Mechanism of suppression of renin secretion by clonidine in the dog

Prolonged clonidine treatment: catecholamines, renin activity and aldosterone following exercise in hypertensives

Eight patients with essential hypertension, WHO grade I-III, were studied under standardized conditions in a metabolic ward, before and after 8-20 weeks of treatment with clonidine in a maintenance dose of 300-600 micrograms/24 h. Before clonidine, plasma noradrenaline concentration (PNA), plasma adrenaline concentration (PA), plasma renin activity (PRA) and plasma aldosterone concentration (PAC) increased in response to standing and submaximal exercise for 20 min. PNA was positively correlated to pulse rate in the supine position (R = 0.74, p 0.05) and the increase in PNA to the increase in pulse rate during exercise (min 10, R=0.73, p less than 0.05; min 15, R = 0.79, p less than 0.05; min 20, R = 0.74, p less than 0.05). No other significant correlations were found between PNA, PA, PRA and PAC on the one hand and blood pressure (BP) and pulse rate on the other. Clonidine reduced BP, pulse rate, PNA and PRA under all conditions studied. PA was reduced in the upright position and in connection with exercise. PAC was reduced during clonidine after exercise but otherwise unaltered. The clonidine-induced decrease in PNA was positively correlated to the decrease in diastolic BP both in the supine (R = 0.76, p less than 0.05) and in the upright (R = 0.80, p less than 0.05) position. thus, long-term clonidine treatment lowered the BP and pulse rate, at least partly by reducing sympathetic activity via a central mechanism. However, clonidine did not block the sympathetic reflex mechanisms engaged in the maintenance of BP in the upright position. During clonidine, the adrenaline values were lower than before treatment in the supine and in the upright position and also following exercise, indicating that clonidine exerts an inhibitory effect on the sympatho-adreno-medullary system.

Preclinical assessment of a new transdermal delivery system for clonidine (M-5041T)

The effects of a new transdermal delivery system for clonidine (M-5041T) on hypotensive effect, urine volume, plasma renin activity (PRA) and antidiuretic hormone (ADH) in spontaneously hypertensive rats (SHRs) were compared to the effects of the continuous infusion of clonidine. Both M-5041T (1.5 and 4.5 mg/kg) and the continuous infusion of clonidine (250 micrograms/kg/24 h) elicited hypotensive effects persisting for 12 hours or more. These effects were based on consistent plasma concentrations of clonidine. These two treatments produced diuresis followed by antidiuresis, which was remarkably observed by continuous infusion of clonidine. Single subcutaneous injection of clonidine (50 micrograms/kg) produced diuresis accompanied by increases in electrolytes corresponding to plasma levels of clonidine. M-5041T at 1.5 mg/kg did not affect PRA until 12 h, and produced an increase in PRA at 24 h. M-5041T at 4.5 mg/kg and the continuous infusion of clonidine resulted in a decrease in PRA at 2 and 1 h followed by an increase at 12 and 24 h, respectively. M-5041T at 1.5 mg/kg did not affect plasma levels of ADH. Plasma ADH did increase at 2 and 4 h accompanied by diuresis following M-5041T at 4.5 mg/kg or the continuous infusion of clonidine, respectively. Clonidine-induced diuresis was not at least due to the inhibition of ADH release. The decrease in urine volume observed by continuous infusion of clonidine may be due to decrease in renal blood flow based on stimulation of peripheral adorenoceptors of clonidine. These findings suggest that the increases in ADH and PRA are due to the compensatory effects related to both diuresis and the long-lasting hypotensive effect induced by high plasma concentrations of clonidine. Thus, it can be expected that M-5041T at 1.5 mg/kg showing the minimum effective plasma concentration of clonidine will not result in tolerance to the hypotensive effect of clonidine associated with the retention of sodium in SHRs.

Brain catecholamines mediate the delayed reduction in renin release after injection of fenfluramine

The present studies examined whether brain or peripheral catecholaminergic mechanisms mediate the fenfluramine-induced reduction in plasma renin activity and concentration. Fenfluramine reduced plasma renin activity and concentration to 35% of basal levels in vehicle pretreated rats. This reduction was prevented in rats whose central catecholaminergic neurons were destroyed by i.c.v. injection of 6-hydroxydopamine (6-OHDA) two weeks prior to the experiment. In contrast, the suppressive effects of fenfluramine on plasma renin activity and concentration was not altered in rats that were surgically adrenal medullectomized and chemically sympathectomized by 6-OHDA. To determine whether alpha 2 adrenoceptors mediate the suppressive effects of fenfluramine, rats were pretreated with increasing doses of the alpha 2 adrenoceptor antagonist yohimbine 1 h before the administration of fenfluramine. Although yohimbine alone increased plasma renin activity and concentration, it did not prevent the suppressive effects of fenfluramine. In conclusion, the data suggest that central, but not peripheral catecholamines mediate the suppressive effect of fenfluramine on renin secretion. This effect is not mediated by alpha 2 adrenoceptors.

Cocaine-induced suppression of renin secretion is partially mediated by serotonergic mechanisms

Acute cocaine reduces renin secretion. To determine whether serotonergic neurons mediate this effect, male Sprague-Dawley rats received the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (75 micrograms/side, ICV) 2 weeks prior to cocaine injections (3.75-15 mg/kg, IP). 5-HT lesions attenuated the cocaine-induced reduction of plasma renin concentration (PRC), suggesting a partial 5-HT role. To determine which receptors mediate this response, rats were pretreated with the partial 5-HT1A agonist 8-[2-[4-(2-methoxyphenyl)-l-piperazinyl]ethyl]-8-azaspirol-[4,5]- decane-7,9-dione (BMY 7378) (1 mg/kg, SC), the 5-HT1C/5-HT2 antagonist ritanserin (0.1 mg/kg, SC), or the alpha 2/5-HT1A antagonist yohimbine (1 mg/kg, SC) prior to cocaine. None of the antagonists altered the cocaine-induced suppression of PRC, although BMY 7378 and yohimbine elevated PRC. The data suggest that cocaine's effect is partially mediated by a serotonergic mechanism, but do not support a role for 5-HT1A receptors, 5-HT2/5-HT1C receptors, or alpha 2-adrenoceptors in mediating the suppressive effect of cocaine on renin secretion.

Repeated injections of cocaine inhibit the serotonergic regulation of prolactin and renin secretion in rats

Alterations in serotonergic function following repeated cocaine injections were examined using neuroendocrine responses to a serotonin (5-HT) releaser and 5-HT agonists. Forty-two hours following administration of cocaine (1-15 mg/kg i.p.) twice daily for 7 or 30 days, male Sprague-Dawley rats were injected with the 5-HT releaser p-chloroamphetamine (PCA; 8 mg/kg i.p.) and blood samples were collected 1 h later for radioimmunoassays of plasma prolactin, plasma renin activity (PRA) and plasma renin concentration (PRC). PCA significantly increased secretion of prolactin and renin. These responses were attenuated in rats pretreated with cocaine for 30 days. In rats receiving cocaine for 7 days, the attenuation of PCA-induced secretion of prolactin and renin was less consistently observed. To determine whether these alterations were due to pre- or postsynaptic effects, rats were injected with cocaine (15 mg/kg i.p.) twice daily for 7 days, and the neuroendocrine responses to the direct 5-HT agonists RU 24969 and m-CPP were examined, 42 h after the last cocaine injection. Pretreatment with cocaine potentiated RU 24969-induced stimulation of plasma prolactin concentration. However, cocaine did not alter the ability of m-CPP to increase plasma prolactin concentrations. The stimulation of renin secretion in response to both 5-HT agonists was not altered by cocaine pretreatment. The data suggest that repeated cocaine impairs the function of serotonergic nerve terminals that regulate these endocrine responses. Furthermore, the 5-HT receptors that mediate prolactin secretion may exhibit supersensitivity.

Comparison of neuroendocrine and behavioral effects of ipsapirone, a 5-HT1A agonist, in three stress paradigms: immobilization, forced swim and conditioned fear

Ipsapirone is an anxiolytic drug and a serotonin1A (5-HT1A) agonist. The aim of the present study was to investigate the effects of low doses of ipsapirone on the hormonal and behavioral response to three stress procedures: immobilization, forced swim and conditioned emotional response (CER). We examined the effect of ipsapirone (0.1, 0.5 or 1.0 mg/kg) on plasma renin concentration (PRC), adrenal corticotropic hormone (ACTH), corticosterone, prolactin and defecation in rats exposed to immobilization, forced swim or CER stress. All three stressors significantly elevated all the hormone levels (P less than 0.01). Immobilization-induced elevations of PRC, and corticosterone were inhibited by the highest doses of ipsapirone (0.5 and 1 mg/kg, i.p.). However, ipsapirone did not modify the immobilization-induced elevations of plasma ACTH, prolactin or defecation. Ipsapirone was relatively ineffective at reducing the endocrine responses to forced swim. Ipsapirone reduced some, but not all of the hormonal responses to CER stress. CER-induced elevations of corticosterone and prolactin were not inhibited by ipsapirone. However, the ACTH response to CER was significantly (P less than 0.01) inhibited by all doses of ipsapirone and the highest dose of ipsapirone attenuated the renin response. In contrast with the hormonal responses, ipsapirone inhibited all of the behavioral responses to CER stress. Ipsapirone inhibited CER-induced freezing behavior and defecation, while dose-dependently reversing the suppressive effect of CER on exploring, grooming and rearing behaviors. In conclusion, there is a dissociation between the influence of ipsapirone on the endocrine and behavioral responses to CER stress. Ipsapirone also has differential effects on the neuroendocrine response to the three stressors studied. Ipsapirone was most effective in attenuating the hormonal responses to CER, followed by immobilization and swim stress. Of the hormones studied, the stimulation of renin secretion after exposure to the three stressors was most sensitive to ipsapirone, while corticosterone and prolactin were the least sensitive to ipsapirone.

Neuroendocrine responses to cocaine do not exhibit sensitization following repeated cocaine exposure

Cocaine-induced enhancement of motor activity and extracellular dopamine concentrations exhibits sensitization upon repeated exposure. In this study, the neuroendocrine responses to cocaine were examined following cocaine pretreatment regimens which have been shown to produce behavioral sensitization. Adult male rats were injected with cocaine (15 mg/kg, IP) once daily for 14 days, followed by a dose-response challenge with cocaine (1-15 mg/kg, IP) either 18 hours or 7 days after the final pretreatment injection. Blood was collected 15 minutes following injections for radioimmunoassay of ACTH, corticosterone, prolactin, and renin. Cocaine increased plasma ACTH and corticosterone, while it decreased prolactin and renin concentrations. Pretreatment with cocaine for 2 weeks did not alter any of these endocrine responses after either the 18 hour or 7 day interval between pretreatment and challenge injections. In contrast, sensitization to the locomotor stimulant effects of cocaine was observed on the final day of pretreatment injections, and 7 days later. These data suggest that these endocrine effects of cocaine do not exhibit sensitization following repeated cocaine exposure.

Evidence from binding studies for alpha 2-adrenoceptors directly associated with glomeruli from rat kidney

Binding of the alpha 2-adrenoceptor radioligands [3H]clonidine and [3H]rauwolscine but not the alpha 1-adrenoceptor radioligand [3H]prazosin was enhanced in membranes prepared from rat isolated renal glomeruli. [3H]Rauwolscine binding to glomeruli was stereoselective with respect to the (-)-isomer of noradrenaline and the order of potency of a series of antagonists for displacement of binding indicated that the alpha 2-adrenoceptors in this preparation differ somewhat from those in some other species and tissues. Chemical sympathectomy produced no significant change in the number of sites labelled by [3H]rauwolscine indicating that few of the alpha 2-adrenoceptors in glomerular membranes are associated with sympathetic nerve terminals.

Central effects of prostaglandin E2 on blood pressure and plasma renin activity in rats. Role of the sympathoadrenal system and vasopressin

This study was designed to determine the roles of the sympathetic nervous system, adrenal medulla, and arginine vasopressin (AVP) in mediating pressor and plasma activity (PRA) responses to intraventricularly (ICV) administered prostaglandin E2 (PGE2) in conscious rats. The ICV PGE2 elevated blood pressure and caused increases in PRA, plasma AVP, and plasma norepinephrine and epinephrine. The pressor effect of ICV PGE2 was not influenced by pretreatment with captopril, but was attenuated by the AVP antagonist, d(CH2)5Tyr(Me)AVP, and by phenoxybenzamine, and was completely abolished by the combination of the AVP antagonist and phenoxybenzamine. The PRA response to ICV PGE2 was not affected by bilateral renal denervation or by phenoxybenzamine alone, but was attenuated by propranolol alone and was completely abolished by the combination of propranolol and phenoxybenzamine. Bilateral adrenomedullectomy did not affect the pressor response to ICV PGE2, whereas it attenuated the increase in PRA and completely abolished the increase in plasma epinephrine. These results suggest that the pressor effect of ICV PGE2 is the result of increased sympathetic nervous system activity and is dependent on the stimulation of alpha-adrenergic receptors and on AVP release. The pressor response to ICV PGE2 is accompanied by but not dependent on an increase in PRA. The renin-stimulating effect of centrally administered PGE2 is, at least in part, dependent on beta-adrenergic receptor stimulation by increased circulating catecholamines.

Conditions for humoral alpha-adrenoceptor stimulation of renin release in anaesthetized dogs

To examine whether an alpha-adrenergic agonist, methoxamine, influences renin release solely by its haemodynamic effect, experiments were performed in anaesthetized dogs with denervated kidneys. Methoxamine was infused intrarenally at rates which reduced renal blood flow (RBF) by 30-40%. At control blood pressure, renin release rose during infusion of methoxamine from 1.4 +/- 0.7 to 31 +/- 11 microgram/min. A beta-adrenergic stimulator, isoproterenol, did not increase renin release significantly when administered alone into the renal artery, but doubled the effect of methoxamine infusion: at control blood pressure renin release rose from 0.5 +/- 0.3 to 71 +/- 17 microgram/min during combined infusion of isoproterenol and methoxamine. Mechanical constriction of the renal artery left RBF unaltered down to a renal perfusion pressure of 90 +/- 4 mmHg during methoxamine infusion, whereas the lowest autoregulating pressure in control experiments averaged 60 +/- 5 mmHg. At renal infusion pressure below the range of autoregulation, renin release was not further increased by intrarenal infusion of methoxamine. Isoproterenol infusion at low renal perfusion pressure doubled renin release, which was not significantly altered by additional infusion of methoxamine. The stimulatory effect of methoxamine on renin release at control blood pressure could be diminished but not prevented by infusing 2.9% NaCl intravenously in large amounts. These data indicate that methoxamine induces autoregulated dilation of afferent arterioles by disproportionate vasoconstriction on pre-afferent arteries. Thereby afferent arterioles are conditioned for stimulation of renin release by isoproterenol.

Inhibition of renin secretion by intrarenal alpha-adrenoceptor blockade

This study was designed to determine whether renal alpha-adrenoceptors can mediate tonic neural stimulation of renin secretion. The effect of alpha-adrenoceptor blockade by phenoxybenzamine (POB) or prazosin on renin secretion rate (RSR) was studied in pentobarbital-anesthetized dogs in which renal perfusion pressure was held constant with an adjustable aortic clamp. POB alone (100 micrograms.kg-1.min-1 iv) did not change arterial plasma renin activity (PRA). However, when beta-adrenoceptors were blocked by intravenous propranolol, intravenous POB infusion (100 micrograms.kg-1.min-1) decreased PRA and RSR to 48 +/- 8 and 21 +/- 9% of previous levels within 30 min. This effect was abolished by acute bilateral renal denervation. Direct intrarenal POB infusion (10 or 3.3 micrograms.kg-1.min-1) decreased RSR, whereas intravenous POB (3.3 micrograms.kg-1.min-1) had no effect on either RSR or PRA in propranolol-pretreated dogs. Prazosin (1 microgram.kg-1.min-1 iv) also significantly decreased PRA. These data indicate that when beta-adrenoceptors are blocked by propranolol, tonic neural stimulation of renin secretion is mediated by renal alpha-adrenoceptors.

Inhibition of renin secretion by clonidine after alpha-adrenoceptor blockade in anesthetized dogs

Dogs anaesthetised with pentobarbital were used to study the effects of the alpha-adrenoceptor antagonists phentolamine and piperoxan on the clonidine-induced suppression of plasma renin activity (PRA). Given alone, clonidine (30 microgram/kg, i.v.) produced an initial rise in mean arterial pressure (MAP) which was followed by a hypotensive response. These changes in blood pressure were accompanied by decreased in PRA and heart rate (HR). Pretreatment with phentolamine, 1 mg/kg i.v. or 3.3 microgram/kg/min infused into the third cerebral ventricle, or piperoxan, 20 microgram/kg stat. + 5 microgram/kg/min infused either i.v. or intraventricularly, did not modify the clonidine-induced falls in PRA, MAP or HR. All pretreatment regimes, with the exception of intraventricular phentolamine, virtually abolished the initial pressor response to clonidine. These results demonstrate that the renin-lowering action of clonidine in the dog is not inhibited by two classical alpha-adrenoceptor antagonists.

Humoral and neurogenic factors in two-kidney renovascular hypertension

A "bolus" dose (110 microgram) of the angiotesin II (A II)-blocker 1-Sar-8-Ala-A II (Saralasin, S) followed by its slow rate infusion (5 microgram/min/rat) for thirty min, was injected before and after the complete ganglionic blockade by pentolinium (P) in unanaesthetized unilaterally clipped renal hypertensive rats (the opposite kidney remained untouched). Pentolinium was also injected like a "bolus" dose (3 mg) followed by slow infusion (0.1 mg/min/rat) for thirty min. The observations were made until the fifth week after clipping the left renal artery. A consistent maximal hypotensive response was observed after the "bolus test" with both drugs. When S was the first drug injected, an inverse correlation was found between the percent decrease in arterial pressure (BP) by S and the percent decrease in BP by P (r = --0.83, P < 0.01, n = 8). Thus whenever a greater hypotensive effect was obtained by S, a smaller neural pressor component remained to be blocked by P. On the other hand, when P was the first drug injected a lesser A II pressor component remained to be blocked by S in the hypertensive rats. The results suggest that a considerable A II pressor effect in two-kidney renovascular hypertension is mediated via neurogenic mechanisms from the first week. A direct pressor vasoconstriction was found to be significant in cases with very high plasma-renin activity.

Influence of raising albumin concentration on renin release in isolated perfused rat kidneys

1. Experiments were conducted in isolated perfused rat kidneys to determine the effect of raising perfusate albumin concentration on renin release.2. Raising albumin concentration in the perfusion fluid from 20 g/l. to 60 g/l. (high albumin concentration) increased renin release and renal perfusate flow rate. The effect was reversible.3. Ureteral occlusion did not prevent the rise in renin release and renal perfusate flow induced by high albumin concentration.4. Propranolol (0.28 mM) did not block the renin release stimulated by high albumin concentration, but it inhibited the release stimulated by isoprenaline (2.43 muM).5. Clonidine (10 muM) and oxymetazoline (10 muM) constricted the renal vasculature and stimulated renin release during high perfusate albumin concentration providing perfusion pressure was kept constant.6. Low renal perfusion pressure (50 mmHg) and isoprenaline (2.43 muM) stimulated renin release in perfusion experiments with both 20 and 60 g/l., but the rate of renin release was substantially greater with 60 g/l.7. On the other hand, perfusion fluid deprived of calcium induced a greater increase in renin release in kidneys perfused with 20 g/l. than in those with 60 g/l.8. We conclude that high albumin concentration stimulates renin release in isolated perfused rat kidneys by a mechanism which does not involve the renal nerve, direct renal vasodilation or sodium excretion. High albumin concentration may increase the sensitivity of the kidney to acute stimulation by a mechanism involving calcium.

Inhibition by alpha-adrenoceptor agonists of renin release in vitro

A variety of alpha-adrenergic agonists encompassing a broad range of concentrations were used to investigate the existence and nature of a putative alpha-adrenergic mechanism inhibitory to renin release, which may operate at the level of the juxtaglomerular apparatus. For this purpose rat renal cortical tissue incubated in vitro was used. Concentrations of noradrenaline, adrenaline and methoxamine of 10(-6), 10(-5), 10(-4) and 10(-3) M caused significant dose-related inhibition of renin release. The inhibition of release by these doses was reversed completely by 10(-4) M phentolamine. In contrast, phenylephrine, oxymetazoline and clonidine did not inhibit renin release. The results support the concept of an alpha-adrenergic mechanism inhibitory to renin release and show that high concentrations of alpha-adrenergic agonist are required for its operation in vitro. The manner in which this inhibitory mechanism affects renin release under physiological circumstances remains to be demonstrated.