María Amelia Enero

Cardiovascular responses of sinoaortic-denervated rats to intracerebroventricular injection of α1- and α2-adrenoceptor agonists

Abstract The aim of the present work was to analyse the cardiovascular responses induced by i.c.v. administration of the α 1 - and α 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 μg) 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 α 2 -adrenoceptors, but the blood pressure responses induced by clonidine in sinoaortic-denervated rats may also have involved α 1 -adrenoceptors. The bradycardia induced by the α 2 -agonist in both groups of rats involved preferentially central α 2 -adrenoceptors but also partially stimulated α 1 -adrenoceptors. Phenylephrine, at a dose of 10–60 μg, induced a rise in blood pressure and a bradycardiac response while 90 μg 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 α 1 -adrenoceptors in every case, but the fall in blood pressure and the bradycardia also involved α 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 α-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.

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

SummaryClonidine (3–30 μg · 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 μg · 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 < 0.005).Pretreatment with 3 μg · kg−1 (i. v.) of clonidine did not alter the pressor response to physostigmine (60 μg · kg−1) in either of the two groups; 10 and 30 μg · 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 μg - kg−1 i.v.) induced a pressor response after pretreatment with clonidine (10 gg · kg−1) in denervated rats.Clonidine (10 μg · kg−1) did not affect the pressor effect of 1,1 dimethyl-4-phenylpiperazinium iodide (DMPP: 50 μg · kg−1 i.v.) or phenylephrine (4 μg · 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.

Effects of acute and short-term treatment with antihypertensive drugs in sinoaortic denervated rats.

Sinoaortic denervation (SAD) produced a marked increase of the systolic blood pressure (SBP). Clonidine (50 micrograms kg-1, i.p.) reduced SBP in SAD but not in sham rats. L-alpha-methyldopa (alpha-MD) (50 mg kg-1, i.p.) also induced a more effective hypotensive action in SAD than sham rats. The withdrawal of alpha-MD in SAD rats after the first treatment was not abrupt and the hypotension persisted for several days, but after the second treatment the withdrawal induced a rapid rebound hypertension. Our results suggest that SAD increases the response to the hypotensive agents. An alteration in the availability of alpha-MD to accumulate or synthesize the active metabolites was also observed after second treatment.

Sinoaortic denervation-induced changes in central serotonergic neurons

Sinoaortic denervation in rats induced changes in endogenous levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brain areas. The midbrain showed a sustained increase for 7 days after the operation, while in pons and medulla oblongata a marked rise was only observed during the first 24 h. No changes were obtained in spinal cord. It is thought that the changes in serotonergic activity described could be related to the neurogenic hypertension induced by sinoaortic denervation.

Decreased brain serotonergic activity after acute propranolol

Acute propranolol reduced the synthesis rate of serotonin in several rat brain structures (hypothalamus, midbrain, pons, frontal cortex and medulla oblongata). Both optical isomers of propranolol were tested. From our results it may be concluded that both the beta-blocking property and the local anaesthetic action of the drug could be involved in its inhibitory effect, except in hypothalamus where only the levorotating isomer was effective. Our results suggest the possibility of a noradrenergic influence on brain serotonergic neurons.

Activation of 5-HT2 receptors inhibits the evoked release of [3H]noradrenaline in the rat spinal cord

1. The participation of 5-HT2 receptors in the modulation of the evoked release of [3H]noradrenaline from rat spinal cord slices has been examined. 2. In rat spinal cord slices preincubated with [3H]noradrenaline, the alpha 2-receptor agonist clonidine (10(-6) mol/l) decreased the release of tritium evoked by field stimulation (600 pulses at 5 Hz, 20 mA, 2 msec), while the alpha 2-antagonist yohimbine (10(-6) mol/l) increased it. 3. The 5-HT2/5-HT1c agonist DOI (3 x 10(-7) mol/l) decreased the evoked release of tritium, an effect which was prevented by ketanserin (10(-7) mol/l). 4. It is suggested that in addition to presynaptic alpha 2-adrenoceptors, there are 5-HT2 receptors which modulate the release of noradrenaline in the rat spinal cord.

Endothelium and Ca2+ in the prostaglandin F2α potentiation of vasoconstrictor responses

Abstract Prostaglandin F2α (PGFP2α), at a concentration that did not induce vascular contraction (10−9 and 10−10 M), potentiated the dose-response curves to norepinephrine (NE) in rat mesenteric ring segments only when the endothelium was present. Moreover, PGF2α, in both unrubbed mesenteric artery and mesenteric vascular bed, was able to increase the contraction to NE as well as the ratio of the amplitude of two NE-induced contractions under previously standardized conditions, in the absence of extracellular calcium. In addition, without extracellular Ca2+, PGF2α increased in both tissues the refilling of Ca2+ induced by 80 mM KCl plus 1.5 mM Ca2+. The mechanism of this potentiation is unknown, but it may be related to cellular events including the intracellular Ca2+ mobilization. This study suggests that the endothelium plays a necessary role in PGF2α potentiation of vasoconstrictor response, possibly through the release of an endothelial vasoconstrictor factor which probably increases the Ca2+ bioavailability for the contraction.

Central serotonergic activity after neurogenic hypertension

Sinoaortic denervated rats, 24 h after operation, showed a 25% increase in the central (hypothalamus, frontal cortex, midbrain, pons and medulla oblongata) tryptophan content and about a 70% rise in plasma free fatty acid (NEFA) concentration. The synaptosomal serotonin (5-HT) uptake and tryptophan hydroxylase activity of slices from these central areas were not significantly different when compared to those from sham-operated rats. There was also an increase in tryptophan and NEFA concentration at 24 h in fasted sham-operated rats. Seven days after neurogenic hypertension, the central tryptophan content had returned to control values. However, a 50% increase in the tryptophan hydroxylase activity of slices from the midbrain area together with a higher turnover rate of serotonin was found in the denervated group of rats. The synaptosomal 5-HT uptake remained unchanged. These results suggest that sinoaortic denervation could induce changes in the serotonergic neurons localized in central nervous system areas during the first week after operation.

Interaction between β2- and α2-adrenoceptor responses in the vascular system: effect of clenbuterol

Subchronic treatment with the β2-adrenoceptor agonist, clenbuterol (0.3 mg · kg−1, twice daily for 14 days), significantly increased the median blood pressure in anaesthetized normotensive rats. The treatment produced a marked reduction in the vasodilator effect of isoproterenol. Acute clenbuterol administration (0.01 mg·kg− i.v.) reduced the contractile response induced by the α2-adrenoceptor agonists, guanabenz or B-HT 920, and the α1- and α2-adrenoceptor agonist, clonidine, whereas it did not affect the vasoconstriction induced by the α1-adrenoceptor agonist, methoxamine, in the pithed rat. Subchronic treatment with clenbuterol attenuated the effect of the β2-adrenoceptor agonist on the vascular α2-adrenoceptor responses and enhanced the α1-adrenoceptor response to phenylephrine in pithed rats. The effect of the β2-adrenoceptor agonist was reduced by 1- but not d-propranolol. These results suggest that subchronic treatment with clenbuterol produces a subsensitivity of the β2-adrenoceptors and reduces the interaction between β2- and α2-adrenoceptors at the vascular wall.

Role of cyclic AMP in the release of noradrenaline from isolated rat atria

SummaryThe possible role of cyclic AMP (cAMP) on tritium overflow evoked by stimulation of the cardioaccelerant nerves was studied in rat atria preincubated with [3H]-noradrenaline. Addition of the activator of adenylate cyclase forskolin (1 µmol/l), or of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMx, 100 µmol/l), did not affect both basal and evoked overflow. However, in the presence of the α2-adrenoceptor antagonist yohimbine (0.03 µmol/l) both forskolin and IBMx increased the stimulation-induced transmitter overflow by 49% and 141%, respectively (compared to yohimbine 0.03 µmol/l). Thus, in rat atria the cAMP-dependent facilitation of noradrenaline release is only present when the autoinhibition exerted by activation of prejunctional α2-adrenoceptors is blocked. Propranolol (0.1 µmol/l) that did not produce any effect on noradrenaline release markedly reduced the facilitatory response induced by forskolin in the presence of yohimbine. When rats were pretreated with the β2-adrenoceptor agonist clenbuterol (0.3 mg · kg−1, s. c., twice daily, 14 days), a treatment which desensitizes β-adrenoceptor-me-diated facilitation of noradrenaline release (Kazanietz and Enero 1989), the facilitatory effect of forskolin and IBMx in the presence of yohimbine was abolished. The results indicate that in rat atria the effect of forskolin and IBMx on noradrenaline release are only to be observed after blockade of presynaptic α2-adrenoceptor autoinhibition. β-adrenoceptor blockade or clenbuterol pre-treatment decreases the facilitatory response to forskolin and hence prejunctional β-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase.