Mohamed A. Bayorh

Neuropeptide Y and peptide YY mediate non-adrenergic vasoconstriction and modulate sympathetic responses in rats

The modulation of cardiovascular sympathetic responses by neuropeptide Y (NPY) and peptide YY (PYY) was assessed in vivo, in pithed rats. Both peptides (0.02-2 nmol/kg) caused similar dose-dependent pressor responses, resistant to adrenergic blockade but antagonized by the calcium channel blocker, nifedipine. Only NPY, at the lowest dose, slightly accelerated heart rate (by 10 +/- 4 beats/min). At the pressor dose (0.6 nmol/kg) but not subpressor dose (0.2 nmol/kg), the increase in blood pressure induced by stimulation of the sympathetic outflow (ST: 0.3 Hz, 50 V, 1 min) was attenuated by PYY (by 40%), whereas ST-evoked tachycardia was reduced by NPY (by 35%). Neither NPY- nor PYY-pretreatment affected ST-induced increments in plasma norepinephrine (NE) and epinephrine concentrations. In addition, regional hemodynamic effects of NPY were studied in conscious rats instrumented with Doppler flow probes. The hypertension caused by NPY was attended by reflex bradycardia and marked rise in peripheral vascular resistance in renal (+ 233 +/- 59%), superior mesenteric (+ 183 +/- 65%) and hindquarter (+ 65 +/- 10%) circulation. The pattern of hemodynamic responses of NPY was similar to that of NE but, unlike the latter, persisted after adrenergic blockade.

Alterations in Aldosterone and Angiotensin II Levels in Salt-Induced Hypertension

Several studies have demonstrated that plasma renin-angiotensin activity is reduced in rats administered a high salt diet. We evaluated changes in plasma and tissue levels of aldosterone (ALDO) and angiotensin II (A-II), as well as the reduced-to-oxidized glutathione ratio. Male Dahl salt-sensitive (SS) rats were placed on either a high-salt (8% NaCl; HS) or a normal-salt (0.3% NaCl; NS) diet for 3 weeks. Prior to and weekly on the diets, systolic blood pressure was measured by tail cuff plethysmography. Levels of A-II and ALDO in plasma, heart, and kidney were analyzed by enzyme immunoassay. Reduced and oxidized gluthatione were simultaneously measured by HPLC fluorescence detection. Heart and kidney tissues were prepared for histological analysis. Systolic blood pressure in animals on a HS diet was significantly elevated above that of those on a NS diet. High salt caused a reduction in both plasma A-II and ALDO levels; while their levels in the heart and kidney were increased. Exposure to a high-salt diet led to the enlargement of both heart and kidney. The reduced-to-oxidized glutathione ratio in plasma, heart and kidney was lowered by exposure to a HS diet. Kidneys from animals on a high-salt diet showed fibroid necrosis associated with wrinkling and thickening of the glomerular capillary wall, while hearts were hypertrophic. Taken together, high dietary salt induces inappropriate activation of the local renin-angiotensin-aldosterone systems. Tissue levels of angiotensin II and aldosterone may be more reflective of the severity of vascular maladaptations than are plasma levels, and may play a greater role in the maintenance of hypertension.

Vasopressin: An essential pressor factor for blood pressure recovery following hemorrhage

Two experimental approaches were used to evaluate the importance of the pressor effects of vasopressin in blood pressure recovery following hypotensive hemorrhage. Experiments using homozygous Brattleboro rats demonstrated that the hemodynamic recovery of these animals was subnormal, even though the activation and efficacy of the sympathetic nervous and renin-angiotensin systems were intact. Experiments using an antipressor vasopressin analogue in normal rats during hypotensive hemorrhage demonstrated significantly blunted blood pressure recovery in the presence of the analogue. Thus, both experiments indicate that the pressor effects of circulating vasopressin play an essential role in blood pressure recovery following hypovolemic hypotension induced by hemorrhage.

Chronic phencyclidine treatment decreases phencyclidine and dopamine receptors in rat brain

Chronic phencyclidine treatment (10 mg/kg/day, SC for 14 days) significantly decreased the number of [3H]phencyclidine and [3H]spiperone binding sites in rat brain. [3H]Dihydromorphine binding was not affected by the same treatment. An acute treatment with phencyclidine (10 mg/kg, SC) did not modify any of the binding sites under study. These results suggest that a chronic phencyclidine treatment induces a down-regulation of phencyclidine and dopamine receptors without affecting opiate receptors. These reductions in the number of phencyclidine and dopamine binding sites might be related to the development of tolerance and/or dependence to phencyclidine.

Effects of Enalapril, Tempol, and Eplerenone on Salt-Induced Hypertension in Dahl Salt-Sensitive Rats

The renin-angiotensin-aldosterone system (RAAS) has been implicated in the pathophysiology of salt-induced hypertension. Angiotensin converting enzyme inhibitors, angiotensin II-type 1 receptor blockers, and aldosterone receptor blockers are used to treat hypertension and congestive heart disease. In addition to their blood pressure lowering effects, they appear to protect against myocardial, renal, and vascular damage. In various models of hypertension, generation of reactive oxygen species is increased in the vasculature and that treatment with antioxidants or superoxide dismutase mimetics (e.g., tempol) improves vascular function and structure and reduces blood pressure. The purpose of this study was to examine the effects of enalapril, an angiotensin II converting enzyme inhibitor; eplerenone, a selective aldosterone receptor antagonist; and tempol, a superoxide dismutase mimetic, on salt-induced hypertension in Dahl Salt-Sensitive rats. The rats were placed on a high salt (HS; 8%) diet for 3 weeks prior to switching to a normal salt (0.3%) diet for an additional 3 weeks. While on the normal salt (NS) diet, rats were treated with enalapril (30 mg/kg/day in the drinking water), eplerenone (100 mg/kg/day by gavage), tempol (1 mM/day in the drinking water), eplerenone + enalapril, eplerenone + enalapril + tempol, or without drug treatment (control). After 3 weeks on HS diet, systolic blood pressure rose from 127 ± 7 to 206 ± 11 mm Hg and remained elevated when switched to NS diet. Subsequently, treatment with eplerenone alone or in combination with enalapril and tempol produced a stepwise reduction in systolic blood pressure reaching −80 mm Hg; however, enalapril and tempol alone produced more modest pressure reduction (∼−35 mmHg). Plasma levels of prostacyclin and nitric oxide were elevated in rats treated with enalapril and eplerenone alone or in combination. Enalapril and eplerenone alone and in combination reduced heart and kidney levels of angiotensin II and aldosterone when compared with control. Renal and heart levels of reduced glutathione were diminished by eplerenone alone; however, enalapril tended to attenuate the effect of eplerenone on reduced glutathione levels in the heart. The findings from this study suggest that eplerenone reduces salt-induced hypertension by increasing endothelium-derived relaxing factors, inhibiting RAAS components and oxidative stress. (353words)

1A-779 attenuates angiotensin-(1-7) depressor response in salt-induced hypertensive rats.

Chronic infusion of angiotensin-(1-7) [Ang-(1-7)] lowers blood pressure in salt-induced and spontaneously hypertensive (SHR) rats. In the present study, we have examined the acute effect of Ang-(1-7) in salt-induced hypertension using Dahl salt-sensitive rats placed on low (0.3%) or high (8.0% NaCl) salt diets for 2 weeks. Rats fed a high salt diet showed a greater rise in BP than those fed a low salt diet. Ang-(1-7) (24 microg/kg) reduced mean arterial pressure (MAP), enhanced the release of prostacyclin and nitric oxide, and suppressed thromboxane A(2) levels. A-779 (48 microg/kg, i.v), a selective Ang-(1-7) antagonist, partially blocked these effects of Ang-(1-7). The Ang-(1-7)-induced depressor response observed in these animals was related to an increase in vasodilatory prostanoids, a decrease in the constrictor prostanoid thromboxane A(2), and an increase in nitric oxide levels in both plasma and isolated aortic rings.

Acute inhibition of glutathione biosynthesis alters endothelial function and blood pressure in rats

The cardiovascular and biochemical responses during acute oxidative stress induced by D,L-buthionine-(S,R)-sulfoximine (BSO) were investigated in Sprague-Dawley rats. Mean arterial pressure, heart rate and vascular reactivity were measured after subcutaneous injection of BSO (4 mmol/kg). Control rats received saline. Levels of GSH and GSSG in blood and tissues as well as renal superoxide were determined. Nitric oxide, prostacyclin and thromboxane A(2) in plasma and aorta, and isoprostane in plasma were also measured. Blood pressure was elevated at 24 h (121+/-2 vs. 104+/-2 mm Hg), with increased reactivity to phenylephrine (by a 59+/-4 vs. 45+/-2 mm Hg change), and impaired response to sodium nitroprusside (by a -35+/-2 vs. -63+/-2 mm Hg change), P<0.05. The GSH:GSSG ratio was reduced at 8 and 24 h in blood (4.1+/-0.6 and 5.1+/-0.3, respectively, vs. 8.5+/-0.2), and at 8 h in the aorta (1.0+/-0.2 vs. 2.9+/-0.5), heart (1.6+/-0.3 vs. 2.3+/-0.1) and kidney (2.1+/-0.2 vs. 3.7+/-0.4), P<0.05. Superoxide fluorescence was increased at 24 h via NADH (4131+/-194 vs. 2853+/-199), NADPH (2874+/-272 vs.1479+/-257) and succinate (2475+/-133 vs. 1594+/-2150), P<0.05. Plasma prostacyclin was reduced at 8 and 24 h (36+/-4 and 52+/-13, respectively, vs. 310+/-44 pg/ml), P<0.001, whereas nitric oxide was reduced at 24 h (6.4+/-1 vs. 22+/-2 microM), P<0.01. Also at 24 h, thromboxane A(2) was increased both in plasma (374+/-154 vs. 61+/-10 pg/ml) and the aorta (174.4+/-38 vs. 27+/-3.4 pg/mg), P<0.05. Thus, acute BSO-induced oxidative stress alters blood pressure and endothelial function by mechanisms involving increased plasma levels and aortic release of thromboxane A(2) and reduced nitric oxide and prostacyclin.

Gender differences in the attenuation of salt-induced hypertension by angiotensin (1-7).

Chronic infusion of angiotensin (1-7) [Ang-(1-7)] lowers blood pressure in spontaneously hypertensive rats (SHR). To assess the role of Ang-(1-7) in salt-induced hypertension, Ang-(1-7) (24 microg/kg/hr) or saline was administered chronically via osmotic minipump into the jugular vein of 5-6 wk-old male (M) and female (F) Dahl salt-sensitive rats placed on a high-salt (8% NaCl) diet for 2 weeks. Blood pressure (BP) and heart rate were measured prior to the start of the diet and weekly thereafter. Ang-(1-7) significantly attenuated the BP increase after 1 wk on the diet in both M and F rats, but after 2 weeks only in F rats. Enhanced release of prostacyclin, (6-keto PGF1 alpha), following Ang-(1-7) treatment was observed in both M and F rats. In addition, significant increases in aortic blood flow and plasma levels of nitric oxide were observed in the F rats following Ang-(1-7) treatment. These findings demonstrate that the reduction in BP is due to both prostacyclin and NO and that there is a gender difference in the attenuation of salt-induced hypertension by Ang-(1-7).

High performance liquid chromatographic assay for the quantitation of total glutathione in plasma

A simple and widely used homocysteine HPLC procedure was applied for the HPLC identification and quantitation of glutathione in plasma. The method, which utilizes SBDF as a derivatizing agent utilizes only 50 microl of sample volume. Linear quantitative response curve was generated for glutathione over a concentration range of 0.3125-62.50 micromol/l. Linear regression analysis of the standard curve exhibited correlation coefficient of 0.999. Limit of detection (LOD) and limit of quantitation (LOQ) values were 5.0 and 15 pmol, respectively. Glutathione recovery using this method was nearly complete (above 96%). Intra-assay and inter-assay precision studies reflected a high level of reliability and reproducibility of the method. The applicability of the method for the quantitation of glutathione was demonstrated successfully using human and rat plasma samples.

The role of gender in salt-induced hypertension.

To evaluate gender differences in salt-induced hypertension, female and male Dahl salt-sensitive rats were fed high (8.0% NaCl, HS) and low (0.3% NaCl, LS) salt diets. During a 3-week treatment period, blood pressure was significantly elevated in both female and male HS groups compared to their respective LS groups. The blood pressure and 4 week mortality rate of the female HS group, however, were significantly lower than those of the male HS group. Renal and aortic blood flows were reduced in male rats on HS diet compared to the LS group, while, in females, renal blood flow was elevated and aortic flow was maintained while on HS diet. Plasma prostaglandin E2 and prostacyclin levels were higher in females than males and unaffected by diet. In contrast, plasma nitric oxide levels were reduced by HS, regardless of gender. In isolated aortic rings, HS diet caused a smaller elevation in the stimulated norepinephrine release ratio in female rats than in males. Thus, salt-induced hypertension is associated with a reduction in levels of nitric oxide regardless of gender. Plasma prostaglandin E2 and prostacyclin levels were higher in females. Taken together, the higher plasma prostaglandin levels and reduced sympathetic activity in females may be contributing factors in their lower blood pressure and reduced mortality.