A novel mechanism of action for hypertension control: moxonidine as a selective I1-imidazoline agonist

Sympathetic hyperactivity in patients with hypertension: pathogenesis and treatment. Part II

The second part of the review considers different classes of drugs affecting blood pressure in increased activity of the sympathetic nervous system. Additional possibilities are discussed on how to reduce the negative effect of sympathetic hyperactivity on cardiovascular system and improve the prognosis.

Nanocarriers as treatment modalities for hypertension

Hypertension, a worldwide epidemic at present, is not a disease in itself rather it is an important risk factor for serious cardiovascular disorders including myocardial infarction, stroke, heart failure, and peripheral artery disease. Though numerous drugs acting via different mechanism of action are available in the market as conventional formulations for the treatment of hypertension but they face substantial challenges regarding their bioavailability, dosing and associated adverse effects which greatly limit their therapeutic efficacies. Various studies have demonstrated that nanocarriers can significantly increase the drug bioavailability thereby reducing the frequency of dosing in addition to minimizing toxicity associated with high dose of the drug. The present review provides an insight into the challenges associated with the conventional antihypertensive formulations and need for oral nanoparticulate systems in order to overcome problems associated with conventional formulations. Hypertension has circadian pattern of blood pressure, therefore chronotherapeutics can play a decisive role for the treatment, and however, nanoparticulate system can play major role in hypertension management. Future prospective for particulate nanocarriers in drug delivery for hypertension includes chronotherapeutics and emerging technique like gene therapy which is also covered in the review.

GABAergic mechanism in the rostral ventrolateral medulla contributes to the hypotension of moxonidine

AIMS

The depressor action of the centrally antihypertensive drug moxonidine has been attributed to activation of I(1)-imidazoline receptor in the rostral ventrolateral medulla (RVLM). The objective of this study was to determine the role of the γ-aminobutyric acid (GABA) mechanisms in the RVLM in mediating the effect of moxonidine in anaesthetized normotensive rats.

METHODS AND RESULTS

The relationship between the effects of microinjection or picoinjection of moxonidine and the functional state of GABA receptors at the level of the RVLM or pre-sympathetic neuron was determined. Microdialysis was performed to detect the effect of moxonidine on the release of GABA in the RVLM. Western blot analysis was carried out to test the effect of chronic intracerebroventricular injection of moxonidine on the protein expression of GABA receptors in the RVLM. Pre-treatment with the GABA(A) or GABA(B) receptor antagonist bicuculline (5 pmol) or CGP35348 (200 pmol), respectively, microinjected into the RVLM significantly attenuated the decrease in blood pressure and renal sympathetic nerve activity induced by moxonidine. In 22 moxonidine-sensitive pre-sympathetic neurons in the RVLM, picoinjection of bicuculline (100 fmol/5 nL) significantly attenuated the neuronal inhibition evoked by moxonidine (100 pmol/5 nL). The release of GABA in the RVLM was increased after intravenous moxonidine (50 μg/kg). Central infusion of moxonidine upregulated the protein expression of both GABA(A) and GABA(B) receptors in the RVLM.

CONCLUSION

The current data demonstrate that GABAergic mechanisms in the RVLM are responsible for the hypotension and sympathoinhibition of moxonidine.

Sympathoexcitation of moxonidine in the caudal ventrolateral medulla is dependent on I1-imidazoline receptors in anesthetized rats

Moxonidine is a second-generation centrally acting antihypertensive drug that has a high affinity for I(1)-imidazoline receptors (I(1)R). The caudal ventrolateral medulla (CVLM), an important region involved in cardiovascular activity, contains binding sites for centrally acting drugs. Our study aimed to determine the effects of moxonidine injected into the CVLM on cardiovascular activity in anesthetized rats. Unilateral microinjection of moxonidine (0.4 and 4 nmol) into the CVLM dose-dependently increased blood pressure (BP) by 8+/-2 and 18+/-2 mmHg and renal sympathetic nerve activity (RSNA) by 19+/-3 and 48+/-5% without modifying heart rate. Microinjection of the I(1)R/alpha(2)-adrenoceptor antagonist efaroxan (4 nmol) into the CVLM produced significant decreases in baseline BP and RSNA, but also completely abolished the increases in BP (2+/-1 versus 18+/-2 mmHg, P<0.01) and RSNA (3+/-2 versus 45+/-10%, P<0.01) evoked by subsequent injection of moxonidine (4 nmol). However, prior injection of yohimbine (500 pmol), a selective antagonist of alpha(2)-adrenoceptors, into the CVLM had no significant (P>0.05) effect on the moxonidine-induced increase in BP (18+/-2 versus 17+/-3 mmHg) and RSNA (45+/-10 versus 42+/-7%). The current data suggest that moxonidine injection into the CVLM has an excitatory effect on cardiovascular activity, which is mediated by an I(1)R dependent mechanism.

Treatment of hypertension in patients with diabetes mellitus

Antihypertensive treatment in diabetes mellitus, especially in diabetics known to have cardiac autonomic neuropathy, may have to consider the status of the autonomic nervous system. In diabetic subjects with cardiac autonomic neuropathy, vagal activity during the night is often reduced. The reduction results in relative or absolute sympathetic activation, which could increase cardiovascular risk. Pathophysiological and clinical data suggests that antihypertensive treatment should reduce rather than induce sympathetic activity in this setting. Beta blocking agents, ACE inhibitors, calcium antagonists of verapamil or diltiazem type and selective imidazoline receptor agonists reduce sympathetic activity and, therefore, may have a beneficial effect in diabetic patients with disturbed sympathovagal balance.

Involvement of central α1- and α2-adrenoceptors on cardiovascular responses to moxonidine

In the present study we compared the effects produced by moxonidine (alpha2-adrenoceptor/imidazoline agonist) injected into the 4th cerebral ventricle and into the lateral cerebral ventricle on mean arterial pressure, heart rate and on renal, mesenteric and hindquarter vascular resistances, as well as the possible action of moxonidine on central alpha1- or alpha2-adrenoceptors to produce cardiovascular responses. Male Holtzman rats (n=7-8) anesthetized with urethane (0.5 g/kg, intravenously-i.v.) and alpha-chloralose (60 mg/kg, i.v.) were used. Moxonidine (5, 10 and 20 nmol) injected into the 4th ventricle reduced arterial pressure (-19+/-5, -30+/-7 and -43+/-8 mmHg vs. vehicle: 2+/-4 mmHg), heart rate (-10+/-6, -16+/-7 and -27+/-9 beats per minute-bpm, vs. vehicle: 4+/-5 bpm), and renal, mesenteric and hindquarter vascular resistances. Moxonidine (5, 10 and 20 nmol) into the lateral ventricle only reduced renal vascular resistance (-77+/-17%, -85+/-13%, -89+/-10% vs. vehicle: 3+/-4%), without changes on arterial pressure, heart rate and mesenteric and hindquarter vascular resistances. Pre-treatment with the selective alpha2-adrenoceptor antagonist yohimbine (80, 160 and 320 nmol) injected into the 4th ventricle attenuated the hypotension (-32+/-5, -25+/-4 and -12+/-6 mmHg), bradycardia (-26+/-11, -23+/-5 and -11+/-6 bpm) and the reduction in renal, mesenteric and hindquarter vascular resistances produced by moxonidine (20 nmol) into the 4th ventricle. Pre-treatment with yohimbine (320 nmol) into the lateral ventricle did not change the renal vasodilation produced by moxonidine (20 nmol) into the lateral ventricle. The alpha1-adrenoceptor antagonist prazosin (320 nmol) injected into the 4th ventricle did not affect the cardiovascular effects of moxonidine. However, prazosin (80, 160 and 320 nmol) into the lateral ventricle abolished the renal vasodilation (-17+/-4, -6+/-9 and 2+/-11%) produced by moxonidine. The results indicate that the decrease in renal vascular resistance due to moxonidine action in the forebrain is mediated by alpha1-adrenoceptors, while the cardiovascular effects produced by moxonidine acting in the brainstem depend at least partially on the activation of alpha2-adrenoceptors.

Antihypertensive responses elicited by central moxonidine in rats: possible role of nitric oxide

In the present study, we investigated the effects of pretreatment with NG-nitro-L-arginine methyl ester (L-NAME) (nitric oxide synthase inhibitor) injected intravenously (IV) on the hypotension, bradycardia, and vasodilation produced by moxonidine (alpha2-adrenergic/imidazoline receptor agonist) injected into the fourth brain ventricle (4th V) in rats submitted to acute hypertension that results from baroreflex blockade by bilateral injections of kynurenic acid (kyn, glutamatergic receptor antagonist) into the nucleus of the solitary tract (NTS) or in normotensive rats. Male Wistar rats (n=5 to 7/group) anesthetized with IV urethane (1.0 g kg(-1) of body weight) and alpha-chloralose (60 mg kg(-1) of body weight) were used. Bilateral injections of kyn (2.7 nmol 100 nL(-1)) into the NTS increased baseline mean arterial pressure (148 +/- 11 mm Hg, vs. control: 102 +/- 4 mm Hg) and baseline heart rate (417 +/- 11 bpm, vs. control: 379 +/- 6 bpm). Moxonidine (20 nmol microL(-1)) into the 4th V reduced mean arterial pressure and heart rate to similar levels in rats treated with kyn into the NTS (68 +/- 9 mm Hg and 359 +/- 7 bpm) or in control normotensive rats (66 +/- 7 mm Hg and 362 +/- 8 bpm, respectively). The pretreatment with L-NAME (25 micromol kg, IV) attenuated the hypotension produced by moxonidine into the 4th V in rats treated with kyn (104 +/- 6 mm Hg) or in normotensive rats (95 +/- 8 mm Hg), without changing bradycardia. Moxonidine into the 4th V also reduced renal, mesenteric, and hindquarter vascular resistances in rats treated or not with kyn into the NTS and the pretreatment with L-NAME IV reduced these effects of moxonidine. Therefore, these data indicate that nitric oxide mechanisms are involved in hypotension and mesenteric, renal, and hindquarter vasodilation induced by central moxonidine in normotensive and in acute hypertensive rats.

Antihypertensive drugs interacting with central imidazoline (I1)-receptors

Central imidazoline (I(1))-receptors have been recognised as targets of a new class of centrally acting antihypertensives. The stimulation of these I(1)-receptors induces peripheral sympatho-inhibition and a reduction of (elevated) blood pressure. Moxonidine and rilmenidine are the prototypes of this new class of centrally acting antihypertensives. These imidazoline receptor stimulants are effective antihypertensives with a haemodynamic profile which is attractive from a pathophysiological point of view. Since both moxonidine and rilmenidine have a much weaker affinity for central (2)-adrenoceptors than classic centrally acting drugs, for example, clonidine and alpha-methyl-DOPA, the side-effects profile of the I(1)-receptor stimulants is significantly better. The imidazoline (I(1))-receptor stimulants are the subject of the current survey. They appear to offer the possibility of developing centrally acting antihypertensives with the same attractive haemodynamic characteristics as the classic alpha(2)-adrenoceptor stimulants, but with clearly better tolerability. Their potential use in the treatment of congestive heart failure and the metabolic syndrome is subject to clinical investigation.

Central blockade of nitric oxide synthesis reduces moxonidine-induced hypotension

1. Nitric oxide (NO) and alpha(2)-adrenoceptor and imidazoline agonists such as moxonidine may act centrally to inhibit sympathetic activity and decrease arterial pressure. 2. In the present study, we investigated the effects of pretreatment with l-NAME (NO synthesis inhibitor), injected into the 4th ventricle (4th V) or intravenously (i.v.), on the hypotension, bradycardia and vasodilatation induced by moxonidine injected into the 4th V in normotensive rats. 3. Male Wistar rats with a stainless steel cannula implanted into the 4th V and anaesthetized with urethane were used. Blood flows were recorded by use of miniature pulsed Doppler flow probes implanted around the renal, superior mesenteric and low abdominal aorta. 4. Moxonidine (20 nmol), injected into the 4th V, reduced the mean arterial pressure (-42+/-3 mmHg), heart rate (-22+/-7 bpm) and renal (-62+/-15%), mesenteric (-41+/-8%) and hindquarter (-50+/-8%) vascular resistances. 5. Pretreatment with l-NAME (10 nmol into the 4th V) almost abolished central moxonidine-induced hypotension (-10+/-3 mmHg) and renal (-10+/-4%), mesenteric (-11+/-4%) and hindquarter (-13+/-6%) vascular resistance reduction, but did not affect the bradycardia (-18+/-8 bpm). 6. The results indicate that central NO mechanisms are involved in the vasodilatation and hypotension, but not in the bradycardia, induced by central moxonidine in normotensive rats.

Moxonidine and central α2 adrenergic receptors in sodium intake

Central injections of the alpha(2) adrenergic/imidazoline receptor agonist moxonidine inhibit water and NaCl intake in rats. In the present study, we investigated the possible involvement of central alpha(2) adrenergic receptors on the inhibitory effect of moxonidine in 0.3 M NaCl intake induced by 24 h sodium depletion. Male Holtzman rats with stainless-steel cannulas implanted into the lateral ventricle (LV) were used. Sodium depletion was produced by the treatment with the diuretic furosemide (20 mg/kg of body weight) injected subcutaneously +24 h of sodium-deficient diet. Intracerebroventricular (icv) injections of moxonidine (20 nmol/1 microl) reduced sodium depletion-induced 0.3 M NaCl intake (6.6+/-1.9 ml/120 min vs. vehicle: 12.7+/-1.7 ml/120 min). Pre-treatment with the alpha(2) adrenoreceptor antagonists RX 821002 (80 nmol/1 microl), SK&F 86466 (640 nmol/1 microl) and yohimbine (320 nmol/3 microl) injected icv abolished the inhibitory effect of icv moxonidine on sodium depletion-induced 0.3 M NaCl intake (13.3+/-1.4, 15.7+/-1.7 and 11.8+/-2.2 ml/120 min, respectively). The results show that the activation of alpha(2) adrenoreceptors is essential for the inhibitory effect of central moxonidine on sodium depletion-induced NaCl intake.

Non-adrenergic exploratory behavior induced by moxonidine at mildly hypotensive doses

Moxonidine is a centrally-active imidazoline compound with preferential affinity for imidazoline receptors (IR) over alpha(2)-adrenoceptors (alpha(2)AR). Clinically, moxonidine has proven advantageous for treating hypertension over pure alpha(2)-adrenergic agonists (i.e., guanabenz) due to its lowered incidence of sedative side effects. The present experiments reveal divergent behavioral effects of low doses of moxonidine and guanabenz in C57Bl/6 mice in an exploratory arena. Low-dose moxonidine (0.05 mg kg(-1) i.p.) elicited an increase in novel object contacts (+36%) and more movement into central space (+56%; P<0.01) compared to saline-injected controls; whereas guanabenz induced only dose-responsive sedative-like behaviors in the same paradigm. Yet, the two agonists were indistinguishable in terms of blood pressure changes over a similar dose range (0.025-0.1 mg kg(-1) i.p.) in consciously free-moving mice (Delta mean+/-S.E.M.=-12.3+/-3.2 mm Hg for moxonidine versus -13.5+/-1.9 mm Hg for guanabenz). As expected of alpha(2)AR involvement, the sedative-like effects of guanabenz were completely blocked by pretreatment with the non-imidazoline alpha(2)AR-antagonist, SKF86466 (0.5 or 1.0 mg kg(-1) i.p.). However, the pro-exploratory effects of low doses of moxonidine (0.05 or 0.1 mg kg(-1)) were not antagonized by SKF86466. These results suggest that moxonidine acts preferentially through a non-adrenergic mechanism, possibly IR-mediated, to elicit pro-exploratory behavior.

Central alpha(2) adrenergic receptors and cholinergic-induced salivation in rats

Salivation induced by intraperitoneal (i.p.) injections of pilocarpine (cholinergic agonist) is reduced by intracerebroventricular (i.c.v.) injections of moxonidine (alpha(2) adrenergic and imidazoline receptor agonist). In the present study, we investigated the involvement of central alpha(2) adrenergic receptors in the inhibitory effect of i.c.v. moxonidine on i.p. pilocarpine-induced salivation. Male Holtzman rats with stainless steel cannula implanted into the lateral ventricle (LV) were used. Saliva was collected using pre-weighted small cotton balls inserted into the animal's mouth under ketamine (100 mg x kg(-1)) anesthesia. Salivation was induced by i.p. injection of pilocarpine (4 micromol x kg(-1)). Pilocarpine-induced salivation was reduced by i.c.v. injection of moxonidine (10 nmol) and enhanced by i.c.v. injections of either RX 821002 (160 nmol) or yohimbine (320 nmol). The inhibitory effect of i.c.v. moxonidine on pilocarpine-induced salivation was abolished by prior i.c.v. injections of the alpha(2) adrenergic receptor antagonists, RX 821002 (160 nmol) or yohimbine (160 and 320 nmol). The alpha(1) adrenergic receptor antagonist prazosin (320 nmol) injected i.c.v. did not change the effect of moxonidine on pilocarpine-induced salivation. The results suggest that moxonidine acts on central alpha(2) adrenergic receptors to inhibit pilocarpine-induced salivation, and that this salivation is tonically inhibited by central alpha(2) adrenergic receptors.

Evidence for the involvement of central I1 imidazoline receptor in ethanol counteraction of clonidine hypotension in spontaneously hypertensive rats

Our previous studies have shown that ethanol counteracts centrally mediated hypotensive responses to clonidine. In this study, we investigated the relative roles of central alpha2-adrenergic and I1 imidazoline receptors in the antagonistic ethanol-clonidine hemodynamic interaction. The effects of selective blockade of alpha2- or I1 receptor by 2-methoxyidazoxan and efaroxan, respectively, on the blood pressure and heart rate responses to clonidine and subsequent ethanol administration were evaluated in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (1.5 microg/kg) produced significant (30 mm Hg; p < 0.05) and sustained (at least 60 min) decreases in blood pressure and heart rate. Systemic ethanol (1 g/kg), administered 10 min after clonidine, counteracted the hypotensive response and restored blood pressure to the preclonidine levels. Treatment with 2-methoxyidazoxan (0.16 microg/kg, intracisternal) or efaroxan (0.45 microg/kg, intracisternal) produced similar attenuation of the hypotensive and bradycardic responses to clonidine. The ability of ethanol to counteract the hypotensive action of clonidine was significantly (p < 0.05) attenuated in rats pretreated with efaroxan. The pressor response to ethanol lasted only 10 min compared with at least 60 min in the absence of efaroxan. In contrast, ethanol counteraction of clonidine-evoked hypotension was not altered when alpha2-adrenoceptors were blocked by 2-methoxyidazoxan. These findings suggest that centrally mediated hypotensive and bradycardic effects of clonidine in conscious spontaneously hypertensive rats involve activation of both alpha2-adrenergic and I1 imidazoline receptors. Furthermore, the findings suggest the dependence of a fully expressed ethanol counteraction of the hypotensive action of clonidine on functional I1 receptor within the central nervous system.

Differential effects of alpha-adrenoceptor agonists on human retinal microvessel diameter

The effects of locally administered brimonidine, clonidine, and p-aminoclonidine on microvessel caliber were compared in human retinal tissues grafted into the hamster cheek pouch. Clonidine and p-aminoclonidine, but not brimonidine, potently constricted human retinal microvessels over a broad concentration range. All three agonists elicited significant vasoconstriction in naive hamster cheek pouch microvasculature. The alpha2-adrenoceptor antagonist, rauwolscine, inhibited p-aminoclonidine-induced constriction in naive hamster cheek pouch microvessels, but not p-aminoclonidine-induced effects in retinal grafts. Selective alpha1-adrenoceptor agonists evoked vasoconstriction in retinal grafts only at relatively high concentrations. These differential effects on the retinal microvasculature could not be readily explained solely on the basis of alpha1- or alpha2-adrenoceptor involvement. Clonidine, p-aminoclonidine and brimonidine are also imidazoline derivatives that interact with putative non-adrenergic imidazoline-sensitive binding sites, the so-called I1-imidazoline binding site subtype implicated by some investigators in mediation of peripheral vasoconstriction. As with p-aminoclonidine, the potent vasoconstriction in human retinal microvasculature elicited by moxonidine, an alpha-adrenergic agonist that has also been reported to exhibit selectivity for putative I1-imidazoline binding sites, was not inhibited by the selective alpha-adrenoceptor antagonist, rauwolscine, nor by idazoxan, an antagonist characterized as having substantial activity at putative I2-imidazoline binding sites. These data suggest the possible involvement of an unconventional non-adrenergic imidazoline-sensitive pathway in regulation of microvascular responses in the inner retina, and that drug activity mediated via such an imidazoline-sensitive component could potentially evoke deleterious effects in the retinal microvasculature.

Antihypertensive drugs and the sympathetic nervous system

The sympathetic nervous system (SNS) plays an important role in the regulation of blood pressure homeostasis and cardiac function. Furthermore, the increased SNS activity is a predictor of mortality in patients with hypertension, coronary artery disease and congestive heart failure. Experimental data and a few clinical trials suggest that there are important interactions between the main pressor systems, i.e. the SNS, the renin-angiotensin system and the vascular endothelium with the strongest vasoconstrictor, endothelin. The main methods for the assessment of SNS activity are described. Cardiovascular drugs of different classes interfere differently with the SNS and the other pressor systems. Pure vasodilators including nitrates, alpha-blockers and dihydropyridine (DHP)-calcium channel blockers increase SNS activity. Finally, central sympatholytics and possibly phenylalkylamine-type calcium channel blockers reduce SNS activity. The effects of angiotensin-II receptor antagonists on SNS activity in humans is not clear; experimental data are discussed in this review. There are important interactions between the pressor systems under experimental conditions. Recent studies in humans suggest that an activation of the SNS with pure vasodilators in parallel increases plasma endothelin. It can be assumed that, in cardiovascular diseases with already enhanced SNS activity, drugs which do not increase SNS activity or even lower it are preferable. Whether this reflects in lower mortality needs to be investigated in intervention trials.

Pharmacology of moxonidine: an I1-imidazoline receptor agonist

The I1-imidazoline receptor is a novel neurotransmitter receptor found mainly in the brainstem, adrenal medulla and kidney. The actions of moxonidine are described at the level of individual biomolecules, cells, tissues, organs and finally with integrative functions. The receptor functions at the cellular level works through arachidonic acid and phospholipid signaling cascades in neuronal cells with the net result of inhibiting sympathetic premotor neurons.

Effect of long-term ethanol feeding on brainstem alpha(2)-receptor binding in Wistar-Kyoto and spontaneously hypertensive rats

Our previous studies have shown that ethanol attenuates baroreflex function in Wistar-Kyoto (WKY) but not in spontaneously hypertensive rats (SHRs). The present study determined the effects of chronic ethanol administration on alpha(2)-binding sites in brainstem areas that modulate baroreflexes. In vitro autoradiography was utilized to evaluate the effect of a 3-month ethanol feeding on the density (B(max)) and affinity (K(D)) of alpha(2)-adrenoceptors in the middle (mNTS) and rostral (rNTS) portions of the nucleus tractus solitarius of SHRs and WKY rats. Autoradiographic examination of brainstem sections preincubated with [125I]p-iodoclonidine revealed no inter-strain differences in alpha(2)-binding in control rats. Ethanol feeding caused strain-dependent changes in alpha(2)-binding activity, which comprised significant (P<0.05) decreases in the density of alpha(2)-binding sites in both areas of the NTS in SHRs versus no effect in WKY rats. These findings do not favor a role for brainstem alpha(2)-adrenoceptors in ethanol-induced attenuation of baroreflexes. Interestingly, the ethanol-evoked reduction in the NTS alpha(2)-receptor density in SHRs may explain reported findings that ethanol abolishes the hypotensive effect of the alpha(2)-adrenoceptor agonist clonidine in this rat model.

Imidazoline and alpha(2a)-adrenoceptor binding sites in postmenopausal women before and after estrogen replacement therapy

BACKGROUND

Platelet alpha(2A)-adrenoceptors (alpha(2A)AR) and imidazoline binding sites (subtype I(1)) have been proposed as peripheral markers of brain stem receptors that mediate sympathetic outflow and are reported to be elevated in major depression.

METHODS

In our study, p[(125)I]-iodoclonidine was used to assess platelet alpha(2A)AR and I(1) binding sites in healthy postmenopausal women (n = 34) compared with healthy women of reproductive age (n = 26). Receptor determinations were repeated in 19 postmenopausal women following 59-60 days of estrogen replacement therapy (ERT; 0.1 mg estradiol transdermal patches).

RESULTS

I(1) binding sites were twofold higher in platelets of postmenopausal women compared with women of reproduction age but were down-regulated (normalized) after 59-60 days of ERT. All other binding parameters, including platelet alpha(2A)AR density, were not different between groups nor were they changed after ERT. Platelet I(1) densities after 59-60 days of ERT were positively correlated with plasma luteinizing hormone concentrations.

CONCLUSIONS

It is suggested that increased imidazoline binding sites might be associated with mood and behavioral changes in postmenopausal women.

Central moxonidine on water and NaCl intake

In this study we investigated: (a) the effects of intracerebroventricular (i.c.v.) injections of moxonidine (an alpha2-adrenergic and imidazoline receptor agonist) on the ingestion of water and NaCl induced by 24 h of water deprivation; (b) the effects of i.c.v. injection of moxonidine on central angiotensin II (ANG II)- and carbachol-induced water intake; (c) the effects of the pre-treatment with i.c.v. idazoxan (an alpha2-adrenergic and imidazoline receptor antagonist) and RX 821002 (a selective alpha2-adrenergic antagonist) on the antidipsogenic action of central moxonidine. Male Holtzman rats had stainless steel cannulas implanted in the lateral cerebral ventricle. Intracerebroventricular injection of moxonidine (5 and 20 nmol/1 microl) reduced the ingestion of 1.5% NaCl solution (4.1 +/- 1.1 and 2.9 +/- 2.5 ml/2 h, respectively vs. control = 7.4 +/- 2.1 ml/2 h) and water intake (2.0 +/- 0.6 and 0.3 +/- 0.2 ml/h, respectively vs. control = 13.0 +/- 1.4 ml/h) induced by water deprivation. Intracerebroventricular moxonidine (5 nmol/1 microl) also reduced i.c.v. ANG II-induced water intake (2.8 +/- 0.9 vs. control = 7.9 +/- 1.7 ml/1 h) and i.c.v. moxonidine (10 and 20 nmol/1 microl) reduced i.c.v. carbachol-induced water intake (4.3 +/- 1.7 and 2.1 +/- 0.9, respectively vs. control = 9.2 +/- 1.0 ml/1 h). The pre-treatment with i.c.v. idazoxan (40 to 320 nmol/1 microl) abolished the inhibitory effect of i.c.v. moxonidine on carbachol-induced water intake. Intracerebroventricular idazoxan (320 nmol/1 microl) partially reduced the inhibitory effect of moxonidine on water deprivation-induced water intake and produced only a tendency to reduce the antidipsogenic effect of moxonidine on ANG II-induced water intake. RX 821002 (80 and 160 nmol/1 microl) completely abolished the antidipsogenic action of moxonidine on ANG II-induced water intake. The results show that central injections of moxonidine strongly inhibit water and NaCl ingestion. They also suggest the involvement of central alpha2-adrenergic receptors in the antidipsogenic action of moxonidine.

The I1-imidazoline receptor and its cellular signaling pathways

Two primary questions are addressed. First, do I1-imidazoline binding sites fulfill all the essential criteria for identification as a true receptor? Second, what are the cellular signaling pathways coupled to this novel receptor? I1-imidazoline binding sites show specificity in binding assays, linkage to physiologic functions, appropriate anatomic, and cellular and subcellular localization. Most important, binding affinities correlate with functional drug responses. I1-imidazoline binding sites meet several additional criteria identified with functional receptors: they show physiologic regulation and endogenous ligands and, most crucially, are coupled to cellular signaling events. A series of studies have identified cellular events triggered by I1-imidazoline receptor occupancy. This receptor is not coupled to conventional pathways downstream of heterotrimeric G-proteins, such as activation or inhibition of adenylyl or guanylyl cyclases, stimulation of inositol phospholipid hydrolysis, or induction of rapid calcium fluxes. The I1-imidazoline receptor is coupled to choline phospholipid hydrolysis, leading to the generation of diacylglyceride, arachidonic acid, and eicosanoids. Additional cellular responses include inhibition of Na+/H+ exchange and induction of genes for catecholamine synthetic enzymes. The signaling pathways linked to the I1-imidazoline receptor are similar to those of the interleukin family, implying that I1-receptors may belong to the family of neurocytokine receptors.

Different modes of action of the imidazoline compound RX871024 in pancreatic beta-cells. Blocking of K+ channels, mobilization of Ca2+ from endoplasmic reticulum, and interaction with exocytotic machinery

The imidazoline compound RX871024 glucose-dependently potentiates the release of insulin in pancreatic islets and beta-cell lines. This activity of the compound is not related to its action by stimulating alpha 2-adrenoceptors and I1- and I2-imidazoline receptors. There are at least three modes of action of RX871024 in beta-cells: (1) RX871024 blocks the ATP-dependent, Ca(2+)-activated, and delayed rectifier K+ channel activity; (2) RX871024 causes mobilization of Ca2+ from thapsigargin-sensitive intracellular stores, the effect probably controlled by cytochrome P450; and (3) the stimulatory activity of RX871024 on insulin release involves interaction of the compound with the exocytotic machinery, unrelated to the changes in membrane potential and cytoplasmic-free Ca2+ concentration, whereas protein phosphorylation plays an important role in this process. The maximal insulinotropic effect of RX871024 is much higher than that of the sulfonylurea glibenclamide. RX871024 stimulates insulin release and normalizes blood glucose levels in rats in vivo without affecting blood pressure and heart rate.

Influence of sino-aortic barodenervation on the cardiovascular effects of imidazoline-like drugs

Earlier findings have shown that hypotensive effects of centrally acting drugs, such as clonidine, are enhanced in animals after denervation of arterial baroreceptors. The purpose of this study was to investigate the dynamic of changes in arterial pressure, heart rate and hypotensive effects of clonidine, rilmenidine and moxonidine in Wistar rats after sino-aortic denervation (SAD) using radio-telemetry. SAD was followed by significant elevation of arterial pressure lability (the standard deviation of the mean arterial pressure), while the baseline mean arterial pressure (MAP) and heart rate in barodenervated rats (12 days after SAD) was similar to intact rats. The hypotension produced by clonidine, rilmenidine and moxonidine was much greater in SAD rats than in intact rats. The study suggests that baroreflex mechanisms are not only important for maintaining levels of blood pressure in the very short term, but also for buffering the effects of centrally acting antihypertensive drugs.

Chronopharmacological dependence of antihypertensive effects of the imidazoline-like drugs in stroke-prone spontaneously hypertensive rats

In the present study we investigated the chronopharmacological dependence of dose-dependent hypotensive and cardiochronotropic effects of the imidazoline-like drugs (clonidine, rilmenidine and moxonidine) in stroke-prone spontaneously hypertensive rats (SHR-SP), using radio-telemetric system (Data Sciences, USA). The 24-h blood pressure, heart rate and locomotor activity profiles showed peak values during the rats' active phase during the night period. The degree of hypotensive and bradycardic effects of all drugs were most evident at this time and occurred in the absence of a change in locomotor activity. These studies show that clonidine, rilmenidine and moxonidine decrease blood pressure and heart rate in a time-dependent manner in SHR-SP. It was demonstrated that the degree and duration of hypotensive action of imidazoline-like drugs vary with the time of drug administration.

Central imidazoline (I1) receptors as targets of centrally acting antihypertensives: moxonidine and rilmenidine

Clonidine, guanfacine, guanabenz and alpha-methyl-dioxyphenylalanine (DOPA), the prototypes of centrally acting antihypertensives, are assumed to induce peripheral sympathoinhibition and a reduction in blood pressure via the stimulation of alpha2-adrenoceptors in the brain stem. More recently, central imidazoline (I1)-receptors have been recognized to be another target of centrally acting antihypertensive drugs. Clonidine is considered to be a mixed agonist that stimulates both alpha2- and I1-receptors. Moxonidine and rilmenidine are considered to be moderately selective I1-receptor stimulants, although it still remains unknown whether these agents act directly on the receptor as genuine agonists. A survey is given on the location, characteristics and functional aspects of imidazoline I1-receptors as targets of centrally acting antihypertensives. Furthermore, the pharmacology and clinical potential of selective I1-receptor agonists such as moxonidine and rilmenidine are discussed. Although far from perfect, these compounds have shown that it may potentially be possible to develop agents with which the well-known side effects caused by alpha2-receptor agonists can be separated from the central antihypertensive mechanism.

Experimental benefit of moxonidine on glucose metabolism and insulin secretion in the fructose-fed rat

OBJECTIVE

Non-insulin-dependent diabetes mellitus (NIDDM) is often associated with hypertension leading to a specifically high cardiovascular risk in these patients. However, there is evidence that insulin resistance and hyperinsulinaemia are not only characteristic for diabetic patients but also for some non-diabetic populations in which a cluster of cardiovascular risk factors is observed (hypertension, hypertriglyceridaemia, obesity). Therefore, hyperinsulinaemia and insulin resistance have been suggested to be of major pathophysiological importance for the development of this syndrome (syndrome X). Since imidazoline receptors are currently considered to be a specific pharmacological target for blood pressure reduction, it is important to know whether and in which way these compounds affect the glucose homoeostasis and insulin release.

DESIGN

The influence of moxonidine on glucose tolerance in vivo was determined in healthy control rats, in rats receiving a high fructose diet for 6 weeks to induce insulin resistance, hyperinsulinaemia and hypertension, and in rats receiving in addition to a high fructose diet moxonidine (1.5 mg/kg body weight daily). In vitro, using isolated pancreatic islets of mice, long-lasting effects (chronic) and immediate (acute) effects of moxonidine on beta-cell function were determined by basal and glucose stimulated insulin release in two different experimental systems: (1) islets were exposed for 24 h (37 degrees C) to various concentrations of moxonidine ranging from 1 nmol/l to 1 mmol/l, followed by a washing procedure to remove excess of moxonidine and then used for the beta-cell function test; (2) islet cultures were incubated again with moxonidine for 24 h (37 degrees C) with either 1 nmol/l or 1 micromol/l. In contrast to the first experiments, however, after the washing procedure moxonidine was added at the same concentration as used for preincubation to test its direct effect on beta-cell function.

RESULTS

In healthy control rats acute administration of moxonidine in vivo impaired the glucose tolerance in high dosages, which effectively reduced the blood pressure (>1 mg/kg body weight). This effect was, however, smaller that that observed by clonidine. In fructose-fed rats, moxonidine completely prevented the development of insulin resistance, hyperinsulinaemia and hypertension. In vitro, pancreatic islets preincubated with moxonidine exhibited dose-dependently both stimulatory and inhibitory chronic effects on beta-cell function compared with that in controls. Preincubation of islet cultures with moxonidine at concentrations between 1 nmol/l and 1 mmol/l resulted in a reduction of basal insulin release which was very pronounced at concentrations higher than 100 nmol/l. The results obtained for glucose-stimulated insulin release opposed in part those for basal insulin release, since the preincubation with moxonidine up to 10 micromol/l gave rise to an increased insulin release. An additional direct effect of moxonidine with a marked reduction of glucose-stimulated insulin release was observed, however, when moxonidine was present during the preincubation (24 h) and the functional test at a concentration of 1 nmol/l or 1 micromol/l.

CONCLUSIONS

Our data suggest that a causal linkage exist between the development of hypertension and insulin resistance/hyperinsulinaemia in the high fructose diet rat model. Since central activation of imidazoline receptors by moxonidine can prevent this syndrome, it follows that an overactivity of the sympathetic nervous system is of major importance. Suppression of this sympathetic overactivity might be an effective approach to reduce hypertension and the concomitant metabolic defect. Therefore, such an interventional strategy could contribute to reduce the cardiovascular risk of NIDDM patients and patients with other forms of insulin resistance/hyperinsulinaemia such as metabolic cardiovascular syndrome.

Relationship between imidazoline/guanidinium receptive sites and monoamine oxidase A and B

Imidazoline binding sites or imidazoline/guanidinium receptive sites (IGRS) recognize bioactive endogenous substances and a variety of pharmacologically active compounds containing imidazoline or guanidinium moieties. The family of imidazoline binding proteins consists of multiple membrane-associated proteins that differ in their tissue/subcellular localization, M(r) and ligand recognition properties. Two of the imidazoline binding proteins are identical to the mitochondrial enzyme monoamine oxidase (MAO) A and B isoforms, which contain imidazoline binding domains distinct from the enzyme active site. The relationship between the imidazoline binding proteins and monoamine oxidases was further characterized in the present report using a covalent probe (2-[3-azido-4[125I]iodophenoxy] methyl imidazoline, [125I]-AZIPI) to label the imidazoline binding proteins in different species and following transient expression of MAO- A and -B in COS 7 cells. Species homologues of MAO-A and -B in rat and human differ in their apparent molecular weight by approximately 2000 Da. In rat and human liver [125I]-AZIPI identified peptides with apparent molecular weights similar to those of the species homologues of MAO. Peptides of M(r) approximately 63,000 (MAO-A) and approximately 59,000 (MAO-B) were also photolabeled in membranes prepared from COS-7 cells transfected with human cDNA clones encoding MAO-A or -B. Additional experiments indicate that the imidazoline binding domains on MAO-A and -B exhibit different ligand recognition properties. The covalent labeling of human liver MAO-B was more sensitive than that of placenta MAO-A to inhibition by the imidazoline 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224). These data indicate that the A and B isoforms of MAO possess imidazoline binding domains that differ in their ligand recognition properties. Allosteric regulation of the activity of MAO via the imidazoline binding domains may be of significance in various disease states associated with elevated enzyme expression or in which the enzyme is a therapeutic target.

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.

Mechanisms of alterations in cardiac membrane Ca2+ transport due to excess catecholamines

The occurrence of excessive catecholamine release is often associated with stress due to the lifestyle of Western societies. Contrary to the general thinking that excess catecholamines produce cardiotoxicity mainly via binding to adrenoceptors, there is increasing evidence that catecholamine-induced deleterious actions may also occur through oxidative mechanisms. In this overview it is shown that a high dose of isoproterenol induces a biphasic change in cardiac Ca2+ transport in the sarcolemma and in sarcoplasmic reticulum. Both sarcolemmal and sarcoplasmic reticular Ca2+-transport activities are initially increased to maintain Ca2+ homeostasis and then are impaired, which may be associated with the occurrence of intracellular Ca2+ overload. On the other hand, mitochondrial Ca2+-transport activities exhibited a delayed increase. Pretreatment with vitamin E partially prevented the deleterious changes in cardiac membranes as well as the depressed energetic status of the heart muscle cell. It is concluded that excess catecholamines affect Ca2+-transport mechanisms primarily via oxidation reactions involving free radical-mediated damage. Thus drug approaches that reduce circulating catecholamines and/or prevent their oxidation should prove beneficial. A combination therapy involving inhibitors of catecholamine release, blockers of adrenoceptors, and antioxidants may be indicated for stress-induced heart disease.

Drug withdrawal and rebound hypertension: differential action of the central antihypertensive drugs moxonidine and clonidine

To examine the antihypertensive action of the centrally acting antiadrenergic drugs moxonidine and clonidine, systolic and diastolic blood pressure as well as heart rate were monitored by radio telemetry in spontaneously hypertensive rats (SHR) with established high blood pressure. Increasing doses were administered with regular rat chow for 6-8 day periods. Moxonidine reduced (p < 0.05) diastolic blood pressure at a dose of 8 mg/kg/day and systolic blood pressure at 13 mg/kg/day. Heart rate was reduced during high activity of rats corresponding to an antitachycardiac action. After withdrawal of 18 mg/kg administered for only 1 day, blood pressure returned to pretreatment values within 8 days. Clonidine reduced systolic and diastolic blood pressure at 0.3 mg/kg/day. At 0.8 and 1.3 mg/kg/day, systolic blood pressure reduction was less pronounced, although heart rate was reduced further, reaching values that were below those of untreated sleeping rats. When 1.3 mg/kg/day clonidine was discontinued, systolic as well as diastolic blood pressure increased above pretreatment values within 1 day. A rebound was also observed in heart rate, which increased by 150 beats/ min. A comparable rebound in blood pressure was observed after withdrawal of 0.3 mg/kg/day. Since a blood pressure rebound occurred also after withdrawal of 0.3 mg/kg/day clonidine in normotensive rats, the rebound phenomenon was independent of the presence of high blood pressure. No blood pressure rebound was observed when moxonidine (8 mg/kg/ day) was administered (chow or gavage) in normotensive rats. These findings in unanesthetized undisturbed rats demonstrate distinct differences in the mode of action of moxonidine and clonidine, which can be accounted for by specific interactions of moxonidine with imidazoline I1-receptors, whereas clonidine would interact not only with I1-receptors but also with alpha2-adrenoceptors, and most probably also with the vagal activity. In view of our previous studies demonstrating a rise in blood pressure and heart rate after a hypercaloric dietary intake, the selective I1-receptor agonist moxonidine appears particularly appropriate for treating overweight hypertension associated with an enhanced sympathetic outflow of the brain. Of importance in this respect is that a moxonidine-induced reduction in sympathetic outflow was not associated with a gain in body weight but resulted in reduced caloric intake.

Sympathetic nervous system in salt-sensitive and obese hypertension: amelioration of multiple abnormalities by a central sympatholytic agent

Excess activity of the sympathetic nervous system (SNS) is linked to human obese hypertension and to salt-sensitive hypertension. Paradoxically, reduced SNS activity has been implicated as a contributor to obesity, particularly in animal models, and salt loading usually inhibits SNS activity. We have investigated the relationship between SNS activity, diet, and hypertension in the obese spontaneously hypertensive rat (SHROB), a model with a recessive obesity trait superimposed on a hypertensive background with multiple metabolic abnormalities resembling human syndrome X. We examined the role of SNS overactivity in the adverse impact of excess dietary salt and the possible beneficial effects of sympatholytic therapy. Mean blood pressure (MBP) was increased in SHROB and SHR fed a 4% NaCl diet. The pressor effect of dietary salt was abolished by ganglionic blockade, suggesting that increased SNS activity contributed to the pressor effect of the high-salt diet. Moxonidine, a second-generation central antihypertensive, controlled hypertension in both SHROB and SHR. Kidney damage in SHROB was accelerated by dietary salt and was reduced by moxonidine. Moxonidine elicited progressive weight loss in SHROB but not in SHR. Food intake in SHROB was reduced to the level of lean SHR. SHROB and SHR treated with moxonidine showed improved glucose tolerance. Additionally, SHROB showed reduced levels of triglycerides, cholesterol, and insulin following moxonidine therapy. Inhibition of the SNS, as with moxonidine therapy, may ameliorate multiple abnormalities and have therapeutic advantages in obese hypertensive syndromes.

Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3H]arginine was demonstrated and endogenous agmatine levels (10(-6)M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an alpha 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 (microM), a synthetic imidazoline2 (I2) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney.

I1-imidazoline receptors. Definition, characterization, distribution, and transmembrane signaling

Data were presented showing that I1-imidazoline sites show a unique ligand specificity that differs markedly from that of any of the alpha 2-adrenergic subtypes or the I2-imidazoline sites labeled by [3H]idazoxan. On the other hand, the ligand specificity of I1-imidazoline sites is maintained across mammalian species (cow, rat, dog, and human) and between different tissues and cell types. I1-Imidazoline sites can be further distinguished from I2 sites because the latter, unlike I1 sites, were not present in RVLM membranes from bovine brain stem. Furthermore, I1-imidazoline sites were modulated by guanine nucleotides with a specificity appropriate for a receptor coupled to G-protein and were mainly localized to plasma membranes. I1-Imidazoline sites show a unique pattern of distribution between diverse tissues and cell types and appear to be a neuroepithelial marker as well as being present in secretory cells of the pancreatic islets. The widespread distribution of I1-imidazoline sites implies that the functional significance of this putative receptor may have been underestimated. The signaling pathway associated with the I1-imidazoline receptor remains to be fully elucidated, but is likely that activation of phospholipase A2 leading to release of arachidonic acid and subsequent generation of prostaglandins plays a major role.

Effect of I1-imidazoline receptor activation on responses of hypoglossal and phrenic nerve to chemical stimulation

Sedation elicited by some centrally acting antihypertensive agents may interfere with respiratory control, and by selectively inhibiting upper airway dilating muscle activity it may facilitate obstructive sleep apnea. Autoradiographic studies with [125I]p-iodoclonidine in the presence of 10 microM epinephrine to block alpha 2-adrenergic sites or 100 nM moxonidine to mask I1-imidazoline sites show that both I1- as well as alpha 2-sites are localized in putative chemosensory areas of the rostral ventrolateral medulla in the cat. We sought to determine the effect of activating I1 and alpha 2-receptors on central chemosensitivity by using moxonidine as a selective I1 agonist, clonidine as a mixed I1/alpha 2 agonist, SK&F-86466 as a specific alpha 2-antagonist, and efaroxan as a mixed I1/alpha 2 antagonist. We recorded responses of phrenic, hypoglossal, and cervical sympathetic nerve activities to progressive hypercapnia after hyperventilation to apnea. Moxonidine (3-100 micrograms/kg i.v.) caused dose-dependent decreases in tonic cervical sympathetic nerve activity and blood pressure, but had no effect on the CO2 threshold (after 30 or 100 micrograms/kg moxonidine, phrenic nerve activity reappeared at 5.8 +/- 0.2% CO2 versus 5.6 +/- 0.3% CO2 in control). Following moxonidine, the slope of the steep portion of the CO2 response tended to increase (10.3 +/- 1.8 versus 7.3 +/- 0.9). Peak phrenic nerve activity was comparable to control at 7.5% CO2 (20 +/- 2 U in control) and at 9.5% CO2 (30 +/- 3 versus 27. +/- 2 U). Similarly, the response of hypoglossal and inspiratory phasic cervical sympathetic nerve activity to a progressive CO2 rise was not affected by moxonidine. By contrast, clonidine in the same doses decreased CO2 sensitivity, because the CO2 threshold was elevated from 5.3 +/- 0.5% to 6.7 +/- 0.4% (p < 0.001). The slope of the CO2 response was decreased from 9.7 +/- 1.9 to 7.4 +/- 1.3 (p = 0.05). Peak phrenic nerve activity was reduced at 7.5% CO2 (11 +/- 5 versus 25 +/- 2 U; p < 0.05) and at 9.5% CO2 (21 +/- 4 versus 33 +/- 2 U; p = 0.06). Clonidine selectively inhibited the response of hypoglossal nerve activity to CO2. The depressive effects of clonidine were reversed by alpha 2-blockade with SK&F-86466 (0.5 or 1 mg/kg). Inspiratory phasic cervical sympathetic nerve activity increased after SK&F-86466 in parallel with phrenic and hypoglossal nerve activity, but the tonic component of cervical sympathetic nerve activity and blood pressure increased only transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of agmatine (clonidine-displacing substance) by diamine oxidase and the possible implications for studies of imidazoline receptors

Clonidine-displacing substance, thought to be the endogenous ligand for imidazoline receptors, has been identified recently as agmatine (1-amino-4-guanidinobutane). The similarity of this compound's structure to that of the diamine oxidase (DAO) inhibitor, aminoguanidine, led us to investigate the possibility that agmatine might be a substrate for this enzyme. The metabolism of agmatine by purified porcine kidney DAO was measured by a peroxidase-linked colorimetric assay. Agmatine was a substrate for this enzyme and, under the experimental conditions used here, was metabolised at a rate of 0.8 mumol agmatine h-1 (unit DAO activity)-1. In contrast, agmatine was a substrate neither for rat brain monoamine oxidase (MAO) -A or -B, nor for rat brown adipose tissue semicarbazide-sensitive amine oxidase (SSAO). The metabolism of agmatine by DAO was inhibited by aminoguanidine (IC50 14.9 nM) and by the antidepressant, phenelzine (IC50 1.95 microM). These results suggest that administration of DAO inhibitors may increase endogenous agmatine levels and thus alter imidazoline receptor densities. A review of the literature documenting ligand affinities for idazoxan-preferring (I2) imidazoline binding site subtypes and drug affinities for DAO enzymes indicates that some of the I2 sites described elsewhere may correspond to DAO and not to an imidazoline receptor.

Heterogeneity of neurokinin1 binding sites in porcine respiratory tract

High-affinity NK1 binding sites for [125I]BH-SP were characterized in adult porcine respiratory tract. The affinity and density of NK1 sites were significantly higher in tracheal epithelium and smooth muscle than in the lung. The potency order for agonists was: SP > neuropeptide-gamma > physalaemin > NKA > eledoisin > septide > SP methyl ester > GR 73632 > NKB > senktide > SP(1-7). For antagonists: CP 99,994 > CP 96,345 > spantide > L 703,606 >> WIN 51,708. The CP compounds discriminated between very high- and high-affinity NK1 sites in all three tissues. The subpopulation of sites with very high affinity for CP compounds also preferentially bound septide. Both binding components were inhibited by guanine nucleotide and showed equal affinities for SP. We propose an NK1B subtype with very high affinity for the antagonists CP 99,994 and CP 96,345 and the agonist septide, and an NK1A subtype with lower affinities for these ligands.