Antihypertensive therapy: Alpha-blockers
Alpha-blockers
Phenoxybenzamine
This is an a-adrenergic blocker that impedes the action of noradrenaline at the neuroeffector junction. It also inhibits the reuptake of catecholamines. The main sexual effects are related to ejaculation. Both failure of emission and retrograde ejaculation have been described, although this latter phenomenon has only been suggested in one report, which described the passage of white urine following orgasm. The incidence of ejaculation disorders varies widely between 4.5 and 100%. Problems have been noted at doses of 5-70mg/day (Buffum, 1982; 1992). The incidence of side effects are highest with high doses. It is likely that the effects on ejaculation are related to inhibition of a-adrenoceptors in the prostate, seminal vesicles and epididymis, which are involved in the production and transport of seminal fluid. Disorders of ejaculation also occur with indoramin, which has a similar mechanism of action to phenoxybenzamine.
Prazosin
Prazosin is another a-adrenergic blocker acting by competitive antagonism at the postsynaptic cxj-adrenoceptor. Failure of ejaculation has been reported but is rare compared with that seen with phenoxybenzamine (only 0.6 to 4%). Unspecified sexual disorders occurred in 9% of patients in one study. This can occur at doses between 3 and 20 mg/day (Buffum, 1982). In a review of over 900 men on prazosin, impotence was only recorded in 0.6% (Smith & Talbert, 1986). In one study, 19 diabetic hypertensive individuals with sexual dysfunction taking clonidine or methyldopa were changed to prazosin; 79% had normal or improved sexual function after 2 months. To rule out a placebo effect, this was later shown to correlate with objective measurements of nocturnal penile tumescence
(Lipson, 1984).
Priapism has also been described in several reports, occurring one to three months after starting therapy and resolving when prazosin was discontinued. Presumably this is secondary to a blockade-induced vasodilatation, leading to increased blood flow to the corpora cavernosum. Several of the reports of prazosin-induced priapism have been in patients with renal failure. It has been suggested that the hypoproteinaemia accompanying uraemia may result in an decrease in the fraction of drug bound to plasma proteins, thereby increasing the active free fraction and potentiating the effect. However, prazosin has been recommended as a good second-line antihypertensive in sexually active individuals.
Treatment premature ejaculation in Canada – www.canadianhealthcaremalll.com.