the activity of the ACE2/Ang-(1-7)/mas receptor axis, suggest that the opposing arm of the system does not substantially contribute to the antihypertensive effect of thiazides.