Physiological effects of macrocycle 1 on the rabbit corpus cavernosum.

When the penis is in the flaccid state, the corpus cavernosum smooth muscle is under adrenergic control in order to maintain the sinusoids in a contracted condition (primarily via alpha-adrenergic stimulation). Penile erection is mediated by relaxation of corporal smooth muscle through a variety of direct and indirect mechanisms. Pharmacological agents that either inhibit alpha-adrenergic transmission or facilitate corporal smooth muscle relaxation will be beneficial for penile erection. Preliminary studies on macrocycle 1, a novel bioactive agent, demonstrated that this compound inhibits receptor-stimulated contraction without significantly altering field-stimulated contraction. The current study was designed to determine the physiological effects of macrocycle 1 on the response of the rabbit corpus cavernosum to various forms of stimulation. Sexually mature male New Zealand White rabbits were used in this study. The corpus cavernosum was dissected sharply from the removed penis and two longitudinal strips were prepared for isometric tension studies. Each strip was prestimulated with 200 mumol/l phenylephrine to produce a maximal contraction. The influences of macrocycle 1 (15.6, 62.5, 250, 1,000 mumol/l) on both the contractile response to phenylephrine and the relaxant effects of field stimulation, carbachol, ATP and nitroprusside were determined. The results demonstrated that although basal tension was not altered, the contractile response to phenylephrine was decreased by 5, 18, 47 and 57%, respectively, by the different concentrations of macrocycle 1. The degree of relaxation induced by field stimulation of various frequencies was significantly enhanced by macrocycle 1 in a concentration-dependent manner. Similarly, the degrees of relaxation induced by bethanechol, ATP and nitroprusside were all significantly enhanced in a concentration-dependent manner. The conclusion from this study is that macrocycle 1 significantly inhibits phenylephrine-stimulated contraction and significantly enhances field-stimulated and pharmacologically induced relaxation. Although the mechanisms of action are not clear, its physiological effects on the rabbit corpus cavernosum implicate a potential for the treatment of erectile dysfunction.