Endothelin receptor antagonists reduce the pressor effects of angiotensin II into the periaqueductal gray area of rats.

Injection of ANGII (0.01, 0.1 and 1 nmol/rat) into the periaqueductal gray (PAG) area significantly (P<0.01) increased, in a dose-dependent manner, the mean arterial blood pressure (MAP). The increases in blood pressure induced by ANGII (1 nmol; 37 +/- 4 mmHg, n=5) were greatly reduced (>85%) by pre-administration of the ET(A) receptor antagonist FR139317 (5 nmol/rat) to the PAG area, but were unaffected by the ET(B) receptor antagonist BQ-788 (5 nmol/rat). SB209670, non-selective ET(A)/ET(B) receptor antagonist, also reduced the effect induced by ANGII. These results suggest that endogenous endothelin-1, via an action on ET(A) receptors, may contribute to the pressor effects of ANGII within the PAG area of rats.     

α1A-adrenergic receptor mediated pressor response to phenylephrine in anesthetized rat

To determine which subtype of α₁-adrenergic receptors plays a role in the regulation of blood pressure, with α_1A-adrenergic receptor-mediated vasoconstriction in perfused hindlimb as a control, we compared the inhibitory effects of various α₁-adrenergic receptor selective antagonists on the vasopressure responses to phenylephrine between the mean arterial pressure and hindlimb perfusion pressure in anesthetized rats. In Normotensive Wistar rats, the results showed that the inhibitory effects (dose ratios of ED₅₀, Dr) of α₁-adrenoceptor selective antagonist (prazosin, Dr 13.5 ± 3.6 vs.15.1 ± 4.3, n = 11), α_1A-adrenoceptor selective antagonist (5-methyl-urapidil, Dr 2.4 ± 0.9 vs. 3.7 ± 2.3, n = 12; RS-17053, Dr 3.2 ± 1.6 vs. 4.4 ± 3.3, n = 12) and α_1D-adrenoceptor selective antagonist (BMY7378, Dr 1.9 ± 0.9 vs. 2.2 ± 0.8, n = 8) on phenylephrine-induced increases of perfusion pressure in the autoperfused femoral beds were the same as that in the mean arterial blood pressure in normotensive Wistar rats. The inhibitory effects of antagonists (RS-17053, Dr 3.4 ± 0.6 vs. 4.3 ± 0.9, n = 5; BMY7378, Dr 1.7±0.5 vs. 1.7 ± 0.5, n = 8) in spontaneous hypertensive rats were similar with the Wistar rats. These results suggest that the mean arterial pressure induced by phenylephrine was mainly mediated by α_1A-adrenergic receptor in both the anesthetized Wistar rats and spontaneous hypertensive rats.     

Intrathecal endomorphin-1 produces antinociceptive activities modulated by alpha 2-adrenoceptors in the rat tail flick, tail pressure and formalin tests

It is known that spinal morphine produces antinociception that is modulated by alpha 2-adrenoceptors. Endomorphin-1, a newly-isolated endogenous opioid ligand, shows the greatest selectivity and affinity for the mu-opiate receptor of any endogenous substance found to date and may serve as a natural ligand for the mu-opiate receptor. We examined the antinociceptive effects of endomorphin-1 administered intrathecally (i.t.) in the rat tail flick, tail pressure and formalin tests. Intrathecal endomorphin-1 produced dose-dependent antinociceptive effects in the three tests. ED50 (CI95) values for antinociception of i.t. endomorphin-1 in the tail flick test and tail pressure test were 1.9 (0.96-3.76) nmol and 1.8 (0.8-4.2) nmol, respectively. ED50 (CI95) values for phase 1 and phase 2 in the formalin test were 12.5 (7.9-19.8) nmol and 17.5 (10.2-30) nmol, respectively. Pretreatment with i.t. beta-funaltrexamine (a mu-opioid receptor selective antagonist) significantly antagonized the antinociceptive effects of endomorphin-1 in the three tests. Beta-funaltrexamine alone had not effects on the three tests. The antinociceptive effects of endomorphin-1 were also antagonized by i.t. yohimbine (an alpha 2-adrenoceptor selective antagonist). The combination of ineffective doses of i.t. clonidine (an alpha 2-adrenoceptor agonist) and endomorphin-1 produced a significant antinociception in the three tests. The results showed that intrathecal endomorphin-1 produced antinociception in a dose-dependent manner in the rat tail flick, tail pressure and formalin tests, which was mediated by spinal mu-opioid receptors and modulated by alpha 2-adrenoceptors.     

Analysis of responses to sympathetic nerve stimulation in the feline pulmonary vascular bed

The adrenergic receptor subtypes mediating the response to sympathetic nerve stimulation in the pulmonary vascular bed of the cat were investigated under conditions of controlled blood flow and constant left atrial pressure. The increase in lobar vascular resistance in response to sympathetic nerve stimulation was reduced by prazosin and to a lesser extent by yohimbine, the respective alpha 1- and alpha 2-adrenoceptor antagonists. Moreover, in animals pretreated with a beta-adrenoceptor antagonist to prevent an interaction between alpha- and beta 2-adrenoceptors, responses to nerve stimulation were reduced by prazosin, but yohimbine had no significant effect. On the other hand, in animals pretreated with a beta-adrenoceptor antagonist, yohimbine had an inhibitory effect on responses to tyramine and to norepinephrine. Propranolol had no significant effect on the response to nerve stimulation, whereas ICI 118551, a selective beta 2-adrenoceptor antagonist, enhanced responses to nerve stimulation and injected norepinephrine. The present data suggest that neuronally released norepinephrine increases pulmonary vascular resistance in the cat by acting mainly on alpha 1-adrenoceptors and to a lesser extent on postjunctional alpha 2-adrenoceptors but that this effect is counteracted by an action on presynaptic alpha 2-receptors. The present studies also suggest that neuronally released norepinephrine acts on beta 2-adrenoceptors and that the response to sympathetic nerve stimulation represents the net effect of the adrenergic transmitter on alpha 1-, alpha 2-, and beta 2-adrenoceptors in the pulmonary vascular bed.     

Functional evidence for the presence of a phosphoramidon-sensitive enzyme in rat brain that converts big endothelin-1 to endothelin-1

Endothelin-1 (ET-1) is produced from its precursor, big endothelin-1 (BigET-1), by a putative endothelin-converting enzyme (ECE), but it is not known whether the enzyme is present in the brain. This study was conducted to examine the central hemodynamic effects of BigET-1 and to indirectly determine the presence of an ECE in rat brain. Cardiovascular effects of centrally administered BigET-1 and ET-1 were examined in anesthetized, ventilated rats. BigET-1 (100 pmol) or ET-1 (10 pmol) applied to the IV ventricle produced similar prolonged decreases in mean arterial pressure (MAP) and renal blood flow (RBF). Thus, peak decreases with BigET-1 were (mean +/- S.E.): MAP = -35 +/- 4%; RBF = -27 +/- 5%, while those with ET-1 were: MAP = -36 +/- 5%; RBF = -29 +/- 9%. Pretreatment with phosphoramidon, a metalloprotease inhibitor (90 nmol), abolished the hemodynamic responses elicited by BigET-1 (MAP = -9 +/- 2%; RBF = -3 +/- 2%) but not those produced by ET-1. These data indicate that; i) conversion of BigET-1 to ET-1 in the brain is essential for the expression of hemodynamic actions and ii) a metalloprotease capable of converting BigET-1 to ET-1 is present in rat brain.     

Interactions of pure beta gamma-subunits of G-proteins with purified beta 1-adrenoceptor

The role of the beta gamma-subunits in the interaction of G-proteins was examined with beta 1-adrenoceptors purified from turkey erythrocytes and pure beta gamma-subunits prepared from turkey erythrocytes and bovine brain. On a non-denaturing polyacrylamide gel, the mobility of beta gamma-subunits was increased when incubated with beta 1-adrenoceptor and the beta 1-adrenergic agonist 1-(-)-isoproterenol, whereas on incubation with the antagonist 1-alprenolol the mobility was unchanged. Furthermore, the beta 1-adrenoceptor was retarded on a Sephadex G-50 column equilibrated with beta gamma-subunits and agonist. No retardation occurred in the presence of antagonist. These data suggest a direct interaction of activated beta 1-adrenoceptors with isolated beta gamma-subunits of G-proteins.     

Effect of nephrectomy and of adrenergic receptor blockers on the cardiorenal actions of endothelin

The cardiorenal actions of endothelin-1 (ET-1) were evaluated in rats following nephrectomy, in rats during alpha-adrenergic blockade with phentolamine, and in rats during beta-adrenergic blockade with propranolol. Female rats were anesthetized with pentobarbital and, following surgery, were allowed 60 min to stabilize before 3 x 20 min-control clearances were collected. ET-1 was then infused at a rate of 100 ng kg-1 min-1 for 30 min, the infusion was stopped, and three additional clearances were collected. Four groups of rats were studied: in Group 1 (n = 10), ET-1 was infused; in Group 2 (n = 5), a bilateral nephrectomy was performed 120 min before infusing ET-1; in Group 3 (n = 5), ET-1 was infused into rats treated with phentolamine (0.015 mg kg-1 min-1); and in Group 4 (n = 5), ET-1 was infused into rats treated with propranolol (0.015 mg kg-1 min-1). At 30 min during infusion of ET-1 into Group 1 rats, mean arterial blood pressure had increased (P less than 0.01) by 27 +/- 2% (SE) and the glomerular filtration rate had decreased (P less than 0.01) by 71 +/- 6% of baseline values. Nephrectomy potentiated and prolonged the ET-1-induced systemic vasoconstriction. Phentolamine had no effect on the cardiorenal actions of ET-1 whereas propranolol enhanced ET-1-induced changes in mean arterial blood pressure; mean arterial blood pressure increased 38 +/- 2% at 30 min during ET-1 + propranolol infusion (P less than 0.01 versus value with ET-1 alone). These data indicate that the kidney affects ET-1-induced systemic vasoconstriction and that beta-adrenergic (but not alpha-adrenergic) receptors are activated during infusion of ET-1 with a resultant attenuation of ET-1-induced changes in systemic blood pressure.     

Clonidine-induced coronary vasodilatation in isolated guinea pig heart is not mediated by endothelial alpha2 adrenoceptors.

Functional role of endothelial alpha(2)-adrenoceptor in coronary circulation remains unclear. Clonidine, an agonist of alpha(2)-adrenoceptors, was reported to induce coronary vasodilatation via stimulation of endothelial alpha(2)-adrenoceptors or coronary vasoconstriction involving vascular smooth muscle alpha(2)-adrenoceptors. Moreover, H(2) receptor-dependent responses to clonidine were described. Here, we reassess the contribution of endothelial alpha(2)-adrenoceptor and H(2) receptors to coronary flow and contractility responses induced by clonidine in the isolated guinea pig heart. We found that clonidine (10(-9) - 10(-6) M) produced concentration-dependent coronary vasoconstriction without a significant change in contractility. This response was inhibited by the alpha(1)/alpha(2)-adrenoceptor antagonist - phentolamine (10(-5) M) and the selective alpha(2)-adrenoceptor antagonist yohimbine (10(-6) M), but it was not changed by the selective alpha(1)-adrenoceptor antagonist prazosin (10(-6) M). In the presence of nitric oxide synthase inhibitor, L-NAME (10(-4) M) the clonidine-induced vasoconstriction was potentiated. Clonidine at high concentrations of 10(-5) - 3 x 10(-5) M produced coronary vasodilatation, and an increase in myocardial contractility. These responses were abolished by a selective H(2)-receptor antagonist, ranitidine (10(-5) M), but not by phentolamine (10(-5) M). We conclude that in the isolated guinea pig heart, clonidine-induced vasoconstriction is mediated by activation of smooth muscle alpha(2)-adrenoceptors whereas clonidine-induced coronary vasodilatation is mediated by activation of vascular H(2) histaminergic receptors. Accordingly, endothelial alpha(2)-adrenoceptors does not seem to play a major role in coronary flow response induced by clonidine.     

Sympathetic activity in early renal posttransplantation hypertension in rats

The contribution of elevated sympathetic activity to the development of renal posttransplantation hypertension was investigated. F1 hybrids (F1H) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were transplanted with either an SHR or an F1H kidney and bilaterally nephrectomized. Three weeks after transplantation, sympathetic activity was assessed by measuring adrenal tyrosine hydroxylase (TH) mRNA content and recording splanchnic nerve activity (SNA) in conscious animals. To investigate the dependence of arterial pressure on sympathetic activity, animals were treated with the alpha(2)-adrenoceptor agonist guanabenz intracerebroventricularly. Mean arterial pressure (MAP) was 143 +/- 4 mmHg in recipients of an SHR kidney (n = 15) versus 110 +/- 3 mmHg in recipients of an F1H kidney (n = 10; P < 0.001). Adrenal TH mRNA content was 1.93 +/- 0.15 fmol/microg total RNA in recipients of an SHR kidney versus 1.96 +/- 0.17 fmol/microg total RNA in recipients of an F1H kidney (not significant). SNA did not differ significantly between recipients of an SHR kidney (n = 8) and recipients of an F1H kidney (n = 7) in terms of frequency and amplitude of synchronized nerve discharges. In response to cumulative intracerebroventricular administration of 10 and 20 microg guanabenz, SNA fell to 51 +/- 5% of control in recipients of an SHR kidney versus 44 +/- 6% of control in recipients of an F1H kidney (not significant) accompanied by a slight fall in MAP in either group. The results suggest that elevated sympathetic activity is not a major contributor to the development of renal posttransplantation hypertension.     

Role of paraventricular nucleus in the cardiogenic sympathetic reflex in rats

Myocardial ischemia stimulates cardiac spinal afferents to initiate a sympathoexcitatory reflex. However, the pathways responsible for generation of increased sympathetic outflow in this reflex are not fully known. In this study, we determined the role of the paraventricular nucleus (PVN) in the cardiogenic sympathetic reflex. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anesthetized rats during epicardial application of 10 microg/ml bradykinin. Bilateral microinjection of muscimol (0.5 nmol), a GABA(A) receptor agonist, was performed to inhibit the PVN. In 10 vehicle-injected rats, epicardial bradykinin significantly increased RSNA 178.4 +/- 48.5% from baseline, and mean arterial pressure from 76.9 +/- 2.0 to 102.3 +/- 3.3 mmHg. Microinjection of muscimol into the PVN significantly reduced the basal blood pressure and RSNA (n = 12). After muscimol injection, the bradykinin-induced increases in RSNA (111.6 +/- 35.9% from baseline) and mean arterial pressure (61.2 +/- 1.3 to 74.5 +/- 2.7 mmHg) were significantly reduced compared with control responses. The response remained attenuated even when the basal blood pressure was restored to the control. In a separate group of rats (n = 9), bilateral microinjection of the ionotropic glutamate antagonist kynurenic acid (4.82 or 48.2 nmol in 50 nl) had no significant effect on the RSNA and blood pressure responses to bradykinin compared with controls. These results suggest that the tonic PVN activity is important for the full manifestation of the cardiogenic sympathoexcitatory response. However, ionotropic glutamate receptors in the PVN are not directly involved in this reflex response.     

ET-1-induced pulmonary vasoconstriction shifts from ET(A)- to ET(B)-receptor-mediated reaction after preconstriction.

Endothelin-1 (ET-1) has been reported to induce pulmonary vasoconstriction via either ET(A) or ET(B) receptors, and vasorelaxation after ET-1 injection has been observed. Our study investigated the effects of ET-1 in isolated rabbit lungs, which were studied at basal tone (part I) and after preconstriction (U-46619; part II). Pulmonary arterial pressure (PAP) and lung weight gain were monitored continuously. In part I, ET-1 (10(-8) M; n = 6; control) was injected after pretreatment with the ET(A)-receptor antagonist BQ-123 (10(-6) M; n = 6) or the ET(B)-receptor antagonist BQ-788 (10(-6) M; n = 6). The same protocol was carried out in part II after elevation of pulmonary vascular tone. ET-1 induced an immediate PAP increase (DeltaPAP 4.3 +/- 0.4 mmHg at 10 min) that was attenuated by pretreatment with BQ-123 (P < 0.05 at 10 min and P < 0.01 thereafter) and that was more pronounced after BQ-788 (P < 0.01 at 10 min and P < 0.001 thereafter). In part II, ET-1 induced an immediate rise in PAP with a maximum after 5 min (DeltaPAP 6.3 +/- 1.4 mmHg), leveling off at DeltaPAP 3.2 +/- 0.2 mmHg after 15 min. Pretreatment with BQ-123 failed to attenuate the increase. BQ-788 significantly reduced the peak pressure at 5 min (0.75 +/- 0.4 mmHg; P < 0.001) as well as the plateau pressure thereafter (P < 0.01). We conclude that ET-1 administration causes pulmonary vasoconstriction independent of basal vascular tone, and, at normal vascular tone, the vasoconstriction seems to be mediated via ET(A) receptors. BQ-788 treatment resulted in even more pronounced vasoconstriction. After pulmonary preconstriction, ET(A) antagonism exerted no effects on PAP, whereas ET(B) antagonism blocked the PAP increase. Therefore, ET-1-induced pulmonary vasoconstriction is shifted from an ET(A)-related to an ET(B)-mediated mechanism after pulmonary vascular preconstriction.     

Endothelin type A receptor antagonist normalizes blood pressure in rats exposed to eucapnic intermittent hypoxia

We have reported that eucapnic intermittent hypoxia (E-IH) causes systemic hypertension, elevates plasma endothelin 1 (ET-1) levels, and augments vascular reactivity to ET-1 and that a nonspecific ET-1 receptor antagonist acutely lowers blood pressure in E-IH-exposed rats. However, the effect of chronic ET-1 receptor inhibition has not been evaluated, and the ET receptor subtype mediating the vascular effects has not been established. We hypothesized that E-IH causes systemic hypertension through the increased ET-1 activation of vascular ET type A (ET(A)) receptors. We found that mean arterial pressure (MAP) increased after 14 days of 7 h/day E-IH exposure (109 +/- 2 to 137 +/- 4 mmHg; P < 0.005) but did not change in sham-exposed rats. The ET(A) receptor antagonist BQ-123 (10 to 1,000 nmol/kg iv) acutely decreased MAP dose dependently in conscious E-IH but not sham rats, and continuous infusion of BQ-123 (100 nmol.kg(-1).day(-1) sc for 14 days) prevented E-IH-induced increases in MAP. ET-1-induced constriction was augmented in small mesenteric arteries from rats exposed 14 days to E-IH compared with those from sham rats. Constriction was blocked by the ET(A) receptor antagonist BQ-123 (10 microM) but not by the ET type B (ET(B)) receptor antagonist BQ-788 (100 microM). ET(A) receptor mRNA content was greater in renal medulla and coronary arteries from E-IH rats. ET(B) receptor mRNA was not different in any tissues examined, whereas ET-1 mRNA was increased in the heart and in the renal medulla. Thus augmented ET-1-dependent vasoconstriction via vascular ET(A) receptors appears to elevate blood pressure in E-IH-exposed rats.     

Potential involvement of a propranolol-insensitive atypical beta-adrenoceptor the vasodilator effect of cyanopindolol in the human pulmonary artery.

The aim of our study was to examine whether non beta(1)-/beta(2)-adrenoceptors participate in the relaxation of the human pulmonary artery. For this purpose the vasodilatory effect of the non-conventional partial beta-adrenoceptor agonist cyanopindolol was examined. Cyanopindolol (1-300 microM), studied in the presence of the beta(1)-/beta(2)-adrenoceptor antagonist propranolol, relaxed the human pulmonary artery preconstricted with serotonin 1 microM in a concentration-dependent manner (maximally by about 80%). This effect was diminished by bupranolol 10 microM (an antagonist of beta(1)-beta(3)-adrenoceptors and the low affinity state of the beta(1)-adrenoceptor) and CGP 20712 10 microM (known to antagonize the low-affinity state of the beta(1)-adrenoceptor at high concentrations). In further experiments, the effect of beta-adrenoceptor ligands on the serotonin-induced vasoconstriction was examined. The concentration-response curve for serotonin was not affected by cyanopindolol 30 microM, bupranolol 10 microM and CGP 20712 10 microM but shifted to the right by cyanopindolol 100 and 300 microM; the serotonin 5-HT(2A) receptor antagonist ketanserin 0.3 microM abolished the maximum contraction elicited by serotonin. In conclusion, the present study reveals that the vasodilatory effect of cyanopindolol in the human pulmonary artery consists of two components, i.e. activation of a propranolol-insensitive atypical beta-adrenoceptor and antagonism against 5-HT(2A) receptors.     

Enalapril attenuates endothelin-1-induced hypertension via increased kinin survival

Recent studies have shown that angiotensin-converting enzyme (ACE) inhibitors attenuate endothelin-1 (ET-1)-induced hypertension, but the mechanisms for this effect have not been clarified. Initial experiments were conducted to contrast the effect of the ACE inhibitor enalapril, the combined ACE-neutral endopeptidase inhibitor omapatrilat, and the angiotensin II receptor antagonist candesartan on the hypertensive and renal response to ET-1 in anesthetized Sprague-Dawley rats. Acute intravenous infusion of ET-1 (10 pmol x kg(-1) x min(-1)) for 60 min significantly increased mean arterial pressure (MAP) from 125 +/- 8 to 145 +/- 8 mmHg (P < 0.05) and significantly decreased glomerular filtration rate (GFR) from 0.31 +/- 0.09 to 0.13 +/- 0.05 ml x min(-1) x 100 g kidney wt(-1). Pretreatment with enalapril (10 mg/kg iv) before ET-1 infusion inhibited the increase in MAP (121 +/- 4 vs. 126 +/- 4 mmHg) before and during ET-1 infusion, respectively (P < 0.05) without blocking the effect of ET-1 on GFR. In contrast, neither omapatrilat (30 mg/kg) nor candesartan (10 mg/kg) had any effect on ET-1-induced increases in MAP or decreases in GFR. To determine whether the effect of enalapril was due to the decrease in angiotensin II or increase in kinin formation, rats were given REF-000359 (1 mg/kg iv), a selective B(2) receptor antagonist, with or without enalapril before ET-1 infusion. REF-000359 completely blocked the effect of enalapril on ET-1 infusion (MAP was 117 +/- 5 vs. 135 +/- 5 mmHg before and during ET-1 infusion, respectively, P < 0.05). REF-000359 alone had no effect on the response to ET-1 infusion (MAP was 117 +/- 4 vs. 144 +/- 4 mmHg before and during ET-1 infusion, respectively, P < 0.05). REF-000359 with or without enalapril had no significant effect on the ability of ET-1 infusion to decrease GFR. These findings support the hypothesis that decreased catabolism of bradykinin and its subsequent vasodilator activity oppose the actions of ET-1 to increase MAP.     

Prevention of endothelin-1-induced increases in blood pressure: role of endogenous CGRP

To determine the role of endothelin-1 (ET-1) and its receptors in the regulation of calcitonin gene-related peptide (CGRP) release, male Wistar rats were divided into six groups and subjected to the following treatments for 1 wk with or without ABT-627 (an ET(A) receptor antagonist, 5 mg.kg(-1).day(-1) in drinking water) or A-192621 (an ET(B)-receptor antagonist, 30 mg.kg(-1).day(-1) by oral gavage): control (Con), ET-1 (5 ng.kg(-1).min(-1) iv), Con + ABT-627, Con + A-192621, ET-1 + ABT-627, and ET-1 + A-192621. Baseline mean arterial pressure (MAP, mmHg) was higher (P < 0.05) in Con + A-192621 (122 +/- 4) and ET-1 + A-192621 (119 +/- 4) groups compared with Con (104 +/- 6), ET1 (106 +/- 3), Con + ABT-627 (104 +/- 3), and ET1 + ABT-627 (100 +/- 3) groups. Intravenous administration of CGRP(8-37) (a CGRP receptor antagonist, 1 mg/kg) increased MAP (P < 0.05) in ET-1 (13 +/- 1), Con + A-192621 (12 +/- 1), and ET-1 + A-192621 (15 +/- 3) groups compared with Con (4 +/- 1), Con-ABT-627 (4 +/- 1), and ET-1 + ABT-627 (5 +/- 1) groups. Plasma CGRP levels (in pg/ml) were increased (P < 0.05) in ET-1 (57.5 +/- 6.1), Con + A-192621 (53.9 +/- 3.4), and ET-1 + A-192621 (60.4 +/- 3.0) groups compared with Con (40.4 +/- 1.6), Con + ABT-627 (40.0 +/- 2.9), and ET-1 + ABT-627 (42.6 +/- 1.9) groups. Plasma ET-1 levels (in pg/ml) were higher (P < 0.05) in ET-1 (2.8 +/- 0.2), ET-1 + ABT-627 (3.2 +/- 0.4), Con + A-192621 (3.3 +/- 0.4), and ET-1 + A-192621 (4.6 +/- 0.3) groups compared with Con (1.1 +/- 0.2) and Con-ABT-627 (1.3 +/- 0.2) groups. Therefore, our data show that ET-1 infusion leads to increased CGRP release via activation of the ET(A) receptor, which plays a compensatory role in preventing ET-1-induced elevation in blood pressure.     

Attenuating effects of intrathecal clonidine on the exercise pressor reflex

We tested the hypothesis that intrathecal injection of clonidine, an alpha 2-adrenergic agonist, attenuated the reflex cardiovascular and ventilatory responses to static muscular contraction in cats. Before clonidine (1 microgram in 0.2 ml), contraction-induced reflex increases (n = 10) in mean arterial pressure and ventilation averaged 25 +/- 3 mmHg and 359 +/- 105 ml/min, respectively, whereas after clonidine these increases averaged 8 +/- 4 mmHg and 200 +/- 114 ml/min, respectively (P less than 0.05). Clonidine had no effect on the heart rate response to contraction. Intrathecal injection of yohimbine (10 micrograms; n = 5), an alpha 2-adrenergic antagonist, but not prazosin (10 micrograms; n = 3), an alpha 1-adrenergic antagonist, prevented the attenuating effects of clonidine on the reflex pressor and ventilatory responses to contraction. Our findings were not due to the spread of clonidine to the medulla, because the reflex pressor and ventilatory responses to contraction were not attenuated by injection of clonidine (1 microgram) onto the medulla (n = 3). In addition, our findings were not due to a clonidine-induced withdrawal of sympathetic outflow, because intrathecal injection of clonidine (1 microgram) did not attenuate increases in arterial pressure and ventilation evoked by high-intensity electrical stimulation of the cut central end of the sciatic nerve (n = 5). Furthermore, our findings were not due to a local anesthetic action of clonidine, because application of this agent to the dorsal roots had no effect on the discharge of group IV muscle afferents. We conclude that stimulation of alpha 2-adrenergic receptors in the spinal cord attenuates the reflex pressor and ventilatory responses to static contraction.     

Late gestational changes in sympathomimetic sensitivity in primagravid rabbit ligaments

The adrenoceptor profile of blood vessels supplying the medial collateral ligament (MCL) of virgin and primagravid (day 29 of pregnancy) rabbit knees was examined. Topical bolus administration of the alpha1-adrenoceptor agonists methoxamine and phenylephrine, and the alpha2-adrenoceptor agonist clonidine, to exposed knee joints resulted in a dose-dependent vasoconstriction of ligamentous vessels. The rank order of potency for the alpha-agonists was found to be phenylephrine > methoxamine > clonidine. Dobutamine, which acts on beta1-adrenoceptors, and ritodrine, which is a beta2-agonist, imparted a mild vasodilatatory effect on MCL blood vessels with the efficacy of ritodrine being greater than that of dobutamine. In primagravid rabbits, the constrictor effects of methoxamine and phenylephrine were significantly attenuated compared with virgin control animals (P < 0.0001), whereas clonidine-mediated vasoconstriction was unaltered in the gravid animal (P = 0.3957). With respect to beta-adrenoceptor activity, the dilatational effect of dobutamine was the same as in controls (P = 0.5294), while ritodrine vasoactivity was completely abolished in primagravid knees (P < 0.005). These findings suggest that the adrenergic control of rabbit MCL blood flow is predominantly mediated by postjunctional alpha1 and beta2-adrenoceptors. Pregnancy leads to either downregulation and (or) desensitisation of alpha1 and beta2-adrenoceptors in the ligament which could disrupt normal joint homeostasis.     

PVN lesions prevent the endothelin 1-induced increase in arterial pressure and vasopressin

Endothelin (ET) acts within the central nervous system to increase arterial pressure and arginine vasopressin (AVP) secretion. This study assessed the role of the paraventricular nuclei (PVN) in these actions. Intracerebroventricular ET-1 (10 pmol) or the ET(A) antagonist BQ-123 (40 nmol) was administered in conscious intact or sinoaortic-denervated (SAD) Long-Evans rats with sham or bilateral electrolytic lesions of the magnocellular region of the PVN. Baseline values did not differ among groups, and artificial cerebrospinal fluid (CSF) induced no significant changes. In sham-lesioned rats, ET-1 increased mean arterial pressure (MAP) 15.9 +/- 1.3 mmHg in intact and 22.3 +/- 2.7 mmHg in SAD (P < 0.001 ET-1 vs. CSF) rats. PVN lesions abolished the rise in MAP: -0.1 +/- 2.8 mmHg in intact and 0.0 +/- 2.9 mmHg in SAD. AVP increased in only in the sham-lesioned SAD group 8.6 +/- 3.5 pg/ml (P < 0.001 ET-1 vs. CSF). BQ-123 blocked the responses. Thus the integrity of the PVN is required for intracerebroventricularly administered ET-1 to exert pressor and AVP secretory effects.     

Presynaptic alpha-adrenoceptors in median preoptic nucleus modulate inhibitory neurotransmission from subfornical organ and organum vasculosum lamina terminalis

The median preoptic nucleus (MnPO) in the lamina terminalis receives a prominent catecholaminergic innervation from the dorsomedial and ventrolateral medulla. The present investigation used whole cell patch-clamp recordings in rat brain slice preparations to evaluate the hypothesis that presynaptic adrenoceptors could modulate GABAergic inputs to MnPO neurons. Bath applications of norepinephrine (NE; 20-50 microM) induced a prolonged and reversible suppression of inhibitory postsynaptic currents (IPSCs) and reduced paired-pulse depression evoked by stimulation in the subfornical organ and organum vasculosum lamina terminalis. These events were not correlated with any observed changes in membrane conductance arising from NE activity at postsynaptic alpha(1)- or alpha(2)-adrenoceptors. Consistent with a role for presynaptic alpha(2)-adrenoceptors, responses were selectively mimicked by an alpha(2)-adrenoceptor agonist (UK-14304) and blockable with an alpha(2)-adrenoceptor antagonist (idazoxan). Although the alpha(1)-adrenoceptor agonist cirazoline and the alpha(1)-adrenoceptor antagonist prazosin were without effect on these evoked IPSCs, NE was noted to increase (via alpha(1)-adrenoceptors) or decrease (via alpha(2)-adrenoceptors) the frequency of spontaneous and tetrodotoxin-resistant miniature IPSCs. Collectively, these observations imply that both presynaptic and postsynaptic alpha(1)- and alpha(2)-adrenoceptors in MnPO are capable of selective modulation of rapid GABA(A) receptor-mediated inhibitory synaptic transmission along the lamina terminalis and therefore likely to exert a prominent influence in regulating cell excitability within the MnPO.     

Epinephrine enhances the sensitivity of rat vagal chemosensitive neurons: role of beta3-adrenoceptor.

This study was carried out to determine whether epinephrine alters the sensitivity of rat vagal sensory neurons. In anesthetized rats, inhalation of epinephrine aerosol (1 and 5 mg/ml, 3 min) induced an elevated baseline activity of pulmonary C fibers and enhanced their responses to lung inflation (20 cm H(2)O, 10 s) and right atrial injection of capsaicin (0.5 microg/kg). In isolated rat nodose and jugular ganglion neurons, perfusion of epinephrine (3 microM, 5 min) alone did not produce any detectable change of the intracellular Ca(2+) concentration. However, immediately after the pretreatment with epinephrine, the Ca(2+) transients evoked by chemical stimulants (capsaicin, KCl, and ATP) were markedly potentiated; for example, capsaicin (50 nM, 15 s)-evoked Ca(2+) transient was increased by 106% after epinephrine (P < 0.05; n = 11). The effect of epinephrine was mimicked by either BRL 37344 (5 microM, 5 min) or ICI 215,001 (5 microM, 5 min), two selective beta(3)-adrenoceptor agonists, and blocked by SR 59230A (5 microM, 10 min), a selective beta(3)-adrenoceptor antagonist, whereas pretreatment with phenylephrine (alpha(1)-adenoceptor agonist), guanabenz (alpha(2)-adrenoceptor agonist), dobutamine (beta(1)-adrenoceptor agonist), or salbutamol (beta(2)-adrenoceptor agonist) had no significant effect on capsaicin-evoked Ca(2+) transient. Furthermore, pretreatment with SQ 22536 (100-300 microM, 15 min), an adenylate cyclase inhibitor, and H89 (3 microM, 15 min), a PKA inhibitor, completely abolished the potentiating effect of epinephrine. Our results suggest that epinephrine enhances the excitability of rat vagal chemosensitive neurons. This sensitizing effect of epinephrine is likely mediated through the activation of beta(3)-adrenoceptor and intracellular cAMP-PKA signaling cascade.