Clonidine-induced increase of renal prostaglandin activity and water diuresis in conscious dogs

Is the Sympathetic System Detrimental in the Setting of Septic Shock, with Antihypertensive Agents as a Counterintuitive Approach? A Clinical Proposition

Mortality in the setting of septic shock varies between 20% and 100%. Refractory septic shock leads to early circulatory failure and carries the worst prognosis. The pathophysiology is poorly understood despite studies of the microcirculatory defects and the immuno-paralysis. The acute circulatory distress is treated with volume expansion, administration of vasopressors (usually noradrenaline: NA), and inotropes. Ventilation and anti-infectious strategy shall not be discussed here. When circulation is considered, the literature is segregated between interventions directed to the systemic circulation vs. interventions directed to the micro-circulation. Our thesis is that, after stabilization of the acute cardioventilatory distress, the prolonged sympathetic hyperactivity is detrimental in the setting of septic shock. Our hypothesis is that the sympathetic hyperactivity observed in septic shock being normalized towards baseline activity will improve the microcirculation by recoupling the capillaries and the systemic circulation. Therefore, counterintuitively, antihypertensive agents such as beta-blockers or alpha-2 adrenergic agonists (clonidine, dexmedetomidine) are useful. They would reduce the noradrenaline requirements. Adjuncts (vitamins, steroids, NO donors/inhibitors, etc.) proposed to normalize the sepsis-evoked vasodilation are not reviewed. This itemized approach (systemic vs. microcirculation) requires physiological and epidemiological studies to look for reduced mortality.

Current status of putative imidazoline (I1) receptors and renal mechanisms in relation to their antihypertensive therapeutic potential

1. A 'second generation' of centrally acting antihypertensive agents has recently been developed. Unlike the 'first generation' of these agents (e.g. alpha-methyldopa, clonidine, guanabenz), which act predominantly by an agonist action at a alpha 2-adrenoceptors, these agents (e.g. rilmenidine, moxonidine) are believed to exert their antihypertensive effects chiefly by an interaction at putative imidazoline (I) receptors of the I1-type, and so have a reduced profile of alpha 2-adrenoceptor-mediated side effects. There is also evidence from studies in experimental animals that activation of I1-receptors mediates a natriuretic effect. This review evaluates the evidence that they mediate renal effects different from those of alpha 2-adrenoceptors that could contribute to their long-term efficacy. 2. Data from binding studies suggest that I1-binding sites are heterogeneous. There is conflicting evidence concerning whether any of these binding sites are truly receptors. Indeed, the best evidence for the existence of I1-receptors comes from in vivo experiments indicating that imidazoline compounds act at non-adrenoceptor receptive sites in the central nervous system to reduce sympathetic drive and blood pressure. 3. There are a wide range of potential sites and mechanisms through which centrally acting antihypertensive agents can affect renal function, including actions mediated within the central nervous system, heart, systemic circulation and within the kidneys themselves. 'First generation' centrally acting antihypertensive agents cause diuresis and natriuresis in rats, while in dogs and humans a diuresis is often seen with variable effects on sodium excretion. 4. Evidence from studies in anaesthetized rats indicates that rilmenidine and moxonidine can promote sodium excretion by interacting with both central nervous system and renal putative I1-receptors. This does not appear to necessarily be the case in other species. At this time there are few or no published data from clinical studies to suggest that 'second generation' centrally acting antihypertensive agents affect salt and water balance differently from 'first generation' agents.

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.

Renal effects of infusion of rilmenidine and guanabenz in conscious dogs: contribution of peripheral and central nervous system alpha 2-adrenoceptors

1. We tested the renal effects of the alpha 2-adrenoceptor agonists, rilmenidine and guanabenz and the antagonists, 2-methoxyidazoxan and idazoxan, in conscious dogs. Our aim was to test the hypothesis that putative imidazoline (I) receptors influence renal function. We reasoned that since rilmenidine and guanabenz are selective for I1- and I2-binding sites respectively, an influence of one of these receptive sites on renal function would be reflected in qualitative differences between the effects of these agents. Moreover, effects mediated by putative I-receptors should be relatively resistant to antagonism by the selective alpha 2-adrenoceptor antagonist, 2-methoxyidazoxan. Since the effects of these drugs on renal function could be mediated in the central nervous system or periphery, the dogs were studied under both normal and ganglion-blocked conditions. 2. In dogs with intact autonomic reflexes, 2-methoxyidazoxan (15 micrograms kg-1 plus 0.6 micrograms kg-1 min-1) produced effects consistent with a generalized increase in sympathetic drive, including increases in mean arterial pressure and plasma renin activity, and a reduction in sodium excretion. In ganglion-blocked dogs, 2-methoxyidazoxan reduced sodium excretion but had no discernible effect on systemic or renal haemodynamics. We conclude that an alpha 2-adrenoceptor-mediated mechanism in the central nervous system tonically inhibits sympathetic drive in the conscious dog. 3. In ganglion-blocked dogs idazoxan (3-300 micrograms kg-1) dose-dependently increased arterial pressure. This was not abolished by concomitant administration of 2-methoxyidazoxan (0.3-30 micrograms kg-1). The pressor effect of idazoxan is therefore probably mediated by an agonist action at alpha 1-adrenoceptors. 4. The effects of infusions of rilmenidine (0.1-1.0 mg kg-1) and guanabenz (10-100 micrograms kg-1) were indistinguishable. They comprised dose-dependent increases in mean arterial pressure, urine excretion, and glomerular filtration rate (the latter in ganglion blocked dogs only), and dose-dependent reductions in heart rate, renal blood flow and sodium excretion (only in dogs with intact autonomic reflexes). All of these effects were antagonized by 2-methoxyidazoxan. 5. We conclude that the renal effects of rilmenidine and guanabenz infusions in conscious dogs are predominantly, if not completely, attributable to activation of alpha 2-adrenoceptors. Our results do not support the hypothesis that putative I-receptors contribute towards the renal effects of these agents.

Effects of alpha-2-adrenoceptor agonists on induced diuresis in rats

The alpha-2 adrenoceptor agonists, clonidine, guanabenz, and guanfacine, injected subcutaneously produced a dose-related diuresis. The maximal effect occurred at 2h after administration of clonidine 192 micrograms/kg or 960 micrograms/kg of guanabenz and guanfacine. The alpha-2 antagonist, yohimbine, in doses of 1-8 mg/kg administered prior to the agonists caused a dose-dependent decrease in urine output. The action of the three agonists at alpha-2 adrenoceptors was supported by the observation that the alpha-1 adrenoceptor agonist, prazosin (0.61-2.5 mg/kg), administered prior to each agonist caused an inconsistent decrease in the elevated urinary output caused by clonidine, guanabenz and guanfacine. These results indicate that stimulation of alpha-2 adrenoceptors causes diuresis in the rat.

The effect of medetomidine, an alpha 2-adrenoceptor agonist, on plasma atrial natriuretic peptide levels, haemodynamics and renal excretory function in spontaneously hypertensive and Wistar-Kyoto rats

1. The effects of the selective alpha 2-adrenoceptor agonist, medetomidine, were assessed on plasma levels of immunoreactive atrial natriuretic peptide (IR-ANP), haemodynamics and on urine water and solute excretion in conscious, chronically cannulated, 7 month-old spontaneously hypertensive (SHR) and age-matched Wistar-Kyoto (WKY) rats, in order to examine the role of alpha 2-adrenoceptors in the control of ANP secretion. 2. A 60 min i.v. infusion of medetomidine (0.2 or 0.6 microgram kg-1 min-1) decreased heart rate dose-dependently in both strains. Medetomidine infusion (0.6 microgram kg-1 min-1) resulted in an increase in mean arterial pressure in WKY, whereas both doses decreased blood pressure in SHR. There was a slight increase in the right atrial pressure in both strains (WKY: +1.18 +/- 0.26 mmHg; SHR: +1.64 +/- 0.64 mmHg, NS) in response to infusion of 0.6 microgram kg-1 min-1 of medetomidine. 3. No differences were found in resting plasma IR-ANP levels between WKY (114 +/- 8 pg ml-1, n = 19) and SHR (117 +/- 10 pg ml-1, n = 21). Infusion of equibradycardic doses of medetomidine increased dose-dependently plasma IR-ANP levels in WKY, but did not affect the plasma IR-ANP concentration in SHR rats. 4. Despite the different effect of medetomidine on ANP release in WKY and SHR rats, i.v. administration of medetomidine affected renal excretory functions similarly in both strains; urine flow and sodium excretion increased and urine osmolality decreased significantly, while there was no consistent change in urinary potassium excretion. Urine osmolality decreased to hypo-osmotic levels during the infusion of 0.6 yg kg-1 min1 of medetomidine, suggesting a possible interaction between alpha 2-adrenoceptor stimulation and the vasopressin system. 5. These results show that the alpha 2-adrenoceptor agonist medetomidine increased plasma levels of ANP in WKY rats, probably through an increase in mean arterial and right atrial pressures, whereas the SHR had attenuated ANP release to alpha 2-adrenoceptor stimulation. Our findings, that medetomidine caused marked natriuretic and diuretic effects in both strains and that these effects on the excretory functions of the kidneys were not related to changes in plasma levels of IR-ANP, demonstrate that changes in plasma ANP levels alone do not account for the diuretic and natriuretic effect of alpha 2-agonists.

Alpha-2 adrenergic agonists inhibit basal and stimulated osmotic water permeability in toad skin

The effect of selective alpha adrenergic agonists and antagonists on osmotic water permeability (Posm) across isolated skins of Bufo arenarum toads was investigated. Clonidine, an alpha-2 agonist, inhibited basal Posm and oxytocin, isoproterenol and theophylline stimulated Posm, but did not alter the hydrosmotic effect of exogenous cAMP. Blockade of the effect of clonidine on basal and stimulated Posm by the selective alpha-2 antagonist yohimbine supports the hypothesis that the inhibitory effect is mediated by the stimulation of alpha-2 adrenergic receptors.

Bumetanide. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic use

Bumetanide is a potent 'loop' diuretic for the treatment of oedema associated with congestive heart failure, hepatic and renal diseases, acute pulmonary congestion and premenstrual syndrome and in forced diuresis during and after surgery. Bumetanide may be given orally, intravenously or intramuscularly and produces a rapid and marked diuresis, and increased urinary excretion of sodium, chloride and other electrolytes (within 30 minutes) which persists for 3 to 6 hours. Its principal site of action is on the ascending limb of the loop of Henle, with a secondary action on the proximal tubule. Pharmacologically, bumetanide is about 40-fold more potent than frusemide (furosemide), with the exception of its effects on urinary potassium excretion where its potency is lower. Studies in patients with oedema due to congestive heart failure, pulmonary oedema or hepatic disease show that oral or intravenous bumetanide 0.5 to 2 mg/day produces results comparable to those with frusemide 20 to 80 mg/day. In acute pulmonary oedema, intravenous bumetanide produces a very rapid diuresis. Higher doses of bumetanide may be required (up to 15 mg/day) in patients with chronic renal failure or nephrotic syndrome. In these patients muscle cramps are not uncommon with bumetanide, but glomerular filtration rates are unaffected. In most studies, diuretic effects were accompanied by decreased bodyweight, abdominal girth and improvements in a variety of haemodynamic parameters. Comparison of bumetanide with frusemide at a dose ratio of 1 : 40 reveals no significant differences in clinical response with the exception of renal disease, where patients with oedema appear to respond better to bumetanide. Combination with thiazide diuretics enhances the clinical response to bumetanide. Potassium supplements and spironolactone may be beneficial additions to bumetanide where patients at risk of hypokalaemia can be identified. Clinically important side effects are infrequent, with audiological impairment occurring to a lesser extent than with frusemide. Bumetanide thus offers an important alternative to frusemide when a 'loop' diuretic is indicated.

A facile reproducible radioimmunoassay of the mixed metabolites of prostaglandins E, suitable for measurement of relative differences of phospholipase/prostaglandin synthetase activity in vivo

A relatively simple, reproducible, radioimmunoassay for the mixed metabolites of prostaglandins E (U-PGE-M) in rat and human urine is described. Results of the assay of treated versus control urine extracts correlate well with differences expected from treatments known to alter in vivo phospholipase/prostaglandin synthetase activity. Cross-reactivity of heterogeneous metabolite antiserum with 5 available endogenous prostaglandins and a single metabolite was determined and showed little or no cross reaction. Sensitivity, within-assay precision, interassay reproducibility, and parallelism were also determined and found acceptable. Excretion rates of U-PGE-M by rats and humans were determined, and statistically significant differences could be shown, although absolute values were smaller than estimated absolute values obtained from mass-spectrometric measurements of single, purified metabolites. Normal human male excretion rates differed significantly from those of females. Injection of prostaglandin E1 caused a significant rise in U-PGE-M excretion in rats whereas aspirin and indomethacin caused it to fall. U-PGE-M excretion rates of spontaneous hypertensive rats were significantly less than rates of normotensive controls. Adrenalectomy resulted in excretion of significantly larger amounts of U-PGE-M than in normal or sham-operated controls. A screen of clinically active pharmacological agents and hormones gave results consistent with previously published reports.

Functional characterization of the alpha adrenergic receptor modulating the hydroosmotic effect of vasopressin on the rabbit cortical collecting tubule

To characterize the type of alpha adrenergic receptor, the effects of specific alpha adrenergic agonists and antagonists on antidiuretic hormone [( Arg8]-vasopressin [AVP])-induced water absorption were evaluated in cortical collecting tubules isolated from the rabbit kidney and perfused in vitro. In the presence of AVP (100 microU/ml), net fluid volume absorption (Jv, nanoliters per minute per millimeter) was 1.39 +/- 0.09 and osmotic water permeability coefficient (Pf, X 10(-4) centimeters per second) was 150.2 +/- 15.0. The addition of 10(-6) M phenylephrine (PE), an alpha adrenergic agonist, resulted in a significant decrease in Jv and Pf to 0.72 +/- 0.11 (P less than 0.005) and 69.9 +/- 10.9 (P less than 0.005). The addition of 10(-4) M prazosin (PZ), an alpha adrenergic antagonist, did not cause any significant change in Jv and Pf, which were 0.71 +/- 0.09 (P = NS vs. AVP + PE) and 67.8 +/- 9.5 (P = NS vs. AVP + PE), respectively. In a separate group of tubules, in the presence of AVP (100 microU/ml) and PE (10(-6) M), Jv and Pf were 0.78 +/- 0.17 and 76.1 +/- 18.0, respectively. The addition of 10(-6) M yohimbine (Y), an alpha 2 adrenergic antagonist, resulted in a significant increase in Jv to 1.46 +/- 0.14 (P less than 0.01) and Pf to 157.5 +/- 22.3 (P less than 0.005). Y (10(-4) M) or PZ (10(-4) M) alone did not significantly affect Jv and Pf in the presence of AVP )100 microU/ml). The effect of the natural endogenous catecholamine norepinephrine (NE) on Jv and Pf in the presence of AVP and propranolol (PR) was next examined. Jv and Pf were 1.53 +/- 0.07 and 176.3 +/- 5.2, respectively, in the presence of AVP (100 microU/ml) and PR (10(-4) M). The addition of NE (10(-8) M) resulted in a significant decrease in Jv to 1.19 +/- 0.11 (P less than 0.05) and Pf to 127.0 +/- 11.3 (P less than 0.02). Increasing the concentration of NE to 10(-6) M resulted in a further decrease in Jv and Pf to 0.70 +/- 0.10 (P less than 0.01 vs. NE 10(-8) M) and 68.5 +/- 10.6 (P less than 0.01 vs. NE 10(-8) M), respectively. The inhibitory effect of NE on AVP-induced water absorption was blocked by Y, but not by PZ. The effect of the alpha 2 adrenergic agonist clonidine (CD) on Jv and Pf was also examined. In the presence of AVP (10 microU/ml) Jv and Pf were 1.65 +/- 0.04 and 175.1 +/- 13.1, respectively. The addition of CD (10(-6) M) resulted in a significant decrease in Jv to 1.08 +/- 0.12 (P < 0.01) and Pf to 108.1 +/- 15.4 (P < 0.01). Increasing the concentration of CD to 10(-4) M resulted in a further significant decrease in Jv and Pf to 0.57 +/- 0.13 (P < 0.02 vs. CD 10(-6) M) and 54.7 +/- 13.8 (P < 0.01 vs. CD 10(-6) M), respectively. Similar results were obtained in the presence of AVP (100 microU/ml). The inhibitory effect of CD on AVP-induced water absorption was blocked by Y. CD did not significantly affect Jv and Pf in the presence of 8-bromo adenosine 3',5'-cyclic monophosphate. These studies indicate that alpha adrenergic agonists directly inhibit AVP-mediated water absorption at the level of renal tubule, an effect that can be blocked by specific alpha2 adrenergic antagonists, but not by specific alpha1 adrenergic antagonists. Alpha2 adrenergic stimulation directly inhibits AVP-mediate water absorption at the level of the tubule, an effect that can be blocked by a specific alpha2 adrenergic antagonist. This effect appears to be exerted at the level of activation of adenylate cyclase, since it is absent in the present of cyclic AMP.

Interaction of prostaglandins and clonidine in the rat vas deferens

Stimulation of presynaptic alpha 2-adrenoceptors by clonidine may lead to local synthesis of prostaglandins which contribute to the inhibition of noradrenaline release observed with this drug. The present investigation was undertaken to determine the role of prostaglandins in the effect of clonidine and xylazine on the rat vas deferens. Both drugs inhibited the twitch response to field stimulation in this preparation. Inhibition was reversed by yohimbine. This effect of clonidine (but not xylazine) was reduced by preincubating vasa deferentia in Krebs containing indomethacin for 1h. Clonidine (but not xylazine) stimulated the synthesis of prostaglandin-like activity in pieces of intact vas deferens incubated in Krebs containing arachidonic acid. Such stimulation was prevented by inclusion of yohimbine (but not prazosin) in the incubation medium. Clonidine did not stimulate prostaglandin synthesis in a cell-free preparation of sheep seminal vesicle microsomes incubated with arachidonic acid or inhibit PGE2 catabolism by purified swine lung 15-PGDH. We conclude that clonidine (but not xylazine) stimulates prostaglandin synthesis possibly by activating phospholipase activity and releasing arachidonic acid from membrane phospholipids. This effect on prostaglandin production is secondary to activation of alpha 2-adrenoceptors.

Effect of medullary tonicity on urinary sodium excretion in the rat

In previous reports from this laboratory we have suggested that a reduction in medullary tonicity decreases the thin ascending loop of Henle sodium reabsorption and is in part responsible for the magnitude for the natriuresis accompanying 10% body weight Ringer loading. According to this postulate, one would expect that the medullary washout associated with water diuresis would also result in a natriuresis, but this does not occur. It is possible, however, that increased delivery from the proximal tubule is necessary to demonstrate an effect of medullary tonicity on urinary sodium excretion. Micropuncture studies were designed to test that possibility by increasing distal delivery by 2% Ringer loading in animals with and without reduced medullary tonicity. In an initial series of experiments the alpha-adrenergic agonist clonidine was used to induce a water diuresis. When given alone, this agent caused a marked decrease in urine osmolality and an increase in urine flow rate but had no effect on proximal reabsorption in either superficial or juxtamedullary nephrons, and did not alter urinary sodium excretion. Volume expansion with 2% body weight Ringer solution resulted in a significant fall in proximal reabsorption and a trivial increment in sodium excretion. When this same degree of volume expansion was conferred on animals undergoing a water diuresis, a marked increase in absolute and fractional sodium excretion occurred. In a second group of studies medullary tonicity was reduced in the left kidney only by removal of the left ureter 1 h before micropuncture. When these animals were infused with 2% body weight Ringer solution, proximal reabsorption was decreased in juxtamedullary nephrons, and a marked increase in sodium excretion was observed only from the left kidney. Finally, the effect of water diuresis on fractional sodium delivery to the early and late distal tubule of superficial nephrons during 2% Ringer loading was evaluated. Delivery to both of these sites was comparable after 2% Ringer loading alone and during 2% Ringer loading plus water diuresis. From these data, we conclude that medullary tonicity does influence renal sodium handling but that this effect is manifest in the final urine only under conditions in which proximal reabsorption is decreased. The data also suggest that this effect is limited to juxtamedullary nephrons and is probably localized to the thin ascending limb of the loop of Henle.

Review of therapeutic modalities acting directly via central pathways

The brain influences arterial pressure through central mediation of a variety of neurotransmitters, including norepinephrine, and translates this action into changes in peripheral autonomic tone. Two opposed adrenergic systems have been described in brain: a hypothalamic pathway in which adrenergic receptor stimulation raises arterial pressure and a brainstem pathway related to the baroreflex arc in which adrenergic receptor stimulation lowers arterial pressure. Antihypertensive drugs with primarily central effects, including clonidine and alpha methyldopa, act as alpha 2 adrenoceptor agonists. The central receptor involved in their antihypertensive action is of the alpha 2 type but is located postsynaptically. Activation of this receptor by either clonidine or alpha methylnorepinephrine, a metabolite of alpha methyldopa, engages the depressor pathway in the brainstem and leads to a decrease in norepinephrine release and a reduction in peripheral sympathetic tone. Clonidine and alpha methyldopa share a similar pattern of peripheral effects, including reductions in preganglionic sympathetic nerve traffic, bradycardia, decreases in plasma renin activity, reductions in blood pressure in the supine position and adverse effects such as depression, sedation and bad dreams. Because of the frequency and severity of these side effects, there is an ongoing search for new centrally acting antihypertensive agents which might be better tolerated.

Clonidine after renal ischemia to lessen acute renal failure and microvascular damage

Clonidine, an antihypertensive drug that inhibits renin release and causes a water diuresis in normal animals, was tested for its ability to reduce the severity of post-ischemic acute renal failure produced in rabbits by clamping the left renal pedicle for 1 hour and removing the opposite kidney. Clonidine significantly lessened renal failure when given during, or 1 hours after, the ischemic insult in dehydrated rabbits. It was also effective when given during the ischemic insult in vasopressin-treated water-drinking rabbits but not in control water-drinking rabbits. In vasopressin-treated rabbits, clonidine lessened renal failure observed 2 days after the ischemic insult despite the fact that in the immediate postischemic period it lowered total renal blood flow, produced hypotension, and did not bring about lower plasma renin levels. Clonidine treatment resulted in less outer medullary microvascular damage (demonstrated by colloidal carbon staining), higher outer medullary blood flow 1 to 2 hours after unclamping, fewer casts, and higher creatinine clearance and free water clearance/creatinine clearance 4 to 6 hours after unclamping compared with controls. The effect of clonidine was unrelated to plasma renin activity. Clonidine did not alter plasma vasopressin concentration. Demeclocycline and lithium, two agents that blunt renal responsiveness to vasopressin, had a beneficial effect in dehydrated animals similar to that of clonidine, but the angiotensin II antagonist saralasin and the angiotensin converting enzyme inhibitor SQ20881 did not. Normal rabbits given a large dose of vasopressin in oil plus clonidine had significantly greater urine output and free water clearance and lower urine osmolality than did rabbits given vasopressin in oil alone. These results suggest that clonidine may be beneficial because it prevents ischemic microvascular injury in the renal outer medulla, an effect that may decrease tubular obstruction by lessening desquamation of damaged tubular cells or cell constituents into the tubular lumen. Clonidine may also decrease formation of obstructive hyaline casts in collecting ducts by blunting the kidney's response to vasopressin and increasing tubular fluid flow rate.

Role of sympathetic nerve inhibition and body sodium-volume state in the antihypertensive action of clonidine in essential hypertension

A simultaneous analysis of the interrelationships between mean blood pressure (MBP), plasma catecholamines, plasma renin activity (PRA) and aldosterone, exchangeable body sodium, and blood volume was carried out before and after acute and chronic administration of clonidine in 15 patients with essential hypertension, in order to further elucidate the mechanism(s) of action of this drug. After a single oral dose of 200 microgram, clonidine produced a significant fall in MBP, heart rate, plasma norepinephrine (NE), PRA, and aldosterone. There were significant correlations (P < 0.01) between NE and MBP, but not between PRA or aldosterone and MBP both before and after the ingestion of clonidine. Furthermore, there was a significant correlation (P < 0.05) between the magnitude of fall of plasma NE and change in MBP. After 6 weeks of treatment, conidine at a dose of 960 +/- 80 microgram/day produced a significant decrement in MBP and NE, but not in PRA and aldosterone; with upright posture, the magnitude of the fall in MBP was significantly greater (P < 0.01), and the rise in plasma NE was significantly smaller (P < 0.01) after chronic clonidine treatment. There was also a significant fall (P < 0.01) in exchangeable sodium and plasma volume. The data provide evidence that inhibition of the sympathetic nervous system by clonidine plays a major role and that the decrease in exchangeable body sodium and blood volume after chronic treatment may contribute to the antihypertensive action of the drug.

Influence of analgesic antipyretics on the central cardiovascular effects of clonidine in rats

The centrally acting antihypertensive drug clonidine has been found to stimulate the synthesis of PGF2 alpha in the brain. Centrally administered PGF2 alpha, in turn, induces rises of blood pressure and heart rate. We therefore studied the influence of inhibitors of prostaglandin (PG) synthesis on the cardiovascular effects of clonidine in urethane-anaesthetised rats. Pretreatment with indomethacin or paracetamol (100 micrograms/rat into the fourth cerebral ventricle) antagonised the central hypotensive effect of clonidine (0.125-16.0 micrograms/rat into the fourth cerebral ventricle). The bradycardic effect of centrally administered clonidine was, however, enhanced by pretreatment with paracetamol but not influenced by indomethacin pretreatment. Sodium meclofenamate (100 micrograms/rat into the fourth cerebral ventricle) did not significantly affect the clonidine-induced changes in blood pressure and heart rate. These results suggest that the clonidine-induced hypotension on one hand and bradycardia on the other hand may be mediated by partly different mechanisms. An interference of the formation of PGF2 alpha with the cardiovascular effects of clonidine cannot be completely excluded since paracetamol pretreatment potentiated the bradycardic effect of clonidine. However, inhibitors of PG synthesis did not enhance but antagonised the hypotensive effect of clonidine. Therefore it is likely that the synthesis of PGF2 alpha does not interfere with the hypotensive effect of clonidine. Moreover, the antagonism of the hypotensive effect by inhibitors of PG synthesis suggests that some hypotensive metabolite of arachidonic acid in the brain could be involved in the central hypotensive effect of clonidine.

The renal effects of clonidine in unanesthetized rats

Clonidine s.c. (0.01-0.3 mg/kg), in unanesthetized rats, caused an initial rise (+20 mm Hg), followed by a continuous fall of BP and a dose-dependent natriuresis and diuresis for up to 2 h. Glomerular filtration rate (GFR) (CIn) increased during the first 20 min, while effective renal plasma flow (ERPF) (CPAH) remained normal. Subsequently, between 20 and 60 min after injection, ERPF (CPAH) decreased considerably while GFR had reverted to its normal value. In saline-infused rats clonidine diuresis was accompanied by an "inappropriate" positive free water clearance. Pentobarbital anesthesia suppressed the initial BP peak and the diuresis. Phenoxybenzamine (1 mg/kg i.v.) was antinatriuretic in saline diuresis; the effect of phenoxybenzamine + clonidine on diuresis and salt excretion represented the sum of the effects of both drugs, but phenoxybenzamine enhanced the clonidine-induced increase of GFR. Neither haloperidol (1 mg/kg i.v.) nor bulbocapnine (3 mg/kg i.v.) interfered with the renal effects of clonidine. Clonidine s.c. caused hyperglycemia and glucosuria which did not account for the natriuresis. Clonidine thus appears to increase the GFR and "filtration fraction" (FF) by a phenoxybenzamine-insensitive rise of glomerular ultrafiltration, to depress ERPF by alpha-adrenergic afferent vasoconstriction, to induce natriuresis by a tubular action not blocked by phenoxybenzamine and to exert an antivasopressin effect, either by depressing pituitary vasopressin secretion or the renal response to vasopressin.

Prostaglandins and the side effects of anti-inflammatory drugs--the kidney

The most important renal side effect of non-steroidal anti-inflammatory therapy in man is analgesic nephropathy. One possible mechanism for this effect is inhibition of renal prostaglandin synthesis. However, an understanding of the regional biosynthesis of renal prostaglandins, of their pharmacological, physiological and pathological properties in the kidney and an understanding of the consequences of their inhibition by drugs is required in order to assess whether such a mechanism is involved. These aspects are reviewed, using much of the early work of the author as a basis for the discussion. The following conclusions can be drawn form a review of published work. "The consequences of the inhbiition of renal prostaglandin synthesis do not seem to bear much relationship to the renal side effects of anti-inflammatory therapy in man". It is further suggested that impaired renomedullary blood flow arising from decreased renomedullary PGE2 synthesis results in increased accumulation of drug in the renal medulla leading to direct toxic damage. Finally, examples of diseases associated with increased or decreased renal PGE2 synthesis are discussed and some examples of drug interactions are presented.

Enhancement of urine prostaglandin excretion by chlorazanil in dogs

In conscious dogs chlorazanil (2.5 mg/kg intravenously) markedly enhanced (5--10 fold) urinary excretion of prostaglandin E2 and F2alpha. The effect came to a peak at 15--30 min. following the administration. Chlorazanil did not modify renal blood flow or inulin clearance in non-pretreated or indomethacin pretreated dogs and the plasma renin activity remained unchanged. A marginal natriuretic and antikaliuretic activity by chlorazanil was similarly observed in non-pretreated dogs and dogs pretreated with indomethacin. The possible effects of the prostaglandins released by chlorazanil remained obscure. In vitro chlorazanil (10(-4) M) exhibited a moderate inhibition of 15-OH-prostaglandin dehydrogenase but an enhanced excretion of 15-keto-13, 14-dihydro PGE2alpha in vivo suggested that chlorazanil increased renal prostaglandin activity by increased prostaglandin synthesis, probably due to increased precursor availability. This was presumably mediated by some as yet unknown factor since chlorazanil ( 4 x 10(-5)M) failed to affect the release of precursor and prostaglandins from isolated Krebs-Henseleit perfused rabbit kidneys.

Prostaglandins, a link between renal hydro- and hemodynamic in dogs

An increase of the intrarenal pressure to 40 mmHg induced by ureteral constriction or by kidney compression is shown to be followed by increased renal blood flow in anesthetized dogs. This hyperemia is probably the result of enhanced intrarenal prostaglandin activity since it is followed by increased urinary prostaglandin E excretion and is abolished by indomethacin pretreatment. The increase of renal blood flow seems to be due to dilation of the afferent arteriole in order to maintain the filtration pressure. The glomerular filtration rate is thus severely depressed in indomethacin pretreated dogs. Urine and electrolyte excretion is comparably redcued during elevated intrarenal pressure in non-pretreated and in indomethacin pretreated dogs, which suggests that factors other than glomerular filtration rate are involved. Urine osmolarity is positively correlated with renal blood flow, and urine osmolarity increases during elevated intrarenal pressure in non pretreated dogs, whilst urine osmolarity remains unchanged in dogs pretreated with indomethacin.

Renal cortical blood redistribution after bumetanide related to heterogenicity of cortical prostaglandin metabolism in dogs

Bumetanide is shown to increase renal blood flow and to augment the proportion of the cortical blood flow to middle cortex. This redistribution still takes place even when renal blood flow is maintained constant by renal artery clamping. Indomethacin pretreatment inhibits the increase of renal blood flow as well as the cortical blood redistribution. In vitro examinations of canine kidney tissue slices suggest that outer cortex and papillar are sites of prostaglandin synthesis. No differences in prostaglandin E degradation are observed within the cortex. This suggests a relative autonomy for prostaglandins in the outer cortex, whilst inner cortical areas are dependent on medullary/papillary prostaglandin E supply. The renal hemodynamic effect of bumetanide is therefore thought to be a result of a stimulation of mainly medullary/papillary prostaglandin synthesis.