Bioisosteric phentolamine analogs as selective human alpha(2)- versus alpha(1)-adrenoceptor ligands.

Phentolamine is known to act as a competitive, non-subtype-selective alpha-adrenoceptor antagonist. In an attempt to improve alpha(2)- versus alpha(1)-adrenoceptor selectivity and alpha(2)-adrenoceptor subtype-selectivity, two new chemical series of bioisosteric phentolamine analogs were prepared and evaluated. These compounds were evaluated for binding affinities on alpha(1)- (alpha(1A)-, alpha(1B)-, alpha(1D)-) and alpha(2)- (alpha(2A)-, alpha(2B)-, alpha(2C)-) adrenoceptor subtypes that had been stably expressed in human embryonic kidney and Chinese hamster ovary cell lines, respectively. Methylation of the phenolic hydroxy group and replacement of the 4-methyl group of phentolamine with varying lipophilic substituents yielded bioisosteric analogs selective for the alpha(2)- versus alpha(1)-adrenoceptors. Within the alpha(2)-adrenoceptors, these analogs bound with higher affinity at the alpha(2A)- and alpha(2C)-subtypes as compared to the alpha(2B)-subtype. In particular, the t-butyl analog was found to be the most selective, its binding at the alpha(2C)-adrenoceptor (Ki=3.6 nM) being 37- to 173-fold higher than that at the alpha(1)-adrenoceptors, and around 2- and 19-fold higher than at the alpha(2A)- and alpha(2B)-adrenoceptors, respectively. Data from luciferase reporter gene assays confirmed the functional antagonist activities of selected compounds from the bioisosteric series on human alpha(1A)- and alpha(2C)-adrenoceptors. Thus, the results with these bioisosteric analogs of phentolamine provide a lead to the rational design of potent and selective alpha(2)-adrenoceptor ligands that may be useful in improving the therapeutic profile of this drug class for human disorders.

alpha-Adrenergic activities of some substituted 2-(aminomethyl)imidazolines.

A series of 2-(aminomethyl)imidazolines related to the alpha-adrenergic antagonist phentolamine was prepared and evaluated for alpha-adrenergic agonist and antagonist activities in the isolated, field-stimulated rat vas deferens. Affinities for alpha-adrenergic receptors were determined by displacement of [3H]clonidine and [3H]prazosin from membrane binding sites of calf cerebral cortex. This series provided a variety of alpha-adrenergic profiles, with some of the (aminomethyl)imidazolines being nonselective alpha 1- and alpha 2-adrenergic antagonists like phentolamine, while others were either nonselective alpha 1- and alpha 2-agonists or mixed alpha 1-agonists/alpha 2-antagonists.

Phentolamine and structurally related compounds selectively antagonize the vascular actions of the K+ channel opener, cromromakalim.

1. The effects of cromakalim, a novel vasodilator agent believed to open K+ channels, were studied in a range of large and small arteries in vitro. In dog isolated coronary artery, precontracted with U46619 (a thromboxane A2-mimetic), cromakalim caused concentration-dependent relaxation which could be inhibited by phentolamine (10-100 microM). 2. The ability of phentolamine to antagonize cromakalim was selective since it did not affect responses to a number of other vasodilators including isoprenaline, nitroprusside or nicorandil. 3. The effect of phentolamine was not related to its alpha-adrenoceptor blocking actions since other alpha-adrenoceptor antagonists (prazosin 10 microM, rauwolscine 10 microM and phenoxybenzamine 1 microM) failed to influence the action of cromakalim. 4. A number of compounds structurally related to phentolamine were also able to block the vaso-relaxant response to cromakalim in the dog isolated coronary artery. The rank order of potency was alinidine = phentolamine = ST91 greater than tramazoline = naphazoline. Clonidine and tolazoline were inactive. The most potent compounds (alinidine and phentolamine) were effective only at concentrations above 1 microM. 5. Electrophysiological studies, in which resting membrane potential and tension were measured simultaneously, were carried out on rat isolated femoral artery. Phentolamine (30 microM) antagonized both the vasorelaxation and hyperpolarization caused by cromakalim. 6. These results suggest that phentolamine and some structurally related compounds, may inhibit K+ channel opening, an action which would account for their ability to antagonize the actions of cromakalim. Such compounds may prove useful in determining the role of K+ channels in regulating vascular smooth muscle tone in vivo and in vitro.

The effect of echothiophate on the biphasic response of rabbit ocular pressure to dipivefrin.

A time-course study was performed on the intraocular pressure response of pigmented rabbit eyes. Dipivefrin administration produced in initial hypertensive phase lasting less than two hours followed by a prolonged hypotensive phase. Echothiophate iodide therapy produced a more pronounced and prolonged hypertensive response; there was no hypotensive phase. Administration of echothiophate plus dipivefrin resulted in a hypertensive phase similar to that from echothiophate alone; as previously reported, this combination was not followed by a hypotensive phase. The alpha-blocker phentolamine mesylate prevented the echothiophate-induced hypertension. When dipivefrin was administered with echothiophate plus phentolamine, there was an immediate hypotensive effect. It was concluded that the hypertensive effect of echothiophate in pigmented rabbit eyes may mask the hypotensive action of dipivefrin. This, rather than an echothiophate-induced inhibition of esterases, may explain why combination therapy with these drugs seemed ineffective.

Evidence for involvement of prostaglandins in central alpha adrenergic activity and LH release in sheep.

Circhoral pulsatile release of immunoreactive luteinising hormone (LH) and prostaglandin F2 alpha (PGF2 alpha) occur synchronously into the jugular vein in ovariectomised sheep. Following a 4-hour control period, intra-carotid injections of phentolamine or intramuscular injections of phenoxybenzamine were given to ovariectomised sheep and the pulsatile release of LH and PGF2 alpha was monitored for a further 6 to 8 hours. Phenoxybenzamine caused a fall in LH and PGF2 alpha in jugular venous plasma. Phentolamine also reduced LH significantly but in this case a marked rise in PGF2 alpha as measured by radioimmunoassay (RIA) occurred after very high doses of phentolamine. Interpretation of the latter results was complicated by the fact that phentolamine at high dose levels interfered with the RIA of PGF2 alpha in plasma. Experiments were repeated in ovariectomised sheep with cannulae placed in the lateral ventricles of the brain for sampling cerebrospinal fluid (CSF). In contrast to the previously observed rise in jugular venous PGF2 alpha following high doses of phentolamine, a fall in CSF levels of immunoreactive PGF2 alpha occurred following intracarotid phentolamine or phenoxybenzamine in 3 out of 7 experiments, while no change was observed in the remaining 4 animals. Phentolamine did not reduce LH significantly in animals with intraventricular cannulae. The work provides support for the view that circhoral pulses of immunoreactive PGF2 alpha in sheep are neural in origin and may be related to sympathetic neurotransmission.

Intracavernous medication for treatment of premature ejaculation.

Sixteen men complained of premature ejaculation during a five-month period between May, 1987, and October, 1987. Eight patients entered this study using intracavernous vasoactive drugs as treatment for their problem. The patients, ages twenty-four through fifty-eight (average 42 years), were all physically healthy and taking no medications. Five patients had normal findings on nocturnal tumescence monitor, while 3 patients did not use the monitor. A mixture of phentolamine mesylate, 1.0 mg/mL, and papaverine hydrochloride, 30 mg/mL, was used. The dosage required was from 0.10 mL to 0.40 mL. All 8 patients responded successfully to this treatment. Three patients stated they were cured and stopped the treatment. The other 5 patients continued using the medication after fourteen months. The drug-induced erection lasted between two and four hours despite ejaculation. There have been no side effects through April, 1988. All patients report satisfaction with the results of this treatment. The study showed that intracavernous medication therapy can be successful in the treatment of premature ejaculation.

Intracavernous pharmacotherapy in psychogenic impotence.

Twenty-five men with psychogenic impotence but without serious psychopathology were considered for intracavernosal therapy with papaverine hydrochloride and phentolamine mesylate. A total of 20 proved suitable and began self-injection in conjunction with sex therapy; 8 patients had return of spontaneous erections without pharmacotherapy, although one of them needs to keep the medication in his refrigerator. The other 12 patients are continuing self-injection therapy. Psychotherapy with self-injection may be helpful in the management of psychogenic erectile impotence.

Mid-term results of autoinjection therapy for erectile dysfunction.

Of over 300 patients with erectile dysfunction, 186 were selected for intracavernosal autoinjection therapy with a standardized papaverine-phentolamine mixture. A total of 156 patients performed 4,813 protocol autoinjections with a minimum of 10 and a maximum of 230 per patient. The dose that induced a full erection at the hospital could be reduced under home conditions by a mean of 35 per cent. Systemic side effects were not observed. The most inconvenient local side effects were prolonged erections in 24 patients in diagnostic use and in 3 patients in therapeutic use. There were treated easily without further consequences.

Utility of Rigiscan and papaverine in diagnosis of erectile impotence.

The Rigiscan machine was used to measure nocturnal penile tumescence and rigidity in 41 men referred for evaluation of erectile impotence. In 6 patients, the Rigiscan was compared with the Tumistore and Snap-Gauge bands. Two patients achieved significant tumescence as measured by Tumistore, and 3 had significant tumescence as indicated by the breaking of two or three of the Snap-Gauge bands. None of these men achieved sufficient rigidity for intercourse as measured by the Rigiscan. Fourteen patients with organic impotence received intracorporeal injections of papaverine and were then monitored by Rigiscan. Six of the 14 were noted to have a Peyronie plaque. Of the remaining 8, 6 of these men achieved lasting erections sufficient for intercourse. The other 2 had erections that lasted ten minutes and were considered to have venous leak impotence. All patients with Peyronie disease had tumescence after papaverine injection, but in 5 the penis became rigid only at the base. Complications of papaverine injection were uncommon and minor. The Rigiscan and papaverine are useful in the diagnosis and management of erectile impotency.

Digital subtraction cavernosography: method to detect venous leakage.

Cavernosography has become an important diagnostic test for detecting venous leakage as a cause of vasculogenic impotence. Digital subtraction cavernosography (DSC) was carried out on 21 patients with a history of venous leakage resulting in impotence. The DSC technique was compared to conventional cineradiography. Major venous leaks were easily identified in 16 patients. DSC was able to detect minor leaks missed by cineradiography in 2 patients. DSC seems to be a reliable technique that is easy to perform. It should be done in conjunction with pharmacologically induced erection.

Effectiveness and safety of multidrug intracavernous therapy for vasculogenic impotence.

A four-drug vasoactive mixture (papaverine hydrochloride, prostaglandin E1, phentolamine mesylate, atropine sulfate) was used for intracavernous injection therapy in 94 patients with vasculogenic impotence as diagnosed by color Doppler sonography and dynamic infusion cavernosometry-cavernosography. At a mean follow-up of twenty months, 66 patients (70%) are using the injections with the initial dose and are satisfied; 14 patients (15%) are using the injections with a smaller dose than initially given; and 14 patients (15%) dropped intracavernous treatment. Only 4 patients (4%) were unable to achieve a sustained rigid erection during the mixture titration phase. Selected cases of vasculogenic impotence can be safely and effectively treated by the association of drugs which rely on different mechanisms of action, producing a pharmacologic synergism which enhances the overall therapeutic effect.

Experience with buccal phentolamine mesylate for impotence.

Satisfactory erection with penetration can be obtained in impotent men by the oral or buccal administration of the alpha-adrenergic antagonist, phentolamine. This agent is also used in conjunction with papaverine HCl for intracavernous injection. The previous observation by Gwinup, that 50 mg of phentolamine HCl po, 1.5 hours before coitus resulted in erection in 11/16 patients, is confirmed. This study, using phenoxybenzamine as the placebo, was repeated with success in 36/85 (42.3%) patients. Because of cost and to decrease the waiting time, a buccal form of phentolamine mesylate was administered (20 mg) with erection and penetration in 21/69 (31.8%). There was no correlation between the degree of penile vascular insufficiency or age and the effectiveness of phentolamine. Buccal phentolamine is shown to increase flow velocity in the dorsal penile artery. Phentolamine produces minimal side effects, including hypertension in the subjects.

Effects of clonidine on alpha-adrenoceptors in the autoperfused hindquarters of the pithed rat.

The effects of clonidine were assessed in pithed rats on responses to spinal cord stimulation of heart rate (HR), systemic blood pressure (BP) and perfusion pressure (PP) in the autoperfused hindquarters. Graded increases in HR, BP and PP were elicited by 10 s periods of spinal stimulation at frequencies of 1, 3 and 10 Hz; the vasoconstrictor responses (BP and PP) were biphasic. Clonidine (3-100 micrograms/kg) caused dose-dependent reductions of HR responses and of the first phase of BP and PP responses to spinal stimulation; the reductions were to a lesser extent with increasing frequency of stimulation. Responses to exogenous noradrenaline were unaffected by clonidine. In contrast, the second phase of the vasoconstrictor responses to 10 Hz stimulation was enhanced by clonidine (3-30 micrograms/kg): this phase of the responses is probably mediated by adrenal catecholamines. Clonidine itself (10-100 micrograms/kg) increased resting levels of BP and PP but not HR. The direct effects of clonidine as well as the effects on responses to spinal stimulation were blocked by phentolamine (1 mg/kg). The results indicate that clonidine selectively activates prejunctional alpha-adrenoceptors in the rat heart and hindquarters in vivo. The results also indicate that clonidine may potentiate the release of adrenal catecholamines, and it is suggested that it may do so by acting as an antagonist at inhibitory alpha-adrenoceptors in the adrenal medulla.

Vasoactive intracavernous pharmacotherapy.

The use of vasoactive intracavernous pharmacotherapy for the treatment of impotence is rapidly expanding because of its effectiveness and the rarity of short-term complications. To minimize complications, careful selection of candidates, cautious dosage determination, and thorough instruction on injection technique are mandatory. Sustained erection should be treated with alpha-adrenergic agents after 4 to 6 hours. Frequent follow-up is mandatory after home injection begins to detect the development of penile abnormalities. Longer follow-up and additional studies are required to determine the types and incidences of long-term adverse effects of the technique.

Lack of circadian variation in the responsiveness of alpha 2-heteroreceptors regulating serotonin release.

We have previously shown that the circadian variation in 5-HT release is not the consequence of a variation in the activity of terminal 5-HT1B autoreceptors. However, recently identified alpha 2-adrenoceptors located on 5-HT nerve terminals may be important in regulating the release of 5-HT from serotonergic neurons. The sensitivity of hippocampal alpha 2-heteroreceptors to both agonist and antagonist was determined at different time points in the light:dark cycle of the rat. No significant circadian differences were evident in either the apparent pD2 values calculated for noradrenaline to inhibit potassium-evoked tritium efflux or in the apparent pA2 values calculated for phentolamine to antagonize the effect of noradrenaline. The corollary of the lack of a circadian rhythm in sensitivity to the alpha 2-heteroreceptor is that this receptor population will accurately reflect any circadian variation in noradrenaline release and in the available concentration of noradrenaline at the receptor sites.

Increased dependency of cardiac pacemaker activity on extracellular Ca after adrenergic blockade in the frog heart.

The frog sinus venosus shows spontaneous regular pacemaker activity, even in the absence of extracellular Ca2+. When an alpha-adrenergic blocking agent (phentolamine) is applied, the rate of pacemaker activity, height of action potential, rate of slow diastolic depolarization, and the maximum diastolic potential become strongly dependent upon the extracellular Ca2+ concentration.

Electrical components contributing to the nerve-mediated contractions in the smooth muscles of the rabbit ear artery.

Adrenergic transmissions were investigated by recording electrical and mechanical responses of the smooth muscle cells in the rabbit ear artery. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization. The latter but not the former was suppressed by prazosin or phentolamine. Both the e.j.p. and slow depolarization were suppressed by tetrodotoxin (TTX) or guanethidine. Facilitation processes of e.j.p.s produced by repetitive stimulation of the nerves were not modified by prazosin, phentolamine, or yohimbine. Increasing the stimulus frequency increased the amplitude of e.j.p.s and slow depolarizations and, at high frequencies (greater than 5 Hz) generated a spike potential. Nicardipine (10(-7) M) blocked the spike potential and reduced the e.j.p. amplitude, but did not affect the slow depolarization. Amplitude of muscle contractions produced by transmural nerve stimulation increased with increase in the stimulus frequency. The nerve-mediated contractions produced by high-frequency stimulation (10 Hz) were reduced to 49% of the control value by prazosin (10(-6) M), to 79% by nicardipine (10(-7) M), to 34% by prazosin (10(-6) M) plus nicardipine (10(-7) M), and to 1.2% by TTX (3 X 10(-7) M). Exogenously applied noradrenaline depolarized the smooth muscle membrane and produced the muscle contraction. These effects of noradrenaline were antagonized by prazosin or phentolamine. Thus, in the rabbit ear artery, perivascular nerve stimulation produced three types of electrical responses, i.e., e.j.p., spike potential, and slow depolarization. The latter but not the former two was produced through activation of alpha 1-adrenoceptors. Nerve-mediated muscle contractions were the results of stimulation of alpha 1-adrenoceptors, generation of spike potentials, and of e.j.p.s.

Ultrastructural study of the effect of acute hypertension on the stria vascularis and spiral ligament.

The effect of acute hypertension, induced in rats by intravenous injection of methoxamine chloride (Mexan), on the stria vascularis and spiral ligament was studied electronmicroscopically with the tracer method of horseradish peroxidase (HRP). Considerable extravasation of HRP occurred in the stria vascularis, due to the increased vesicular transport. The leaked HRP spread into intercellular spaces, but was prevented from spreading towards the endolymph by zonulae occludentes between marginal cells and towards the perilymph by zonulae occludentes between basal cells. The reaction product was occasionally found between basal cells. No leakage of HRP from capillaries was observed in the spiral ligament, although some labelled micropinocytotic vesicles were present in the endothelium. It is suggested that, under acute hypertensive conditions, areas of zonulae occludentes bordering the stria vascularis play an important role as a barrier to HRP, whereas capillaries in the spiral ligament themselves act as a barrier to it.

[Corpus cavernosum autoinjection therapy: initial experiences in erectile dysfunction]. / Schwellkörper-Autoinjektionstherapie (SKAT): erste Erfahrungen bei erektiler Dysfunktion.

The cause of erectile dysfunction was evaluated by a multidisciplinary approach in 120 impotent patients. Out of these, the CCAT was proposed to 20 patients with vasculogenic erectile dysfunction, 19 of them accepted this therapy. The dosage of phentolamine mesylate-papaverin hydrochloride injection required was determined in each patient individually. Usually we injected initially 1 cc in one corpus cavernosum. Eighteen patients followed the regimen successfully at home. In 2 patients prolonged erections were observed, which were successfully relieved by simple corpus puncture, however, discontinuation of therapy was not necessary. CCAT failed in 1/19 patients with a severe venous leak. Follow up was carried out weekly.

[Metaraminol--an antidote in corpus cavernosum autoinjection therapy-induced prolonged erection]. / Metaraminol--ein Antidot bei SKAT-bedingter prolongierter Erektion.

Up to now 65 patients with erectile dysfunction were treated by means of corpus cavernosum-autoinjection-therapy (CAT). The only side effect observed were 6 prolonged erections, which could successfully be treated in 3 cases by puncture and aspiration of the corpora cavernosa. In another 3 cases prolonged erection subsided after an intracavernous injection of 2 mg metaraminol. Erection induced by CAT was immediately interrupted by metaraminol in 10 other patients.