Atypical -Adrenoceptor Subtypes Mediate Relaxations of Rabbit Corpus Cavernosum

This study was performed to characterize the -adrenoceptor population in rabbit isolated corpus cavernosum (RbCC) by using nonselective and selective -adrenoceptor agonists and antagonists in functional assays. Metaproterenol, ritodrine, fenoterol, and 8-hydroxy-5-[(1R)-1-hydroxy-2-[N-[(1R)-2-( -methoxyphenyl)-1-methylethyl]amino]ethyl]carbostyril (TA 2005) (3–100 nmol each) dose dependently relaxed the RbCC preparations. These relaxations were markedly reduced by N -nitro-L-arginine methyl ester (L-NAME; 10 M) and 1H-[1,2,4]-oxadiazolo-[4,3,a]quinoxalin-1-one (ODQ) (10 M), whereas the adenylyl cyclase inhibitor SQ 22,536 [9-(2-tetrahydrofuryl) adenine] (10 M) had no effect. In contrast, neither L-NAME nor ODQ affected the isoproterenol-induced RbCC relaxations, but SQ 22,536 abolished this response. Sildenafil (1 M) significantly potentiated the relaxations induced by 2-agonists without affecting the isoproterenolevoked relaxations. Rolipram (10 M) enhanced the relaxations elicited by isoproterenol but had no effect on those induced by the selective 2 agonists. Propranolol and ( )-1-[2,3-(dihydro-7methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride (ICI 118,551) determined a rightward shift in the concentration-response curves to isoproterenol in a noncompetitive manner with a reduction of maximum response at the highest antagonist concentration, with the slope values significantly different from unity. Propranolol and ICI 118,551 had no effect on the relaxations elicited by fenoterol, TA 2005, metaproterenol, and ritodrine. Atenolol and 1-[2-((3-carbamoyl-4-hydroxy)phenoxy) ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]-2-propanol methanesulfonate (CGP 20712A) (0.1–10 M) failed to affect the relaxations induced by all tested -adrenoceptor agonists. Our study revealed the existence of two atypical -adrenoceptors in the rabbit erectile tissue. Isoproterenol relaxes the rabbit cavernosal tissue by activating atypical -adrenoceptors coupled to adenylyl cyclase pathway, whereas the selective 2-adrenoceptor agonists relax the RbCC tissue through another atypical -adrenoceptor subtype coupled to nitric oxide release from the sinusoidal endothelium. The erectile tissue is contained within the corpora cavernosa and consists of endothelium-lined sinusoidal spaces surrounded by smooth muscle bundles. Penile erection, which follows arterial and corpus cavernosum smooth muscle relaxation, is regulated by a sequence of coordinated physiological, neurological, and vascular events (Lue, 2000). The pattern of contraction and relaxation of penile cavernosal smooth muscle is complex and regulated by sympathetic, parasympathetic, and nonadrenergic noncholinergic fibers, and by endothelium-derived vasoactive substances that diffuse to the underlying muscle and influence smooth muscle tone (Andersson and Wagner, 1995). Activation of adrenergic receptors in corpus cavernosum produces either contractile response mediated by -adrenoceptors (Diederichs et al., 1990; Costa et al., 1993) or relaxant responses mediated by -adrenoceptors (Carati et al., 1985; Dhabuwala et al., 1985; Hedlund and Andersson, 1985; Recio et al., 1997). The participation and characterization of 1-adrenoceptors by using selective agonists and antagonists are well established (Traish et al., 1999). Activation of -adrenoceptors is involved in maintenance of corpus cavernosum tone in the flaccid state and suppression of erectile activity (Andersson et al., 2000). The relaxant response mediated by -adrenoC.E.T. is supported by Fundação de Amparo à Pesquisa do Estado de São Paulo. Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. DOI: 10.1124/jpet.103.062026. ABBREVIATIONS: RbCC, rabbit corpus cavernosum; TA 2005, 8-hydroxy-5-[(1R)-1-hydroxy-2-[N-[(1R)-2-( -methoxy-phenyl)-1-methylethyl]amino]ethyl]carbostyril; BRL 37344, ( )-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid; L-NAME, N -nitro-L-arginine methyl ester; ODQ, 1H-[1,2,4]-oxadiazolo-[4,3,-a]quinoxalin-1-one; SQ 22,536, 9-(2-tetrahydrofuryl) adenine; CGP 20712A (1-[2-((3-carbamoyl4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]-2-propanol methanesulfonate); ICI 118,551, ( )-1-[2,3(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride; GTN, glyceryl trinitrate; NO, nitric oxide; ACh, acetylcholine; TTX, tetrodotoxin; PDE, phosphodiesterase. 0022-3565/04/3092-587–593$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 309, No. 2 Copyright © 2004 by The American Society for Pharmacology and Experimental Therapeutics 62026/1143746 JPET 309:587–593, 2004 Printed in U.S.A. 587 at A PE T Jornals on N ovem er 4, 2017 jpet.asjournals.org D ow nladed from ceptors in corpus cavernosum is poorly studied and the -adrenoceptor subtypes involved in this response are still a matter of controversy. An early study showed that -adrenoceptor agonists isoproterenol and salbutamol cause relaxation of human cavernosal preparations that is blocked by nonselective -adrenoceptor antagonist (propranolol), but not by 1(practolol) or 2 (butoxamine)-receptor antagonists, thus suggesting the existence of atypical -adrenoceptors in this tissue (Adaikan and Karim, 1981). Other studies suggested that -adrenoceptors in human (Dhabuwala et al., 1985; Hedlund and Andersson, 1985; Cirino et al., 2003) and canine (Carati et al., 1985) corpus cavernosum are of the 2or 3-subtypes. On the other hand, a mixed 1and 2adrenoceptor population predominantly mediates the corporeal relaxations in the horse (Recio et al., 1997). Therefore, the purpose of the present study was to characterize the population of -adrenoceptors that mediate the relaxation of rabbit cavernosal tissue by using selective and nonselective -agonists and antagonists, in both bioassay cascade and organ bath experiments. Materials and Methods Isolation and Preparation of Rabbit Corpus Cavernosum (RbCC). Male New Zealand White rabbits (2.5–3.0 kg) were anesthetized with pentobarbital sodium (Sagatal, 30–40 mg/kg i.v.) and exsanguinated via the carotid artery. After penectomy, the RbCC was rapidly removed and immersed in Krebs’ solution of the following composition: 118 mM NaCl, 25 mM NaHCO3, 5.6 mM glucose, 4.7 mM KCl, 1.2 mM KH2PO4, 1.17 mM MgSO4 7H2O, 2.5 mM CaCl2 2H2O. Tissues were dissected and cleared of the tunica albuginea and surrounding tissues. All procedures were designed in accordance with the guidelines for animal care of the State Univer-