Alpha1-adrenergic receptor subtypes in peripheral blood lymphocytes of essential hypertensives

OBJECTIVE: The expression of alpha1-adrenergic receptor subtypes in peripheral blood lymphocytes was investigated in 28 essential hypertensive patients as well as in the peripheral blood lymphocytes and aorta of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. METHODS: Alpha1-adrenergic receptors were quantified by radioligand binding assays, employing [3H]-prazosin as the radioligand in association with compounds displaying different degrees of selectivity for alpha1A-, alpha1B- and alpha1D-adrenergic receptor subtypes. RESULTS: The affinity of [3H]-prazosin binding was similar in peripheral blood lymphocytes of different stage essential hypertensive and normotensive subjects or of SHR and age-matched normotensive WKY rats as well as in the aortas of SHR and WKY rats. The radioligand binding assay revealed no change in the expression of alpha1-adrenergic receptors in peripheral blood lymphocytes of essential hypertensives compared with normotensive subjects; a moderate decrease of alpha1B-adrenergic receptors and an increase of alpha1D-adrenergic receptors. The relative densities of the alpha1-adrenergic receptor subtypes were similar in the three groups of essential hypertensives. In peripheral blood lymphocytes and in aorta of SHR, [3H]-prazosin binding was significantly reduced compared with normotensive WKY rats. The expression of alpha1-adrenergic receptor subtypes in peripheral blood lymphocytes of SHR was similar to that found in peripheral blood lymphocytes of essential hypertensives. CONCLUSIONS: Changes of lymphocyte alpha1-adrenergic receptor subtypes in essential hypertensives are similar to those observed in lymphocytes and vascular tissues of animal models of hypertension. This suggests that assays of lymphocyte alpha1-adrenergic receptors may represent an indirect marker of their involvement in essential hypertension.     

Specific changes in hypothalamic alpha-adrenoceptors in young spontaneously hypertensive rats

Changes in the activity of hypothalamic and brain-stem adrenergic neurons have been reported in young spontaneously hypertensive rats (SHR) prior to the development of hypertension. We have measured central alpha- and beta-adrenoceptor concentrations in 4-week-old SHR and Wistar-Kyoto (WKY) controls by direct radioligand binding studies using [3H]prazosin (alpha 1), [3H]clonidine (alpha 2), and [125I]iodohydroxybenzlpindolol (beta). The concentration of alpha 2-adrenoceptors was significantly elevated in the hypothalamus of the SHR, 156.9 +/- 10.4 compared with WKY, 119.4 +/- 10.0 fmole/mg protein (n = 7, mean +/- SEM, p less than 0.0125). Alpha 2-adrenoceptor concentrations in both the brain stem and cerebral cortex were similar in the two groups of animals. The increase in hypothalamic adrenoceptors was specific for alpha 2-adrenoceptors, since similar concentrations of alpha 2- and beta-adrenoceptors were found in this region.     

Upregulation of renal alpha 2-adrenergic receptors is not indicative of salt-related increases in blood pressure in spontaneously hypertensive rats

The aim of the present study was to determine if elevations in salt intake were coupled to increases in renal alpha 2-adrenergic receptors in SHR that differ in their blood pressure response to high salt diets. Salt-resistant spontaneously hypertensive rats (SHR-R), which do not increase their blood pressure in response to high salt intake, and salt-sensitive spontaneously hypertensive rats (SHR-S), which do exhibit significant elevations in blood pressure on high salt diets (3.15% NaCl), were used. Radioligand binding studies using [3H]rauwolscine were performed on 6- and 11-week-old SHR-S and Wistar-Kyoto (WKY) rats to determine the effects of age, strain, and salt intake on alpha 2-adrenergic receptor number and affinity. One week of high salt intake significantly increased blood pressure 22% in 6-week-old SHR-S and increased the blood pressure of 11-week-old SHR-S 12% without altering WKY rat controls. This treatment did not significantly increase renal alpha 2-adrenergic receptors in either SHR-S or WKY rats. SHR-S had significantly higher numbers of renal alpha 2-adrenergic receptors than WKY rats on the high salt diets. One week of high (3.15%) or low (0.05%) salt intake did not significantly alter renal alpha 2-adrenergic receptor number in 11-week-old SHR-S or WKY rats; however, blood pressure was significantly elevated in the SHR-S (175.0 +/- 3.5 versus 196.0 +/- 3.0 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)     

A pertussis toxin substrate regulates alpha 1-adrenergic dependent phosphatidylinositol hydrolysis in cultured rat myocytes

The chronotropic response of the heart to alpha 1-adrenergic catecholamines influenced by pertussis toxin under certain conditions. In view of the fact that alpha 1-adrenergic action is mediated by the phosphatidylinositol pathway of hormone action in many cells, we examined the hypothesis that alpha-adrenergic agonists stimulate phosphatidylinositol hydrolysis in cardiomyocytes and that this effect is sensitive to pertussis toxin. Addition of norepinephrine to cultured rat ventricular myocytes prelabeled with myo-[2-3H]inositol resulted in rapid and significant accumulation of inositol phosphate (IP1) and inositol biphosphate. Norepinephrine-stimulated IP1 formation was not inhibited by propranolol, but was inhibited by alpha-adrenergic antagonists with an order of potency indicating alpha 1-adrenergic receptor subselectivity: prazosin (alpha 1; 3 nM) greater than yohimbine (alpha 2; 10 microM). The effect of norepinephrine to enhance IP1 formation was markedly attenuated in cells pretreated with pertussis toxin. Pertussis toxin also induced the transfer of ADP-ribose from NAD to a 41,000-dalton membrane protein in these cells. The concentration of pertussis toxin resulting in maximal inhibition of norepinephrine-stimulated IP1 formation correlated well with the concentration of pertussis toxin necessary to completely ADP-ribosylate a 41,000-dalton membrane protein (1 ng/ml). The range over which pertussis toxin inhibited norepinephrine-dependent IP1 formation and ADP-ribosylated the 41,000-dalton substrate was virtually identical. These observations establish a role for a 41,000-dalton pertussis toxin substrate in coupling the alpha 1-adrenergic receptor to phosphoinositol hydrolysis in myocardial cells.     

Comparison of Endothelin-1 and Noradrenaline Stimulated Inositol Phosphate Formation in Cultured Aortic Smooth Muscle Cells from Spontaneously Hypertensive and Wistar Kyoto Rats

Total [³H]-inositol phosphate formation was measured in cultured aortic smooth muscle cells from 6 and 14 week spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) normotensive rats. Basal inositol phosphate formation was significantly increased in cells cultured from SHR compared to WKY at both 6 and 14 weeks as was basal phosphatidylinositol formation. This difference in basal values was apparent after 9 h or more incubation with [³H]-myoinositol. Both endothelin-1 and noradrenaline stimulated inositol phosphate formation was unchanged in cultured smooth muscle cells from 6-week SHR compared to WKY. In cultured smooth muscle cells from 14-week SHR a decrease was observed in endothelin-1 stimulated inositol phosphate formation compared to controls. Noradrenaline stimulated inositol phosphate formation was increased in cultured cells from 14 week SHR. Endothelin-1 and noradrenaline stimulated inositol phosphate formation does not appear to be involved in the development (at 6 weeks) of hypertension in this model. However, in established hypertension (14 weeks) cells from SHR have altered total [³H] inositol phosphate formation in response to stimulation with noradrenaline and endothelin-1 although these changes are in opposite directions. Therefore, in cultured smooth muscle cells from 14-week rats noradrenaline and endothelin-1 appear to be regulated independently with regard to their effects on the phosphatidylinositol cycle.     

Phospholipase A2 and phospholipase C are activated by distinct GTP-binding proteins in response to alpha 1-adrenergic stimulation in FRTL5 thyroid cells.

In FRTL5 rat thyroid cells, norepinephrine, by interacting with alpha 1-adrenergic receptors, stimulates inositol phosphate formation, through activation of phospholipase C, and arachidonic acid release. Recent studies have shown that GTP-binding proteins couple several types of receptors to phospholipase C activation. The present study was undertaken to determine whether GTP-binding proteins couple alpha 1-adrenergic receptors to stimulation of phospholipase C activity and arachidonic acid release. When introduced into permeabilized FRTL5 cells, guanosine 5'-[gamma-thio]triphosphate (GTP[gamma-S]), which activates many GTP-binding proteins, stimulated inositol phosphate formation and arachidonic acid release. Neomycin inhibited GTP[gamma-S]-stimulated inositol phosphate formation but was without effect on GTP[gamma-S]-stimulated arachidonic acid release, suggesting that separate GTP-binding proteins mediate each process. In addition, pertussis toxin inhibited norepinephrine-stimulated arachidonic acid release but not norepinephrine-stimulated inositol phosphate formation. Norepinephrine-stimulated arachidonic acid release but not inositol phosphate formation was also inhibited by decreased extracellular calcium and by TMB-8, suggesting a role for a phospholipase A2. To confirm that arachidonic acid was released by a phospholipase A2, FRTL5 membranes were incubated with 1-acyl-2-[3H]arachidonoyl-sn-glycero-3-phosphocholine. GTP[gamma-S] slightly stimulated arachidonic acid release, whereas norepinephrine acted synergistically with GTP[gamma-S] to stimulate arachidonic acid release. The results show that phospholipase C and phospholipase A2 are activated by alpha 1-adrenergic agonists. Both phospholipases are coupled to the receptor by GTP-binding proteins. That coupled to phospholipase A2 is pertussis toxin-sensitive, whereas that coupled to phospholipase C is pertussis toxin-insensitive.     

Alterations in the plasma membrane properties of the myocardium of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were used to investigate the adaptive biochemical changes in the myocardium in response to chronic afterload. Ouabain-inhibited Na+,K+-adenosine triphosphatase (ATPase) activity was decreased by 40% in myocardium of SHR compared with that from WKY, which may lead to increased intracellular Ca2+ through Na+-Ca2+ exchange. Similarly, alpha 1-adrenergic receptor density, estimated by [3H]prazosin binding, was decreased by 42% in myocardial membranes of SHR, while the affinity for the agonist and the antagonist was not altered. In contrast, the number of Ca2+ channels estimated by [3H]nitrendipine binding was increased by 45% in myocardial membranes of SHR, while the affinity was comparable between SHR and WKY. These differences between WKY and SHR in the membrane properties were not due to differential contamination of plasma membranes because the activities of other putative plasma membrane marker enzymes were comparable between WKY and SHR. There were no differences between WKY and SHR in the myosin ATPase activity estimated using myofibrils, actomyosin, and myosin. These results suggest that specific alterations have occurred in the plasma membrane properties of myocardium of SHR that result in altered intracellular Ca2+ metabolism. These alterations may have an important bearing on excitation-contraction coupling in myocardium of SHR.     

Pressor mechanisms linked obligatorily to spontaneous hypertension in the rat

To identify genetic factors linked obligatorily to hypertension in the rat, pithed spontaneously hypertensive rats (SHR) were compared with genetically similar (Wistar-Kyoto rats; WKY) and different (Sprague-Dawley) normotensive strains. The only variables that distinguished SHR from both WKY and Sprague-Dawley rats were a greater maximum pressor response to electrical stimulation of sympathetic outflow and decreased sensitivity to submaximal doses of the alpha 1-adrenergic agonist methoxamine (i.e., higher ED50). SHR had in common with Sprague-Dawley rats basal blood pressure after pithing plus adrenalectomy and the maximum pressor response to methoxamine; both these values were higher than those in WKY. All strains demonstrated equal sensitivity of the vasoconstrictor response to endogenous norepinephrine released by electrical simulation at submaximal frequency, even though sensitivity to the alpha 1-adrenergic receptor agonist was lower in SHR. The alpha 2-adrenergic receptor antagonist rauwolscine attenuated the pressor response to electrical stimulation in SHR and WKY but increased it in Sprague-Dawley rats. The alpha 1-adrenergic receptor antagonist prazosin attenuated the response more in SHR and WKY than in Sprague-Dawley rats. We conclude that 1) sympathetic hyperactivity is linked obligatorily to hypertension in SHR; 2) increased basal blood pressure and noradrenergic vasoconstrictor response are present in SHR, but they are not obligatorily linked to hypertension; 3) feedback inhibition of norepinephrine release is comparable in SHR or WKY and poorly developed compared with that in Sprague-Dawley rats; 4) decreased sensitivity of the pressor response to stimulation of vascular alpha 1-adrenergic receptors in SHR compensates partially for increased sympathetic activity or hyperinnervation, or both.     

Modulation of noradrenergic transmission by neuropeptide Y and presynaptic alpha 2-adrenergic receptors in the hypothalamus of spontaneously hypertensive rats.

Neuropeptide Y (NPY) has a wide and specific distribution in the central nervous system, and is colocalized with catecholamines in specific neuronal systems. In this study, in order to investigate the regulatory mechanisms of NPY and presynaptic alpha 2-adrenergic receptors on central noradrenergic transmission in hypertension, we have examined the effects of NPY and the alpha 2-agonist, UK 14,304, on (3H)-noradrenaline (NA) release from hypothalamic slices of spontaneously hypertensive rats (SHR). Electrical stimulation (1 Hz)-evoked (3H)-NA release was significantly greater in the hypothalamic slices of SHR than in those of Wistar Kyoto rats (WKY). NPY and the alpha 2-agonist, UK 14,304, inhibited the stimulation-evoked (3H)-NA release in a dose-related manner. The inhibitory effects of NPY and UK 14,304 on NA release were significantly attenuated in SHR compared with WKY. These results suggest that NPY and alpha 2-adrenoceptors might be involved in the regulation of central sympathetic nervous activity in hypertension.     

Normal ontogeny of alpha 1-adrenergic receptor in rat liver is thyroid hormone dependent

Postnatal ontogeny of rat liver alpha 1-adrenergic receptor was examined using alpha 1-specific radioligand [3H]prazosin in control and propylthiouracil-treated congenital hypothyroid rats at various ages. Partially purified rat liver membranes prepared by the Neville method had 8-fold purification of 5'-nucleotidase from the crude homogenates from postnatal day 5 to adulthood. [3H]Prazosin binding was typical of an alpha 1-adrenergic receptor, and (-)epinephrine affinity for the [3H]prazosin-binding sites was not altered in the presence of 10(-5) M guanylyl-imidodiphosphate. The receptor density was lower in 5- and 15-day-old rats than in 28-day-old or older rats in both control and hypothyroid groups. (P less than 0.01). At 28-34 days of age, hypothyroid pups had significantly lower alpha 1-receptor density than controls (399 +/- 10 vs. 869 +/- 40 fmol mg protein-1; P less than 0.01). Replacement therapy with daily T4 injection from postnatal days 16-27 restored 54% of the deficit in PTU-treated hypothyroid pups at 28 days. The dissociation constant of [3H] prazosin did not change with advancing age or with different treatment and was consistent at 0.1 nM. These findings indicate that the normal ontogeny of plasma membrane alpha 1-adrenergic receptors is dependent upon thyroid hormone and matures postnatally in rat liver.     

Selective modification of renal alpha 2-adrenergic receptors in Milan hypertensive rat strain

Cerebral and renal alpha-adrenergic receptors play an important role in the control of blood pressure. We studied alpha-adrenergic receptors in the cerebral and renal cortex of Milan hypertensive strain (MHS) and normotensive strain (MNS) rats, a genetic model of spontaneous hypertension linked to a kidney abnormality. Binding of the selective alpha 1-adrenergic antagonist [3H]prazosin and the alpha 2-adrenergic antagonist [3H]rauwolscine was used for receptor studies in tissues of prehypertensive (24-day-old) and hypertensive (60-day-old) rats. In the cerebral cortex, no between-strain differences in alpha 1-adrenergic and alpha 2-adrenergic receptor density and affinity were observed in prehypertensive and hypertensive periods. The density of these receptors increased similarly with age in MHS and MNS rats. In the renal cortex, the differences between MHS and MNS rats concerned alpha 2-adrenergic receptors only. Compared with their age-matched normotensive controls, MHS rats showed 1) a lower affinity for the antagonist (p less than 0.05) in the prehypertensive period, 2) absence of the normal age-related increase in receptor density, and 3) a lower density of [3H]rauwolscine binding sites (p less than 0.001) in the hypertensive period. In this period, studies of competitive inhibition of [3H]rauwolscine binding showed that l-epinephrine bound to one class of sites in MHS rats (pseudo-Hill plot, 0.90) and to two classes in MNS rats (pseudo-Hill plot, 0.68). In addition, the lack of any guanylylimidodiphosphate effect on the l-epinephrine competition curve observed in MHS rats suggests the uncoupling of these receptors from the guanosine 5'-triphosphate binding protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of norepinephrine release by galanin in rat medulla oblongata.

Galanin, a 29-amino acid peptide, is widely distributed in both the central and peripheral nervous systems and is colocalized with catecholamines, although its physiological significance remains to be elucidated. In the present study we investigated the regulatory mechanisms of galanin on norepinephrine release in rat medulla oblongata. In slices of medulla oblongata of Sprague-Dawley rats, galanin inhibited the stimulation-evoked [3H]norepinephrine release in a concentration-dependent manner (fractional release ratio during electrical stimulation: control 0.937 +/- 0.043, mean +/- SEM, n = 6; galanin 1 x 10(-7) M 0.501 +/- 0.037, n = 6, p less than 0.05; and galanin 1 x 10(-6) M 0.299 +/- 0.018 n = 6, p less than 0.05). Galanin potentiated inhibition of [3H]norepinephrine release by the alpha 2-agonists (UK 14,304 and clonidine). The blockade of alpha 2-adrenergic receptors by RX 781094 diminished the inhibition of norepinephrine release by galanin. Pretreatment of pertussis toxin, which interferes with the coupling of inhibitory guanosine triphosphate-binding proteins to adenylate cyclase, significantly attenuated the suppressive effects of galanin on norepinephrine release. In slices of medulla oblongata obtained from spontaneously hypertensive rats (SHR), the inhibitory effect of galanin on norepinephrine release was significantly less than in those from age-matched Wistar-Kyoto rats. These results show that galanin might inhibit the stimulation-evoked norepinephrine release in rat medulla oblongata, at least partially mediated by alpha 2-adrenergic receptors and the pertussis toxin-sensitive guanosine triphosphate-binding proteins. Moreover, less suppression of norepinephrine release by galanin in SHR suggests that galanin might be involved in the regulation of central sympathetic nervous activity in hypertension.     

Permissive Role of Calcium in α1-Adrenergic Stimulation of Pineal Phosphatidylinositol Phosphodiesterase (Phospholipase C) Activity

Activation of alpha 1-adrenergic receptors increases [Ca+2]i and phosphatidylinositol phosphodiesterase (phospholipase C) activity in the pinealocyte. In this report the receptor involved in the stimulation of phospholipase C activity was further characterized, and the role of Ca2+ in this effect was investigated in some detail. Phospholipase C activity was estimated by measuring the production of [3H]inositol phosphates by [3H]inositol-labelled dispersed pinealocytes in suspension culture. Norepinephrine stimulated [3H]inositol monophosphate production severalfold; this was blocked by alpha 1-adrenergic antagonists, including prazosin, WB 4101, and phenoxybenzamine, but by neither an alpha 2- nor a beta-adrenergic antagonist, confirming that an alpha 1-adrenoceptor is involved in the regulation of phosphatidylinositol hydrolysis. Treatment with the Ca2+ chelator, EGTA, or with inorganic Ca2+ blockers, including Co2+, Mn2+, and La3+, reduced the norepinephrine-stimulated response, suggesting that the alpha 1-adrenergic stimulation of phospholipase C activity is Ca2+ dependent. However, phospholipase C activity was not increased by elevating intracellular Ca2+ with either the Ca2+ ionophore A23187 or with depolarizing concentrations of K+. These results indicate that although Ca2+ is necessary for alpha 1-adrenergic stimulation of phospholipase C activity, an increase in [Ca2+]i alone is not sufficient to stimulate the activity of this enzyme, and that effects which A23187 and depolarizing concentrations of K+ have on pineal function probably do not involve stimulation of phospholipase C activity.     

Sodium responsiveness of central alpha 2-adrenergic receptors in spontaneously hypertensive rats

The responsiveness of central nervous system alpha 2-adrenergic receptors in the neural control of renal function was compared in conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) on normal or high sodium intake (3-4 weeks of 1% NaCl for drinking). The responsiveness of central alpha 2-adrenergic receptors was determined by comparing among groups the dose-response curves for the effects of cumulative intracerebroventricular injections of guanabenz (5, 25, and 125 micrograms) on changes in mean arterial pressure, renal sympathetic nerve activity, and urinary sodium excretion. Guanabenz altered mean arterial pressure similarly in SHR on normal or high sodium intake and in WKY on normal or high sodium intake. High sodium intake shifted the guanabenz-renal sympathetic nerve activity and guanabenz-urinary sodium excretion dose-response curves to the left in SHR and to the right in WKY. The dose-response curves between SHR and WKY on normal sodium intake were similar. Surgical renal denervation or pretreatment with an alpha 2-adrenergic receptor antagonist (rauwolscine, 30 micrograms i.c.v.) attenuated the ability of guanabenz to inhibit renal sympathetic nerve activity or increase urinary sodium excretion in SHR and WKY on either normal or high sodium intake. We conclude that the responsiveness of central nervous system alpha 2-adrenergic receptors regarding the neural control of renal function is increased by high sodium intake in conscious SHR, but not in conscious normotensive WKY.     

Enhanced DNA synthesis in heart and kidney of newborn spontaneously hypertensive rats

Studies comparing relative organ weight (per gram of body weight) in spontaneously hypertensive rats (SHR) and their normotensive controls, Wistar-Kyoto rats (WKY), have shown that the heart and kidney of SHR are enlarged at birth. In addition, higher DNA concentrations in these organs provide evidence of cardiac and renal cellular hyperplasia in newborn SHR. The rate of DNA synthesis was estimated with [methyl-3H]thymidine labeling in vivo for heart, kidney, liver, adrenals, and aorta of newborn WKY and SHR, and significantly higher relative organ weights, protein levels, and DNA contents were observed only for heart and kidney from SHR. In contrast, relative weight was significantly lower for liver from SHR and the same for adrenals from WKY and SHR despite lower adrenal protein and DNA contents for SHR. Relative weight and protein content of aorta were higher in SHR, possibly indicating hypertrophy of this organ. Several differences in [methyl-3H]thymidine labeling were observed in organs from SHR. Significantly higher organ accumulation (cpm/mg organ weight) was observed for heart from SHR (p less than 0.01), and higher incorporation into DNA (cpm/cpm) was observed for kidney and liver from SHR (p less than 0.001). Most significant was the finding of higher DNA specific activity for heart (+33%; p less than 0.05) and kidney (+39%; p less than 0.001) from SHR. These data provide evidence of enhanced DNA synthesis in the heart and kidney of newborn SHR.     

Cardiovascular responsiveness to parathyroid hormone (PTH) and PTH-related protein in genetic hypertension.

Hypertension is often accompanied by abnormalities of calcium homeostasis, including hyperparathyroidism with reduced target organ responses to PTH in kidney and bone. Due to this association between PTH and hypertension and since PTH and the paracrine factor PTH-related protein (PTHrp) have both been shown to exert marked changes in cardiovascular activity, these actions of PTH and PTHrp were examined in spontaneously hypertensive rats (SHR) and in control normotensive Wistar-Kyoto rats (WKY). Fourteen-week-old SHR [systolic blood pressure (SBP), 201 +/- 4.4 mm Hg] and WKY (SBP, 141 +/- 2.5 mm Hg) were studied. Renal cortical membranes were prepared and assayed for radioligand binding with [125I]PTH-(1-34) and [125I]PTHrp-(1-34). There was no apparent alteration in the affinity of the binding sites to either peptide in the SHR, but specific binding in SHR renal tissue was only 60% of that observed in WKY tissue for both peptides. Serum immunoreactive PTH levels were 4-fold higher in SHR than WKY, while serum total calcium and 1,25-dihydroxyvitamin D3 levels were not different. The iv administration of both PTH and PTHrp produced dose-dependent reductions in SBP and increases in heart rate in conscious unrestrained SHR and WKY. Both peptides caused greater absolute reductions in blood pressure in SHR than in WKY. However, when the hypotensive response was normalized for the higher baseline pressure in the SHR, the blood pressure reductions caused by PTH and PTHrp were not different in SHR and WKY. Conversely, the chronotropic responses to PTH and PTHrp were lower in SHR compared to WKY. These findings indicate that the SHR exhibits elevated PTH levels, with a reduced number of renal PTH/PTHrp receptors and a depressed chronotropic response to either PTH or PTHrp. In contrast, the hypotensive response to PTH or PTHrp was not altered, indicating possible tissue-specific receptor subclasses or tissue-specific regulation of PTH and PTHrp receptors.     

Alterations in catecholamine release in the central nervous system of spontaneously hypertensive rats.

The aim of the present study was to investigate alterations in catecholamine release in the central nervous system of spontaneously hypertensive rats. Slices of hypothalamus, medulla oblongata and striatum were prepared from spontaneously hypertensive rats (SHR: 9-10 weeks old) and age-matched Wistar Kyoto rats (WKY). The slices were incubated with (3H)norepinephrine (NE) or (3H)dopamine (DA), superfused with Krebs-solution in vitro, and the release of the catecholamines was compared between the two strains. The basal release of hypothalamic (3H)NE did not differ between SHR and WKY slices. However, stimulation (1 Hz)-evoked (3H)NE release was significantly greater in SHR than in WKY (percent fractional release of total tissue NE: WKY 0.494 +/- 0.019%, n = 6, SHR 0.730 +/- 0.053%, n = 6, p less than 0.05). The stimulation-evoked (3H)NE release from the medulla oblongata did not differ significantly between SHR and WKY slices. Finally stimulation-evoked release of striatal (3H)DA was significantly depressed in SHR (percent fractional release of total tissue DA: WKY 2.048 +/- 0.24%, n = 6, SHR 1.460 +/- 0.068%, n = 6, p less than 0.05). These results indicate that the release of hypothalamic NE and striatal DA are altered in SHR. It is suggested that enhanced hypothalamic noradrenergic activity and reduced striatal dopaminergic activity can increase sympathetic outflow to the periphery, which may play a role in the pathogenesis of this form of hypertension.     

Inhibitory effect of melatonin on alpha1-adrenergic-induced vasoconstriction in mesenteric beds of spontaneously hypertensive rats

BACKGROUND: The aim of this study was to assess the effects of melatonin on alpha1-adrenergic pathway in mesenteric arteries of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. METHODS AND RESULTS: The SHR are characterized by a higher vasoconstriction (P < .001) and inositol phosphate formation (P < .001) in response to phenylephrine (PHE) and an increased superoxide anion production (P < .001) in mesenteric arteries. Melatonin and 2-iodomelatonin produced a significant inhibition of the PHE-induced vasoconstriction in isolated and perfused mesenteric beds (P < .001) with the same magnitude in SHR and WKY rats. Melatonin significantly decreased the inositol phosphate (IPs) formation in isolated mesenteric arteries from SHR compared to WKY rats (P < .001). The inhibitory effect of melatonin was increased by the removal of endothelium (P < .001). No effects of superoxide dismutase (SOD), tempol, or catalase were observed on the PHE-induced vasoconstriction. Moreover, no superoxide anion scavenging effect of 2-iodomelatonin was observed in isolated mesenteric vascular muscle cells using lucigenin. CONCLUSIONS: The present study showed that high melatonin concentrations inhibit the alpha1-adrenergic-induced vasoconstriction and inositol phosphate formation in mesenteric arteries from SHR and WKY rats. The vasorelaxant effect of the melatonin receptors agonist, 2-iodomelatonin, and the absence of any vasoactive effect of antioxidants such as SOD, tempol, and catalase suggest that melatonin exerts its inhibition on alpha1-adrenergic-induced vasoconstriction of mesenteric arteries through a low-affinity membrane receptor negatively coupled to the IPs formation and that this effect is independent of its antioxidant properties.     

Central alpha-adrenoceptors during the development of hypertension in rats on high and low salt intake

Our purpose was to investigate the binding characteristics of central alpha-adrenoceptors during the early stages of the development of hypertension in rats on high and low salt (NaCl) intake. We measured alpha 1-[( 3H]prazosin) and alpha 2-[( 3H]rauwolscine) binding in membranes of the hypothalamus and medulla oblongata of six groups of young Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHR) and subtotally nephrectomized WKY (SN) rats with mean arterial blood pressure (MAP) ranging from normotensive to hypertensive levels after 1 week of salt restriction or loading. In the hypothalamus the SN-high salt rats and both SHR groups had elevated alpha 1-number but there was no change in alpha 2-number. Moreover, MAP was positively correlated with mean hypothalamic alpha 1-number in the six groups. In the medulla oblongata alpha 1-number was unaffected. However, high salt diet influenced medullary alpha 2-binding in the opposite manner in WKY rats versus SN rats and SHR. In these latter groups the affinity was increased and the number decreased in response to high salt intake. Furthermore, a positive correlation between MAP and mean alpha 1:alpha 2 ratio existed in both the hypothalamus and the medulla of the six groups. The data suggest that hypothalamic alpha 1-binding capacity was increased in SHR due principally to a genetic condition which is mimicked by salt loading in the SN rats. Medullary alpha 2-adrenoceptors of WKY, which remained normotensive despite salt loading, responded differently to high salt intake than those of the SN and SHR, whose blood pressure rose significantly.(ABSTRACT TRUNCATED AT 250 WORDS)     

Renal alpha 2-adrenergic receptors multiply and mediate sodium retention after prazosin treatment

Renal nerve stimulation-induced antinatriuresis normally is mediated through post-synaptic alpha 1-adrenergic receptors; however, prazosin-induced alpha 1-adrenergic receptor blockade is associated clinically with sodium retention and not natriuresis. To study whether alpha 2-adrenergic receptors mediate renal nerve stimulation-induced antinatriuresis after chronic prazosin treatment, Sprague-Dawley rats were pretreated for 3 days with prazosin (3 mg/kg/day i.p. plus 0.15 mg/ml drinking water) or vehicle (untreated). In isolated perfused (Krebs-Henseleit; Ficoll, 3.5 g/dl, + albumin, 1.0 g/dl at 36 degrees C) kidneys from untreated rats, subpressor levels of renal nerve stimulation (approximately 1 Hz, 10 V, 1 msec) decreased (p less than 0.05) sodium (from 4.50 +/- 0.42 to 1.71 +/- .23 muEq/min) and urinary excretion rate (from 87.2 +/- 4.1 to 57.9 +/- 3.9 microliter/min). Adding prazosin (30 nM) to the perfusate completely (approximately 90%) reversed this effect (p less than 0.05), while alpha 2-adrenergic receptor blockade with yohimbine (300 nM) had no effect. In perfused kidneys from prazosin-treated rats, renal nerve stimulation decreased (p less than 0.05) sodium (from 3.24 +/- .40 to 1.32 +/- .27 muEq/min) and urinary excretion rate (from 78.7 +/- 5.0 to 54.1 +/- 5.3 microliter/min). However, adding prazosin (100 nM) to the perfusate produced only a slight, insignificant reversal of these effects; prazosin plus yohimbine were required to completely reverse the effects. These results suggest that renal nerve stimulation-induced sodium reabsorption was activated by alpha 1-adrenergic receptors in untreated rats and in part by alpha 2-adrenergic receptors in rats pretreated for 3 days with prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)