I1 agents: a new approach to the treatment of hypertension

Brain stem adenosine receptors modulate centrally mediated hypotensive responses in conscious rats: A review

Adenosine is implicated in the modulation of cardiovascular responses either at the peripheral or at central level in experimental animals. However, there are no dedicated reviews on the involvement of adenosine in mediating the hypotensive response of centrally administered clonidine in general and specifically in aortically barodenervated rats (ABD). The conscious ABD rat model exhibits surgically induced baroreflex dysfunction and exaggerated hypotensive response, compared with conscious sham-operated (SO) rats. The current review focuses on, the role of adenosine receptors in blood pressure (BP) regulation and their possible crosstalk with other receptors e.g. imidazoline (I₁) and alpha (α_2A) adrenergic receptor (AR). The former receptor is a molecular target for clonidine, whose hypotensive effect is enhanced approx. 3-fold in conscious ABD rats. We also discussed how the balance between the brain stem adenosine A₁ and A_2A receptors is regulated by baroreceptors and how such balance influences the centrally mediated hypotensive responses. The use of the ABD rat model yielded insight into the downstream signaling cascades following clonidine-evoked hypotension in a surgical model of baroreflex dysfunction.

Synthesis and pharmacological evaluation of imidazoline sites I1 and I2 selective ligands

Several series of 2-aryl or heterocyclic-imidazoline compounds have been prepared and evaluated in vitro as imidazoline sites (I1 and I2) and alpha-adrenergic (alpha1 and alpha2) receptor ligands. Their pKi values indicate that linkage of the imidazoline moiety at the 2-position with an aromatic substituent dramatically decreases alpha-adrenergic affinity. I1 sites are more accessible by phenyl imidazolines substituted by a methyl or a methoxy group at the ortho or meta position. Indeed, 2-(2'-methoxyphenyl)-imidazoline (17) is one of the best I1 ligands ever reported (pKi = 8.53 and I1/I2 > 3388). On the other hand, I2 selectivity increases in the presence of a methyl group in the para position. The original compound, 2-(3'-fluoro-4'-tolyl)-imidazoline (31) is a new potent ligand for the I2 sites with high selectivity (pKi = 8.53 and I2/I1 > 3388).

Atrial natriuretic peptide is involved in renal actions of moxonidine

Moxonidine, an antihypertensive imidazoline compound, reduces blood pressure by selective activation of central imidazoline I(1)-receptors and inhibition of sympathetic nerve activity and by direct actions on the kidney, with both mechanisms resulting in diuresis and natriuresis. We hypothesized that the hypotensive and renal actions of moxonidine may be mediated by atrial natriuretic peptide (ANP), a cardiac peptide involved in pressure and volume homeostasis through its vasodilatory, diuretic, and natriuretic actions. Renal parameters were measured on an hourly basis over a period of 4 hours in conscious rats that received bolus intravenous injections of moxonidine (1 to 150 microg/300 microL saline). During the first hour, moxonidine dose-dependently stimulated diuresis, natriuresis, kaliuresis, and urinary cGMP, the index of ANP activity. Moxonidine (50 microg) significantly (P<0.001) stimulated urinary volume (0.35+/-0.04 versus 1.05+/-0.09 mL/h per 100 g), sodium (14. 3+/-2.5 versus 51.8+/-6.5 micromol/h per 100 g), potassium (10.5+/-2. 3 versus 32.3+/-3.2 micromol/h per 100 g), and cGMP (325+/-52 versus 744+/-120 pmol/h per 100 g). Pretreatment with a selective imidazoline receptor antagonist, efaroxan, dose-dependently inhibited moxonidine-stimulated renal parameters. Efaroxan (25 microg per rat) significantly inhibited moxonidine-stimulated diuretic and natriuretic effects and urinary cGMP excretion (744+/-120 versus 381+/-137 pmol/h per 100 g, P<0.02). The alpha(2)-adrenoceptor antagonist yohimbine (50 microg per rat) partially yet significantly inhibited moxonidine-stimulated diuresis and natriuresis but not cGMP excretion. Plasma ANP was dose-dependently increased by moxonidine and was inhibited by pretreatment with efaroxan (220.8+/-36.9 versus 100.3+/-31.7 pg/mL, P<0.03) but not by yohimbine. In conclusion, selective in vivo activation of imidazoline receptors by moxonidine is associated with dose-dependent diuresis, natriuresis, and kaliuresis as well as stimulated plasma ANP and urinary cGMP excretion, thus implicating ANP in the renal actions of moxonidine.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

'Seeing through a glass darkly': casting light on imidazoline 'I' sites

Although imidazoline sites have been the subject of research for several years, there is still controversy about their structure, diversity and physiology. The I1 site is thought to exist principally as a binding site and is widely purported to play a role in controlling systemic blood pressure, although this is still unclear. The majority of I2 sites are widely accepted as being allosteric sites on monoamine oxidase; however, even with selective ligands, their exact function remains to be determined. A putative I3 site modulates insulin secretion and could represent the first functional site to be pharmacologically defined with selective agonists and antagonists. The structure and relevance of the proposed endogenous ligand 'clonidine-displacing substance' remains elusive. A potential candidate for this substance is agmatine; however, although it is capable of displacing bound clonidine from imidazoline sites, it lacks the functionality ascribed to the clonidine-displacing substance. In this review, Richard M. Eglen and colleagues assess our knowledge of imidazoline sites in the light of recent data.

1,5-Bis(4-amidinophenoxy)pentane (pentamidine) is a potent inhibitor of [3H]idazoxan binding to imidazoline I2 binding sites

The aromatic diamidine 1,5-bis(4-amidinophenoxy)pentane (pentamidine) is used for treatment and prophylaxis of Pneumocystis carinii pneumonia in patients with Acquired Immune Deficiency Syndrome. Clinical use of pentamidine has been restricted by significant toxicity, that includes hypotension, and hypoglycemia. Although clinical toxicity is well described, the mechanisms are still poorly understood. Competitive binding analyses using [3H]idazoxan as the radioligand, and cirazoline to define non-specific binding, demonstrate that pentamidine binds to an imidazoline I2 binding site on rat liver membranes with a Ki of 1.4+/-0.22 nM. The Ki indicates that pentamidine inhibits radioligand binding at imidazoline I2 sites with an affinity approximating the most potent known ligands and may be related to pentamidine toxicity. Moreover, pentamidine analogs inhibit radioligand binding with a range of affinities that vary according to their structure. Two candidate drugs, Compounds 5 and 6, are more active than pentamidine in the corticosteroid-suppressed rat model of P. carinii pneumonia, yet have different affinities for the imidazoline I2 site (Ki 5 = 50.1+/-1.06 nM and Ki 6 = approximately 3500 nM). Affinity for this site does not correlate with antimicrobial activity (r = 0.60; p = 0.09) or the calculated log of the octanol:water partition coefficient (ClogP) (r = -0.38; p = 0.22).

The imidazoline receptor in control of blood pressure by clonidine and allied drugs

Clonidine, moxonidine, and rilmenidine are centrally acting antihypertensive agents that lower arterial pressure by inhibiting the tonic activity of sympathoexcitatory neurons in the rostral ventrolateral medulla. Competing hypotheses have been put forward by different investigators to explain the sympathoinhibition evoked by "imidazoline drugs": either via central actions at alpha 2-adrenergic receptors or novel I1-imidazoline receptors. These different perspectives are presented in the accompanying reviews.

Effect of rilmenidine on arterial pressure and urinary output in the spontaneously hypertensive rat

Rilmenidine is an antihypertensive agent acting at the imidazoline receptor that may have both central effects in the ventral lateral medulla and direct effects on the kidney to alter Na+ excretion. The present experiments examined whether rilmenidine induces a leftward shift or change in the slope of the pressure-natriuresis curve in the spontaneously hypertensive rat (SHR). A single oral gavage dose indicated that 3 and 10 mg/kg rilmenidine significantly lowers arterial pressure at 4-12 h after administration by oral gavage. The effect of rilmenidine on pressure-natriuresis was studied using twice daily doses of 1 and 3 mg/kg for control and treated SHR drinking tap water or 1% NaCl for 3 days. Na+ excretion was measured over 24 h, and mean arterial pressure was measured 6-8 h after the morning dose of rilmenidine. The results indicate that 1 mg/kg had no effect, while the pressure-natriuresis relationship for the rats receiving the 3 mg/kg dose was shifted to the left and was not significantly different from the vertical slope of the untreated SHR. This experiment also suggested that rilmenidine may attenuate the salt preference of the rats. This was confirmed in an additional series of experiments in which the rats had access to both tap water and 1% NaCl. Thus, rilmenidine shifts the pressure-natriuresis relationship to the left and reduces salt preference in SHR.

Rilmenidine and reflex renal sympathetic nerve activation in Wistar and hypertensive rats

1. This study sets out to examine the effect of rilmenidine administered systemically on basal and reflexly activated renal nerve activity in Wistar and stroke prone spontaneously hypertensive rats (SHRSP). 2. Animals were anaesthetized with chloralose/urethane, stimulating electrodes were placed on the brachial plexi and the renal nerves were isolated and put on recording electrodes. Both brachial nerves were stimulated electrically at 0.8, 1.6 and 3.2 Hz (15 V, 0.2 ms) in the absence and in the presence of rilmenidine given at 100 and 200 micrograms kg-1 i.v. in a cumulative manner. 3. Stimulation of the brachial nerves caused graded increases in blood pressure, heart rate and integrated renal nerve activity (P < 0.05) in both Wistar and SHRSP. Fast Fourier transformation of the renal nerve activity signal to generate a power spectrum demonstrated that both total power and percentage power at heart rate was higher in the SHRSP than Wistar (P < 0.05). Total power was raised during brachial nerve stimulation in both Wistar and SHRSP by some 200-300% (P < 0.05) but the percentage power at heart rate was decreased by some 60% (P < 0.01) in the Wistar but was raised by some 40-50% (P < 0.05) in the SHRSP. 4. Administration of rilmenidine caused dose-related decreases in blood pressure and heart rate and integrated renal nerve activity in both Wistar and SHRSP (all P < 0.05). Both doses of rilmenidine decreased (P < 0.05) the total power in the signal in both strains of rat by about one-half but the power occurring at heart rate only fell at the higher dose of compound in the Wistar, whereas in the SHRSP it was decreased by both doses by approximately 60-70%. In the presence of rilmenidine, coherence of the renal nerve signal was reduced in the Wistar and SHRSP and although the drug had no effect on phase difference in the Wistar, this parameter was decreased in the SHRSP by the low and high doses of rilmenidine (P < 0.05). 5. In the presence of 100 micrograms kg-1 rilmenidine, stimulation of the brachial nerves caused increases in total power in the Wistar and SHRSP (two to three fold, P < 0.05), together with a decrease (P < 0.05) in the percentage power occurring at heart rate in the Wistar, of some 60%, and an increase (P < 0.01) in the SHRSP, of some two to three times, which were very similar in magnitude and pattern to those obtained in the absence of the drug. Following the 200 micrograms kg-1 dose of rilmenidine, brachial nerve stimulation increased total power in the Wistar and SHRSP groups (P < 0.05) and whereas in the Wistar the percentage power at heart rate did not change in the SHRSP it was again increased in response to the electrical stimulation of the brachial plexus (P < 0.001) by between two to three fold. 6. These results showed that in both the Wistar and SHRSP rilmenidine depressed blood pressure, heart rate and integrated renal nerve activity. Moreover, rilmenidine did not affect the reflex activation of renal nerve activity via the somatosensory system although the characteristics within the power spectra underwent certain changes which might have a functional impact at the level to the kidney.

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.