Tempol-nebivolol therapy potentiates hypotensive effect increasing NO bioavailability and signaling pathway.

Nebivolol is a third generation beta blocker with endothelial nitric oxide synthase (eNOS) agonist properties. Considering the role of reactive oxygen species (ROS) in the uncoupling of eNOS, we hypothesized that the preadministration of an antioxidant as tempol, could improve the hypotensive response of nebivolol in normotensive animals increasing the nitric oxide (NO) bioavailability by a reduction of superoxide (O2(•-)) basal level production in the vascular tissue. Male Sprague Dawley rats were given tap water to drink (control group) or tempol (an antioxidant scavenger of superoxide) for 1 week. After 1 week, Nebivolol, at a dose of 3 mg/kg, was injected intravenously to the control group or to the tempol-treated group. Mean arterial pressure, heart rate, and blood pressure variability were evaluated in the control, tempol, nebivolol, and tempol nebivolol groups, as well as, the effect of different inhibitor as Nß-nitro-l-arginine methyl ester (L-NAME, a Nitric oxide synthase blocker) or glybenclamide, a KATP channel inhibitor. Also, the expression of α,ß soluble guanylate cyclase (sGC), phospho-eNOS, and phospho-vasodilator-stimulated phosphoprotein (P-VASP) were evaluated by Western Blot and cyclic guanosine monophosphate (cGMP) levels by an enzyme-linked immunosorbent assay (ELISA) commercial kit assay. We showed that pretreatment with tempol in normotensive rats produces a hypotensive response after nebivolol administration through an increase in the NO bioavailability and sGC, improving the NO/cGMP/protein kinase G (PKG) pathway compared to that of the nebivolol group. We demonstrated that tempol preadministration beneficiates the response of a third-generation beta blocker with eNOS stimulation properties, decreasing the basal uncoupling of eNOS, and improving NO bioavailability. Our results clearly open a possible new strategy therapeutic for treating hypertension.

Central insulin-angiotensin II interaction in blood pressure regulation in fructose overloaded rats.

The aim of the present study was to determine if insulin is able to modulate the pressor response to intracerebroventricularly administered angiotensin II in insulin resistant fructose overloaded rats. Male Sprague-Dawley rats were divided into two groups: 1) Control group (C) with tap water to drink for 6 weeks (n=36); and 2) fructose treated (F), with fructose solution (10% w/v) to drink for 6 weeks (n=36). On the day of the experiment, anesthetized male C and F rats were intracerebroventricularly infused with insulin (12 mU/h, n=15) or Ringer's solution as vehicle (n=15) for 2h. Immediately, changes in mean arterial pressure (MAP) in response to an intracerebroventricular subpressor dose of angiotensin II (5 pmol, n=10) or vehicle (n=5) were measured for 10 min. Then, hypothalami were removed and Akt and ERK1/2 phosphorylation levels were determined. In a subset of C (n=10) and F (n=20) animals, PD98059 (p44/42 MAPK inhibitor) or vehicle was administered intracerebroventricularly at a flow rate of 5 µl/min for 1 min. Ten minutes later, insulin (12 mU/h, n=5 for each group) or vehicle (Ringer's solution, only in the F group, n=5) was perfused for 2h at a flow rate of 4 µl/h, and cardiovascular parameters were measured every 15 min. Immediately, changes in MAP and HR in response to a subpressor dose of Ang II (5 pmol/2 µl) were evaluated for 10 min (n=5 for each group). In other subset of animals (n=6 for each group), AT1 and AT2 hypothalamic receptor levels were measured by Western blotting. Intracerebroventricular insulin pre-treatment increased the pressor response to angiotensin II in C rats. In F rats (with or without insulin pretreatment), the pressor response to angiotensin II was higher than that in vehicle pre-treated C animals, but similar to that observed in C after insulin infusion. In C rats phospho-ERK 1/2 hypothalamic levels significantly increased after angiotensin II injection in insulin pretreated animals compared to vehicle pre-treated rats, suggesting that MAPK activation might be involved in insulin potentiation of blood pressure response to angiotensin II in the brain. Phospho-ERK 1/2 hypothalamic levels were significantly increased in vehicle treated F rats compared to C, suggesting that basal MAPK activation might play a role in the enhanced response to angiotensin II observed in these animals. Finally, in F rats, either after vehicle or insulin infusion, angiotensin II injection was associated with a similar increase in phospho-ERK 1/2 hypothalamic levels, comparable to that observed after angiotensin II injection in insulin pre-treated C animals. ERK 1/2 blockade significantly reduced MAP in F rats compared to C. Moreover, ERK 1/2 inhibition completely abolished the Ang II pressor response in F rats and in insulin pre-treated C animals. All these findings suggest that insulin-angiotensin II interaction at hypothalamic level might be involved in the increase in blood pressure observed in the insulin resistant state.

Pharmacokinetic and pharmacodynamic alterations of methyldopa in rats with aortic coarctation. A study using microdialysis.

A pharmacokinetic-pharmacodynamic study of methyldopa (MD) was made in anesthetized sham operated (SO) and aortic coarctated (ACo) rats by using a vascular shunt probe for arterial microdialysis and simultaneous blood pressure recording. Anesthetized Wistar rats were used 7 days after aortic coarctation or sham operation. A vascular shunt probe was inserted into the carotid artery and a concentric probe was placed into the striatum or posterior hypothalamus. MD and 3,4-dihydroxyphenylacetic acid (DOPAC) were determined in the dialysates by HPLC-EC. MD (50 mg kg(-1)i.p.) induced an increase of heart rate in SO (Delta HR: 108 +/- 22 bpm, n= 6) and in ACo rats (Delta HR: 55 +/- 10 bpm, n= 6, P< 0.05, one way ANOVA). Moreover, MD also reduced the mean arterial pressure (MAP) of SO rats (Delta MAP: -10 +/- 4 mmHg, n= 6) and ACo animals (Delta MAP: -51 +/- 9 mmHg, n= 6, P< 0.05, one way ANOVA). Analysis of the arterial blood dialysates showed a lower half-life of MD in ACo rats (t(1/2): 1.5 +/- 0.3 h, n= 6, P< 0.05, 't' test) than in SO rats (t(1/2): 3.7 +/- 1.0 h, n= 6). A low accumulation and a fast decay of striatal MD levels were seen in ACo rats. However, peak levels of drug were greater in the hypothalamic dialysates of ACo rats than in SO animals samples. On the other hand, MD also induced an increase of DOPAC levels in the hypothalamic dialysates of ACo rats. In conclusion, the aortic coarctation modifies the pharmacokinetic and cardiovascular effect of MD in the rat. The action of this drug on dopaminergic neurotransmission is also altered in the ACo animals.

Pharmacokinetic study of methyldopa in aorta-coarctated rats using a microdialysis technique.

The pharmacokinetics of methyldopa (12.5, 25 and 50 mg kg(-1), i.p.) was studied in anesthetized sham-operated (SO) and abdominal aorta-coarctated (ACo) rats using a microdialysis technique. A non-linear relationship between the area under the curve (AUC) and dose was observed in SO rats. However, in ACo rats the AUC showed a proportional increase with dose. Abdominal aortic coarctation produced significant differences in the estimates of clearance (Cl) and the elimination rate constant from the dialysate (K(ed)) after the administration of 50 mg kg(-1)of methyldopa (K(ed)SO, 0.31 +/- 0.09; ACo, 0.66 +/- 0.09(*)h(-1): Cl SO, 30.8 +/- 10.1; ACo, 78.6 +/- 13.3(*)mlkg(-1)min(-1);n= 6,(*)P< 0.05 vs SO). In conclusion, this study, by using a microdialysis technique, suggests that abdominal aortic coarctation seems to produce changes in the pharmacokinetics of methyldopa in rats.

Study of the evolution of blood and striatal levels of methyldopa: a microdialysis study in sinaortic denervated rats.

Pharmacokinetic of methyldopa (50 mg kg(-1)i.p.) was studied in anesthetized sham operated and sinoaortic denervated (SAD) rats by using the microdialysis technique. Vascular shunt probe was inserted into the carotid artery and concentric probe was placed in the striatum. Levels of methyldopa were measured by HPLC-EC. The number of animals in each group was six and normal distribution of the variables of the study was assumed. Peak concentrations in arterial blood of methyldopa were similar in both groups of rats but the elimination rate constant was 0.31+/-0.09 h(-1)for sham rats (n =6) and 1.28+/-0.31 h(-1)for SAD rats (n =6, P<0.05). Low levels of methyldopa and a more pronounced decrease were seen in striatum of sinoaortic denervated rats. In conclusion, by using a microdialysis technique, different kinetic profiles of methyldopa were observed in sham operated and sinoaortic denervated rats.

Central benzodiazepine involvement in clonidine cardiovascular actions.

It is well known that the GABAergic and noradrenergic systems play an important role in blood pressure and heart rate regulation. Benzodiazepines and beta-carbolines, respectively, increase or decrease the probability of chloride-channel opening induced by GABA. The aim of this study was to determine, in conscious rats, the interaction existing between the central alpha2-adrenoceptor stimulation induced by clonidine and the facilitation or impairment of benzodiazepine receptor activity through the administration of either diazepam, a benzodiazepine receptor agonist, or methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse benzodiazepine agonist. Clonidine (5-10 microg, intracerebroventricularly) reduced heart rate and increased mean blood pressure by activation of central alpha2-adrenoceptors. Diazepam (2 mg/kg, intravenously (i.v.)) induced an increase in heart rate, while DMCM (0.3 mg/kg, i.v.) elicited a bradycardic effect. The bradycardic effects induced by both clonidine and DMCM were antagonized by the prior administration of methylatropine (1.5 mg/kg, i.v.). DMCM (0.3 mg/kg, i.v.) prevented the clonidine effects on heart rate and mean blood pressure, while diazepam (2 mg/kg, i.v.) failed to modify these effects. Our results suggest that the bradycardic effects of clonidine are mediated by a vagal stimulation and are related to the activation of a GABAergic pathway.

Adrenoceptor involvement in the cardiovascular responses to B-HT 920 in sinoaortic denervated rats.

1. A study was made relating the involvement of alpha-adrenoceptors in the cardiovascular responses to intracerebroventricular (i.c.v.) injection of B-HT 920, a clonidine-type drug, in conscious sham-operated and sinoaortic-denervated rats. 2. Wistar rats were used, 7 days after the sham operation or sinoaortic denervation. For i.c.v. injection of drugs, a guide cannula had been previously implanted in the left lateral ventricle. 3. In sham-operated rats, cardiovascular responses to B-HT 920 (10-60 microg) were increased blood pressure and bradycardia; but, in sinoaortic-denervated rats, after the pressor response, a decrease in blood pressure also was seen. The responses to this agent were greater in sinoaortic-denervated rats than in sham-operated animals. Treatment with the alpha2-adrenoceptor antagonist yohimbine (30 microg), the imidazoline receptor antagonist idazoxan (15 microg) and the alpha1A-adrenoceptor antagonist 5-methylurapidil (15 microg) blocked the responses to B-HT 920 (30 microg). The alpha1-adrenoceptor antagonist prazosin (15 microg) and the alpha1B-adrenoceptor antagonist chloroethylclonidine (100 microg) did not modify the responses to agonist. 4. Sinoaortic denervation enhances the cardiovascular responses to B-HT 920. Moreover, the effects of i.c.v. administration of B-HT 920 could be mediated by several types of brain receptors: imidazoline receptors and alpha1A- and alpha2-adrenoceptors.

[The central cholinergic stimulation in sinoaortic denervation. Effect of intracerebroventricular administration of cholinomimetic agents]. / La estimulación colinérgica central en la desnervación sinoaórtica. Efecto de la administración i.c.v. de agentes colinomiméticos.

It was decided to made a pharmacological study of the cholinergic participation in the sinoaortic denervation experimental model by analyzing the cardiovascular effects of several muscarinic agonists and the anticholinesterase neostigmine administered either by intravenous via or by the intracerebroventricular via. The activity of the enzyme acetylcholinesterase was also evaluated in diverse structures of the central nervous system after the intracerebral administration of neostigmine. Sinoaortic denervation increases the pressor response to the i.c.v. administration of the agonist bethanechol and of the anticholinesterase neostigmine but it diminishes the bradycardic effect. However it would not alter the cardiovascular responses to the i.v. injection of the agonist oxotremorine and to the i c v. administration of the agonist McNeil-A-343. After the i c v. administration of neostigmine, the enzymatic activity oscillated among 24%-30% in hypothalamic structures and among 42%-52% in the remaining tissues, without differences between the rats with sham operation and those with sinoaortic denervation. The results suggest that M, muscarinic receptor subtype would not be involved in the cardiovascular effects of the central cholinergic stimulation On the other hand, it would support the idea of an involvement of muscarinic receptors in the observed changes. Because the used route of administration of neostigmine, a greater degree of inhibition of the hypothalamic acetylcholinesterase is observed, suggesting then that the hypothalamic structures could be involved in the cardiovascular effects induced by the intracerebral administration of the anticholinesterase.

Muscarinic receptor subtype involvement in brain cholinergic stimulation by intracerebroventricular neostigmine in sinoaortic denervated rats.

1. The present studies evaluated the participation of central muscarinic receptors in the cardiovascular effects of centrally injected neostigmine, a quaternary anticholinesterase, in conscious, sham-operated rats and in sinoaortic denervated animals. 2. The dose-dependent pressor effect of neostigmine (0.1 to 1 microg i.c.v.) was greater in sinoaortic denervated rats than in sham-operated animals, but only a dose-dependent bradycardic effect was seen in sham-operated rats. 3. Doses of 3.3 nmol (i.c.v.) of both the M1 muscarinic antagonist, pirenzepine, and the M3 muscarinic antagonist, 4-DAMP, prevented the pressor response to 1 microg of neostigmine in sham-operated rats and in sinoaortic denervated animals; however, the M2 muscarinic antagonist, AF-DX116, partially blocked this response in sham-operated rats while failing to do so in sinoaortic denervated rats. In sham rats, doses of 3.3 nmol (i.c.v.) of both pirenzepine and 4-DAMP prevented the bradycardic response to 1 microg (i.c.v.) of neostigmine, whereas AF-DX116 induced a partial blockade. 4. 4-DAMP, at the dose of 0.3 nmol (i.c.v.), but not pirenzepine at the same dose, prevented the pressor effect of neostigmine (0.1 to 1 microg i.c.v.) in both groups of rats. Both muscarinic antagonists at this dose prevented the bradycardia elicited by the anticholinesterase (0.1 to 1 microg i.c.v.), but 4-DAMP showed a greater antagonistic action on this cardiac effect than pirenzepine. In sham-operated rats, i.c.v. injection of 0.3 nmol of AF-DX116 failed to modify the cardiovascular responses to 0.3 microg of neostigmine. 5. Results suggest mainly an involvement of brain M3-subtype muscarinic receptors in the cardiovascular effect of intracerebroventricular administration of anticholinesterase neostigmine in both groups of rats.

Anticholinergic action of clonidine in rats with sinoaortic denervation.

A study was carried out relating to the anticholinergic action of clonidine on the cardiovascular responses to i.c.v. injection of neostigmine, a quaternary anticholinesterase, in conscious sham-operated animals and rats with sinoaortic denervation, 7 days after the corresponding operation. Neostigmine (0.1-1 micrograms i.c.v.) induced a dose-dependent pressor and bradycardic responses in sham-operated rats but induced only an increase in blood pressure in sinoaortic-denervated animals. However, the pressor response in sinoaortic-denervated rats was significantly greater than in sham-operated animals. Clonidine (10 micrograms kg-1 i.v.) induced a fall in mean arterial pressure in sinoaortic-denervated rats but not in sham-operated animals. Moreover, sinoaortic denervation reduced the bradycardic action of this antihypertensive drug. The anticholinesterase activity of clonidine (10 micrograms kg-1 i.v.), given 30 min previously, prevented the bradycardic action of neostigmine (0.1-1 micrograms i.c.v.) but failed to modify the pressor effect in sham-operated rats. This alpha2-adrenergic agent reduced the pressor response to i.c.v. administration of neostigmine in sinoaortic-denervated rats. Alternatively, the i.c.v. administration of clonidine (3 micrograms i.c.v.), given either 15 or 30 min before neostigmine, only prevented the bradycardic effect of the anticholinesterase (0.3 micrograms i.c.v.) in sham-operated rats but not the pressor action of this drug. In sinoaortic denervated rats, 3 micrograms of clonidine i.c.v. reduced an increase in blood pressure by i.c.v. injection of the anticholinesterase. The results suggest different central cholinergic mechanisms and different cholinergic-adrenergic interactions on the cardiovascular responses elicited by centrally injected neostigmine in sinoaortic denervated rats.

[Effect of treatment with dexamethasone on cardiovascular responses of adrenergic agents]. / Efecto del tratamiento con dexametasona sobre las respuestas cardiovasculares de agentes adrenérgicos.

Cardiovascular responses to several agents should be modified by glucocorticoid administration in the rat. We investigate the response to adrenergic agonists such as phenylephrine, noradrenaline, clonidine and isoproterenol and ganglionic blocking agent such as hexamethonium in conscious rats treated during 7 days with dexamethasone. Wistar rats were treated with either Dex (150 micrograms daily x 7 days, p.o.) or water. Mean arterial pressure were calculated from the intraarterial recordings of blood pressure. No differences in basal mean arterial pressure were seen between dexamethasone and control groups of rats. Phenylephrine and noradrenaline showed a pressor effect in control rats that was reduced by dexamethasone treatment. Clonidine showed similar pressor effect in both groups of rats but ten minutes after drug administration, a light hypotension was seen in dexamethasone rats. Isoproterenol and hexamethonium showed a similar hypotensive effect on control and dexamethasone rats. In conclusion, dexamethasone treatment should reduce the pressor responses to phenylephrine and noradrenaline. Moreover, the alpha adrenergic agonist clonidine showed a hypotensive effect in dexamethasone treated rats, although the response of isoproterenol and hexamethonium remains unchanged.

Brain alpha 1-adrenoceptor in the cardiovascular responses to BHT-920 and phenylephrine.

1. It is well known that alpha 1A-adrenoceptors have binding sites for imidazolic and for phenylethylaminic drugs. A study was made relating alpha 1A-adrenoceptor involvement in cardiovascular responses to intracerebroventricular (ICV) injection of BHT-920, an imidazoliclike drug, and phenylephrine, a phenylethylaminic drug, in conscious sham-operated and sinoaortically-denervated rats. 2. In sham-operated rats, cardiovascular responses to BHT-920 (30 micrograms, ICV) were increase of blood pressure and bradycardia but in sinoaortically denervated rats, after the pressor response, a decrease of blood pressure was also seen. The pressor and bradycardic responses to agonist were greater in sinoaortically denervated rats than in sham-operated rats. Phenylephrine (90 micrograms, ICV) showed a biphasic effect on blood pressure: an increase followed by a decrease, and bradycardia. The cardiovascular responses to phenylephrine in sinoaortic-denervated rats were greater than in sham-operated rats. 3. In sinoaortically denervated and sham-operated rats subchronically treated with the alpha 1-adrenoceptor antagonist prazosin (0.5 mg kg-1, intraperitoneally twice daily, for 6 days), an increase of cardiovascular responses to ICV administration of BHT-920 and phenylephrine was seen. 4. Baroreceptor deafferentation by sinoaortic denervation enhances the cardiovascular responses to BHT-920 and phenylephrine. The effects of BHT-920 could be mediated by brain alpha 1A adrenoceptors because this agonist has an imidazoliclike structure; phenylephrine could also be activating central alpha 1A-adrenoceptors. The enhanced cardiovascular responses after prazosin treatment could also be due to a supersensitivity of brain alpha 1A-adrenoceptors.

[Anticholinergic action of clonidine on cardiovascular effects of the central cholinergic estimulation]. / Acción anticolinérgica de clonidina sobre los efectos cardiovasculares de la estimulación colinérgica central.

A study was made relating the anticholinergic action of alpha 2-adrenoceptor agonist clonidine on the cardiovascular responses to i.c.v. injection of neostigmine, a quaternary anticholinesterase, in conscious rats. The cholinergic agent neostigmine (0.1-1 microgram i.c.v.) induced a dose-dependent pressor and bradycardic responses in the rats. Clonidine (10 micrograms.kg-1 i.v.), given 30 min previous the anticholinesterase, failed to modify the pressor effect of neostigmine (0.1-1 microgram i.c.v.) but prevented the bradycardia. On the other side, the i.c.v. administration of clonidine (3 micrograms), given 15 min before neostigmine, only prevented the bradycardic effect of neostigmine (0.3 microgram i.c.v.) but not the central pressor action of this drug. The results support the idea that clonidine has an anticholinergic action in the rat. However, prevention by clonidine of central cholinergic bradycardia suggests different cholinergic-adrenergic interaction on the cardiovascular responses elicited by central administration of cholinergic agents.

Central alpha 1- and alpha 2-adrenoceptors and brain cholinergic stimulation in sinoaortic denervated rats.

The central alpha-adrenoceptor role in cardiovascular responses to intracerebroventricular (i.c.v.) injection of neostigmine, a tertiary anticholinesterase, was studied in conscious sham-operated and sinoaortic-denervated rats. Neostigmine (0.1-1 micrograms i.c.v.) showed dose-dependent pressor and bradycardiac effects in vehicle-pretreated sham-operated rats but only an increased pressor effect in sinoaortic-denervated animals. The pretreatment with the catecholaminergic neurotoxin, 6-hydroxydopamine (250 micrograms i.c.v.), given 72 h previous to the corresponding operation, blunted the cardiovascular effects of neostigmine in both groups of rats. Prazosin (10 and 30 micrograms i.c.v.), an alpha 1-adrenoceptor antagonist, prevented the pressor response to neostigmine (0.3 micrograms i.c.v.) in sham-operated and sinoaortic-denervated rats. Yohimbine, a alpha 2-adrenoceptor antagonist (10 and 30 micrograms i.c.v.), only prevented the bradycardia induced by neostigmine (0.3 micrograms i.c.v.) in the sham-operated rats. 6-Hydroxydopamine pretreatment lowered the norepinephrine content in hypothalamus, midbrain, medulla oblongata and spinal cord, but did not modify it in the pons, in sham-operated rats and sinoaortic-denervated animals. The present results suggested that brain alpha 1-adrenoceptors would mediate the pressor response to neostigmine (i.c.v.) in sham-operated and sinoaortic-denervated rats and central alpha 2-adrenoceptors mediate the bradycardia in sham-operated rats. This work lends support to the view that cardiovascular responses to brain cholinergic stimulation in sham-operated and sinoaortic-denervated rats could be mediated by a central catecholaminergic activation.

Cardiovascular responses to cholinergic agonists in sinoaortic denervated rats.

1. The cardiovascular effect of systemic nicotinic and muscarinic cholinergic stimulation were studied in conscious sham operated and sinoaortic denervated (SAD) rats, 7 days after the corresponding operation. 2. The administration of the nicotinic ganglionic agent, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 50-100 micrograms.kg-1, i.v.) induced a fall of heart rate that was significantly higher in SAD rats than in sham rats. DMPP induced in sham rats an increase of arterial pressure but in SAD animals a biphasic response: an initial hypotension followed by an increase of arterial pressure. 3. Under muscarinic blockade, DMPP only showed a pressor and tachycardic action in both groups of rats without differences between them. 4. The muscarinic agonist, carbachol (0.1-10 micrograms.kg-1, i.v.) showed the same hypotensive and bradycardic action in both groups of rats. 5. Our results suggest that after 7 days of SAD, differences in the response to DMPP between sham and denervated animals could be due to the loss of baroreflex mechanisms. The increased bradycardic effect of DMPP in SAD rats could be mediated by a supersensitivity of parasympathetic ganglionic nicotinic receptors, whilst the sympathetic ganglionic nicotinic receptors remained unaltered. On the other side, the cardiovascular muscarinic responses to carbachol remain unaffected in SAD rats.

Cardiovascular responses of sinoaortic-denervated rats to intracerebroventricular injection of alpha 1- and alpha 2-adrenoceptor agonists.

The aim of the present work was to analyse the cardiovascular responses induced by i.c.v. administration of the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and clonidine, respectively, in conscious normal and sinoaortic-denervated rats. Sinoaortic denervation involves changes in central and peripheral catecholaminergic pathways. Clonidine (1-10 micrograms) produced a dose-dependent rise in blood pressure and a bradycardiac response in sham-operated animals, whereas in sinoaortic-denervated rats it provoked a brief rise in blood pressure followed by a marked fall as well as bradycardia. The responses involved mostly activation of central alpha 2-adrenoceptors, but the blood pressure responses induced by clonidine in sinoaortic-denervated rats may also have involved alpha 1-adrenoceptors. The bradycardia induced by the alpha 2-agonist in both groups of rats involved preferentially central alpha 2-adrenoceptors but also partially stimulated alpha 1-adrenoceptors. Phenylephrine, at a dose of 10-60 micrograms, induced a rise in blood pressure and a bradycardiac response while 90 micrograms produced a biphasic pressure response (early transient rise followed by a fall) as well as bradycardia in both sham-operated and sinoaortic-denervated animals. Phenylephrine activated alpha 1-adrenoceptors in every case, but the fall in blood pressure and the bradycardia also involved alpha 2-adrenoceptors. The responses were significantly higher in the sinoaortic-denervated rats than in the sham-operated. Our findings suggest that arterial baroreceptor reflexes can modify the effects of alpha-agonists initiated in the central nervous system. Sinoaortic denervation preparations enable one to unmask the depressor response to clonidine and also demonstrate the true magnitude of the phenylephrine response.

Cardiovascular responses to adrenergic agonists in sinoaortic denervated rats.

1. The cardiovascular effects of systemic alpha and beta adrenergic stimulation were studied in conscious sham-operated and sinoaortic denervated (SAD) rats, 7 days after the corresponding operation. 2. SAD does not modify the pressor response to phenylephrine (0.125-8 micrograms.kg-1, i.v.). The blood pressure increase was compensated for bradycardia in the sham-operated rats but not in the SAD. Only 8 micrograms.kg-1 of phenylephrine induced a slight fall of heart rate in the denervated animals. 3. The beta adrenergic agonist isoproterenol (7.8 ng.kg-1-2 micrograms.kg-1, i.v.) showed a greater hypotensive and a lesser tachycardic action in SAD than in sham-operated rats. However, these differences disappeared after autonomic ganglionic blockade (hexamethonium, 10 mg.kg-1, i.v.). 4. Our results suggest that after 7 days of SAD the pressor response to alpha adrenergic agonists may be unaltered and that the differences in the cardiovascular beta adrenergic response of SAD rats may be due to loss of the baroreflex mechanisms of cardiovascular compensation.

Interaction between clonidine and physostigmine in normal rats and in rats after sinoaortic denervation.

Clonidine (3-30 micrograms.kg-1, i.v.) induced a fall in mean arterial pressure in rats after sinoaortic denervation but not in sham-operated animals. Moreover, sinoaortic denervation reduced the bradycardic action of this antihypertensive drug. Pressor and tachycardic response to physostigmine (60 micrograms.kg-1, i.v.) were greater in denervated than in sham-operated rats. The increase of mean arterial pressure was 26.2 +/- 2.2 mm Hg in sham-operated rats (n = 12) and 53.8 +/- 2.0 mm Hg in denervated rats (n = 12, P less than 0.005). Pretreatment with 3 micrograms.kg-1 (i.v.) of clonidine did not alter the pressor response to physostigmine (60 micrograms.kg-1) in either of the two groups; 10 and 30 micrograms.kg-1 of clonidine reduced the physostigmine-induced increase of mean arterial pressure in sham-operated rats but enhanced the pressor response in denervated animals. Furthermore, an ineffective dose of physostigmine (30 micrograms.kg-1, i.v.) induced a pressor response after pretreatment with clonidine (10 micrograms.kg-1) in denervated rats. Clonidine (10 micrograms.kg-1) did not affect the pressor effect of 1,1 dimethyl-4-phenylpiperazinium iodide (DMPP: 50 micrograms.kg-1, i.v.) or phenylephrine (4 micrograms.kg-1, i.v.) in either group. The anticholinergic effect of clonidine in sham-operated rats may be explained by an inhibitory action on the release of acetylcholine in several brain structures but the facilitatory effect of clonidine observed in denervated animals is not clear. The results did not suggest a peripheral involvement in this facilitatory effect.