Guanabenz: a centrally acting, natriuretic antihypertensive drug

The novel guanidine ME10092 protects the heart during ischemia-reperfusion

The novel guanidine N-(3,4-dimethoxy-2-chlorobenzylideneamino)-guanidine [ME10092; a metabolite to the strongly cardioprotective hydroxyguanidine N-(3,4-dimethoxy-2-chlorobenzylideneamino)-N'-hydroxyguanidine (PR5)] was administered intravenously to rats subjected to left coronary artery clamping followed by reperfusion. Administration of 1-10 mg/kg of ME10092 1 or 5 min before 10 min of coronary artery occlusion followed by 20 min reperfusion significantly and dose-dependently inhibited the reperfusion-induced burst of arrhythmia, and markedly improved the survival of the animals. This dose schedule also dose-dependently and significantly inhibited the ST-segment elevation seen on the ECG during the artery occlusion, and attenuated the secondary rise in ST-segment during the reperfusion. Even when ME10092 was administered 5 min after the start of the reperfusion, the ST-segment elevation became significantly attenuated. Administration of ME10092 (3 plus 1.5 mg/kg) to animals subjected to 1 h left coronary occlusion followed by 2 h reperfusion reduced the heart infarction size by about 40%. ME10092 also dose-dependently reduced the heart rate, both during normal conditions and during ischemia and reperfusion. Moreover, the highest dose of ME10092 used (10 mg/kg) strongly attenuated the reduction in blood pressure seen during 10 min left coronary occlusion, as well as it attenuated the rebound rise in blood pressure seen during the 20 min reperfusion phase; that is, resulting in a normalisation of the blood pressure disturbances caused by the ischemia-reperfusion. We also showed that after its p.o. administration, the PR5 hydroxyguanidine became completely metabolised to its guanidine ME10092, with no detectable traces of PR5 being present 30 and 60 min after the administration. Moreover, after the p.o. administration of ME10092, no signs of the formation of PR5 were seen on analysis of the rats' plasma. In view of the practically indistinguishable pharmacological effects of ME10092 and PR5, we suggest the strong cardioprotective effects of these compounds to be mediated by a direct effect by ME10092 per se.

Chronic activation of central alpha2-adrenoceptors prevents hypertension in DOCA-salt rats

The effects of chronic intracerebroventricular (i.c.v.) injections of the alpha2-adrenoceptor agonist, xylazine, on blood pressure were examined in DOCA-salt rats. Acute studies also examined the renal sympathetic nerve activity (RSNA) and renal excretory responses produced by i.c.v. xylazine in rats with established DOCA-salt hypertension. Rats implanted with a chronic i.c.v. cannula for drug injection were used. In chronic studies, four groups were investigated: control rats treated with s.c. soybean oil and i.c.v. saline; DOCA-salt rats (s.c. deoxycorticosterone acetate) receiving i.c.v. saline, xylazine or the alpha2-adrenoceptor antagonist, yohimbine. During vehicle or DOCA-salt treatment, xylazine (0.2 ng/microg) or yohimbine (10O microg/kg) was injected i.c.v. daily (three times). In DOCA-salt rats receiving i.c.v. saline, resting mean arterial pressure (MAP) was elevated on days 15 and 30 (135 +/- 5 and 160 +/- 6 mmHg, respectively). Chronic i.c.v. xylazine significantly attenuated the rise in MAP produced by DOCA-salt (day 15, 118 +/- 5 mmHg; day 30, 121 +/- 4 mmHg). Alternatively, chronic i.c.v. yohimbine shortened the onset (day 15, 152 +/- 7 mmHg) and augmented the hypertension in DOCA-salt rats (0 survival by day 30). In acute studies, i.c.v. xylazine elicited a profound natriuresis and diuresis as well as a reduction in RSNA without altering MAP. This study demonstrates that the ongoing (tonic) activity of central alpha2-adrenoceptor mechanisms are critically involved in regulating blood pressure in the DOCA-salt treated rat. In this manner, an enhanced activity of central alpha2-adrenoceptor systems acts to protect against a rise in blood pressure. In contrast, the attenuation of central alpha2-adrenoceptor stimulation evokes hypertension. The central action of xylazine to prevent hypertension may be associated with the inhibition of sympathetic outflow to the kidneys and evokes an enhanced natriuresis. By inhibiting the avid sodium retention elicited by DOCA-salt treatment, the central activation of alpha2-adrenoceptors delays the onset and the severity of hypertension in this pathological model.

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.

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.

Influence of antihypertensive therapy on renal function

Antihypertensive therapy influences kidney function by different mechanisms depending on the mode of action of the drug used. The GFR is improved by calcium entry blockers and ACE inhibitors, unaffected by vasodilators, alpha-blockers and centrally acting sympatholytics and impaired by beta-blockers. The same is true for renal blood flow and is due to changes of renal vascular resistance. Renal sodium excretion is impaired mostly by vasodilators, by alpha-blockers, sympatholytics and beta-blockers; in contrast, calcium entry blockers and ACE inhibitors acutely induce natriuresis. The RAAS is stimulated by vasodilators, unaffected by alpha-blockers and sympatholytics and suppressed by beta-blockers. Plasma catecholamines are stimulated by vasodilators and suppressed by centrally acting sympatholytics and unaffected by the others. Induction of acute renal functional impairment is reported for ACE inhibitors under conditions of compromised renal perfusion pressure such as in renal artery stenosis. These data from the literature reviewed are supported by our own experimental data on sodium balance under different drugs and micropuncture data in experimental renal artery stenosis. To achieve effective antihypertensive treatment with a low profile of side effects, careful monitoring of renal function seems to be mandatory.

The effect of an alpha-2 agonist on bladder function and cord histology after spinal cord injury

Spinal injury in cats is accompanied by urinary bladder and hind limb dysfunction. Ten cats subjected to spinal contusion at the ninth thoracic segment were treated with guanabenz (an alpha-2 agonist) intraperitoneally (0.65 mg./kg.) three hours after injury, and twice daily for eight weeks. An additional six spinal cats were untreated and served as controls. Urodynamic studies were performed on a weekly basis on all animals. Guanabenz modified the vesico-somatic reflex: detrusor-sphincter dyssynergia was either ablated or abolished. In contrast, the controls demonstrated detrusor-sphincter dyssynergia, high residual urine, and spasticity below the lesion. Histological evaluations of the spinal cords revealed that the six paraplegic animals (untreated) suffered marked cavitation of the cord and complete destruction of the grey matter. The five incomplete paraplegic animals (treated) showed minimal cavitation with some preservation of the grey matter. The five ambulators (treated) demonstrated some distortion of grey matter with preservation of white matter. Treatment with guanabenz post traumatic cord injury results in decreased cord cavitation. Detrusor-sphincter dyssynergia is diminished and hind limb function is improved in treated animals.

Clonidine inhibits fluid absorption in the rabbit proximal convoluted renal tubule

Previous studies have shown that norepinephrine (NE) and the beta-adrenoceptor agonist, isoproterenol (I), enhance fluid absorption (JV) in isolated, perfused proximal convoluted tubule segments (PCT). Pretreatment of PCT with the beta-adrenoceptor antagonist, propranolol, inhibited the action of NE and produced a significant decline in JV, suggesting modulation of JV by both alpha- and beta-adrenoceptors. The present studies further characterize the alpha-adrenoceptor control of JV in isolated perfused PCT using specific agonists and antagonists. Basal JV declined significantly with the addition of the alpha 2-adrenoceptor agonist, clonidine (10(-4) M), to the bath; however, it was unchanged with the addition of the alpha 1-adrenoceptor agonist, methoxamine (10(-6) or 10(-4) M). With the addition of 10(-6) M isoproterenol JV increased significantly, and returned to control values with the subsequent addition of clonidine (10(-6) or 10(-4) M). Pretreatment of PCT with the alpha 2-adrenoceptor antagonist, yohimbine (10(-5) M), or with pertussis toxin (100 ng/ml) did not interfere with the stimulation of JV by isoproterenol, but abolished the inhibition of isoproterenol-stimulated JV by clonidine. Thus, clonidine inhibits JV in PCT via an alpha 2-adrenoceptor. This effect is mediated by a pertussis toxin inhibitable GTP-binding protein, but not one that is coupled to adenylyl cyclase.