The blood pressure effects of alpha-adrenoceptor antagonists injected in the medullary site of action of clonidine: The nucleus reticularis lateralis

We administered a series of alpha-blocking drugs to the nucleus reticularis lateralis (NRL) of the medulla oblongata, the main site for the hypotensive action of clonidine. These experiments were performed on pentobarbital anaesthetized cats. Drugs were injected through a needle which was stereotaxically inserted. Prazosin (6 nmol) was hypertensive (MBP=+25±8%), corynanthine had no effect and AR-C239 (7 nmol), another alpha₁- blocker, was hypotensive (MBP=−16±3.5%).The alpha₂-blockers, yohimbine and idaxozan, were hypotensive. The blood pressure effects of alpha-blocking drugs directly microinjected in the nucleus reticularis lateralis cannot be simply related to their selectivity for a particular subtype of alpha-receptors.     

Role of presympathetic C1 neurons in the sympatholytic and hypotensive effects of clonidine in rats

The rostral ventrolateral medulla (RVLM) may play an important role in the sympatholytic and hypotensive effects of clonidine. The present study examined which type of presympathetic RVLM neuron is inhibited by clonidine, and whether the adrenergic presympathetic RVLM neurons are essential for clonidine-induced sympathoinhibition. In chloralose-anesthetized and ventilated rats, clonidine (10 microg/kg iv) decreased arterial pressure (116 +/- 6 to 84 +/- 2 mmHg) and splanchnic nerve activity (93 +/- 3% from baseline). Extracellular recording and juxtacellular labeling of barosensitive bulbospinal RVLM neurons revealed that most cells were inhibited by clonidine (26/28) regardless of phenotype [tyrosine hydroxylase (TH)-immunoreactive cells: 48 +/- 7%; non-TH-immunoreactive cells: 42 +/- 5%], although the inhibition of most neurons was modest compared with the observed sympathoinhibition. Depletion of most bulbospinal catecholaminergic neurons, including 76 +/- 5% of the rostral C1 cells, by microinjection of saporin anti-dopamine beta-hydroxylase into the thoracic spinal cord (levels T2 and T4, 42 ng. 200 nl(-1). side(-1)) did not alter the sympatholytic or hypotensive effects of clonidine. These data show that although clonidine inhibits presympathetic C1 neurons, bulbospinal catecholaminergic neurons do not appear to be essential for the sympatholytic and hypotensive effects of systemically administered clonidine. Instead, the sympatholytic effect of clonidine is likely the result of a combination of effects on multiple cell types both within and outside the RVLM.     

Apparent resistance to hypotensive effect of clonidine.

Clonidine failed to reduce the blood pressures of two patients with essential hypertension. On was given 5-4 mg/day and the other 6 mg/day, and their respective peak plasma clonidine concentrations were 26-2 ng/ml and 14-4 ng/ml. Several months after the end of clonidine treatment a single oral dose of 0-3 mg of clonidine produced maximum falls in blood pressure of 30/22 mm Hg and 88/41 mm Hg with peak plasma clonidine concentrations of 1-4 ng/ml and 0-9 ng/ml. Resistance to the hypotensive effect of high doses of clonidine may be due to stimulation of peripheral alpha-adrenoceptors causing vasoconstriction, which maintains a raised blood pressure.     

Role of ventral surface of brain stem in control of arterial pressure and in hypotensive action of clonidine]

Superficial destructions of the brain stem have been performed in cats at the level of the chemosensitive areas "S". These destructions produced a drop in blood pressure, which was transient in 9 and definitive in 4 animals. In 6 sham-operated animals, clonidine (15 mug/kg), injected intravenously, always induced a marked fall in blood pressure, whereas in the 10 animals which have maintained or regained normal blood pressure after the destruction of the areas S, clonidine no longer induced any decrease in blood pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at that level of the ventral surface of the brain stem, through an inhibition of a vasopressive structure.     

Contribution of AMPA/kainate receptors in the rostral ventrolateral medulla to the hypotensive and sympathoinhibitory effects of clonidine

The depressor and sympathoinhibitory effect of the imidazoline drug clonidine is reported to be associated with functional states of the central glutamate receptors. The rostral ventrolateral medulla (RVLM) has been recognized as a specific target area for mediating the central depressor mechanism of clonidine. The objective of this study was to determine the role of the glutamate receptor subtype alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor within the RVLM in clonidine-induced depressor and sympathoinhibitory action in anesthetized normotensive rats. Unilateral microinjection of 200 pmol of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a potent AMPA/kainate receptor antagonist, into the RVLM completely abolished the pressor action evoked by AMPA (5 pmol) without affecting the pressor action of N-methyl-D-aspartate (20 pmol). Pretreatment with intra-RVLM injection of CNQX (20 and 200 pmol) dose dependently attenuated the reduction in blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) elicited by intra-RVLM clonidine (5 nmol) or intravenous clonidine (10 microg/kg), while 2 pmol of CNQX did not alter clonidine-induced cardiovascular action. Furthermore, the decreases in BP, HR, and RSNA evoked by intravenous clonidine (10 microg/kg) or intra-RVLM clonidine (5 nmol) were reversed when CNQX (20 and 200 pmol) was subsequently injected into the RVLM. In conclusion, these data show that blockade of AMPA/kainate receptors in the RVLM significantly antagonizes decreases in BP, HR, and sympathetic activity induced by clonidine, suggesting that the AMPA/kainate receptors within the RVLM contribute to the depressor and sympathoinhibitory effect of clonidine.     

The hypotensive action of bradykinin in the guinea pig and cat]

In the guinea-pig as in the cat, the duration of the bradykinin general arterial hypotension is reduced by indometacine. This decrease is due to the inhibition of the synthesis of endogenous vasodilatator prostaglandins. The guinea-pig and the cat present the same type of reactivity for bradykinin as the rabbit, the dog and the rat.     

Hemodynamic aspects of the hypotensive action of captopril

Captopril was tested in acute experiments on anesthetized cats subject to cardiopulmonary bypass. The drug was shown to reduce total peripheral vascular resistance and to increase vascular bed capacity.     

The influence of propranolol on the hypotensive action of ketanserin in normotensive rats

In normotensive rats the effect of different doses of propranolol (1.0, 5.0 and 10.0 mg/kg i.p.) and ketanserin (10.0 mg/kg p.o.) on mean blood pressure and heart rate and on cardiovascular response to noradrenaline (0.1, 0.3, 0.5, 0.7 and 1.0 micrograms/kg i.v.) was examined. The drugs were given separately or together. Propranolol slightly reduced the hypotensive effect of ketanserin. On the other hand a decrease in heart rate caused by propranolol was not affected by ketanserin. Our results show that propranolol given with ketanserin did not change the effect of the latter on the cardiovascular system.     

Ventrobulbular hypotensive action of muscimol]

Muscimol, a rigid analogue of GABA has been injected in the CNS of urethane anesthetized, normotensive cats. Injected either intracisternally (1 or 2 microgram/kg, 0.05 ml) or directly by microinjection in a restricted ventrolateral region of the brain stem (0.5 or 1 microgram/kg, 0.5 microliter), muscimol induced hypotension and bradycardia. These central cardiovascular effects of muscimol were antagonized by bicuculline, a "specific" GABA antagonist agent. These data emphasize the involvement of gabergic mechanisms in the central cardiovascular control, at least in the ventrolateral part of the medulla oblongata.     

Analysis of the inotropic action of ouabain in rat ventricles: two apparent ouabain inotropic responses

Two distinct inotropic effects of ouabain were observed in 68 of 82 right ventricular strips of rats with ED50s of 0.5 and 20 microM referred to as "low-dose" and "high-dose" effects, respectively. The other 14 strips showed monophasic dose-response curves, with an apparent ED50 of 0.3 microM; the inotropic response to ouabain or isoproterenol developed by these atypical strips did not exceed 50% over control. In the strips showing the biphasic inotropic response curves, the proportion of the "low-dose" effect varied from 6 to 89% of the maximum response. After treatment with 160 microM ouabain, followed by 60 min of washout, the maximum developed tension in response to ouabain was unchanged, but the "low-dose" effect was abolished or dramatically reduced. In those cases with a remaining "low-dose" response, the response was only 12% or less of the maximum and the ED50 was shifted from 0.3 to about 3 microM ouabain by the washout. The atypical strips, which showed only the "low-dose" effects during the first ouabain exposure, revealed after washout a consistent "high-dose" effect with an ED50 of about 12 microM ouabain. The data show that the two inotropic responses exist in all ventricular strips and that in some strips, after ouabain washout, a residue of the "low-dose" effect remains.     

Kallikrein-thrombolytic and hypotensive action in cats--preliminary results.

An intravenous injection of kallikrein produced hypotensive and thrombolytic effects in anesthetized cats, using the blood superfused tendon technique. The thrombolytic action of kallikrein was mediated by an unstable substance. The generation of this substance was abolished by either acetylsalicylic acid (ASA) or aprotonin and enhanced by captopril. The hypotensive action of kallikrein was only partially inhibited by ASA. It is proposed that both these pharmacological effects of kallikrein are mediated by bradykinin which in turn releases prostacyclin from the endothelium. However, in contrast to the thrombolytic effect of kallikrein which is totally mediated by prostacyclin the hypotensive action of kallikrein depends not only on prostacyclin but also on another endothelium-derived vasorelaxant, e.g. EDRF.     

The effects of clonidine on the kidney function in the anesthetized dog

Studies were performed to determine the mechanism by which the antihypertensive agent clonidine increased urine flow. The response of the kidney has been examined in four combinations. The parameters of renal function have been compared during volume expansion by 1.5-2.0% body weight Ringer solution. In the control animals, volume expansion by 2% body weight, resulted in a slight increase in sodium excretion and urine flow. In 10 anesthetized dogs 1.0 microgram/kg/min of clonidine infused i.v. during 30 minutes (the total amount of clonidine infused was 30 micrograms/kg) decreased the arterial blood pressure from 136 +/- 13 mmHg to 127 +/- 12 mmHg and elevated urine flow from 2.95 +/- 1.65 ml/min to 4.34 +/- 1.77 ml/min while the urine osmolality diminished from 399 +/- 107 mosm/l to 265 +/- 90 mosm/l and the glomerular filtration remained constant. In 5 animals 0.1 microgram/kg/min of clonidine was infused into the left renal artery (this dose is corresponding to the renal fraction of the cardiac output) without any effects in the left kidney. 1.0 microgram/kg/min of clonidine infused directly into the left renal artery produced vasoconstriction in the ipsilateral kidney, decreased the glomerular filtration rate and the urine flow. By contrast in the right kidney the urine flow rose without hemodynamic changes, and the urine osmolality became hypoosmotic compared to the plasma. In ten dogs 1.0 microgram/kg/min of clonidine and 1 mU/kg/min of arginine-vasopressin were infused intravenously. The vasopressin infusion superimposed on the clonidine could not inhibit the increase of the urine excretion, and the fall of the urine osmolality. The results suggest that the clonidine increases the renal medullary blood flow possibly via a direct mechanism, decreases the sympathetic outflow to the kidney and via an indirect pathway, mediated by the renin-angiotensin system. The renal medullary flow increase produces a washout of the medullary osmotic gradient, and the water reabsorption diminishes.