Endothelin-1 as a Putative Modulator of Erectile Dysfunction: I. Characteristics of Contraction of Isolated Corporal Tissue Strips

Normal erectile physiology is heavily dependent on a delicate balance between the effects of endogenous vasoconstricting and vasorelaxing hormones on the tone of the corporal smooth muscle. Recent studies indicate that endothelin-1 (ET-1) is present and physiologically active in the human corpora. The primary goal of the present investigation was to further define the role of ET-1 in corporal physiology and to ascertain whether it might play a role in augmenting corporal tone in vivo, as reported in other vascular tissues. Thus, we conducted pharmacological studies of ET-1-induced steady-state contractions in isolated human corporal smooth muscle strips to determine if there were any detectable age- or diabetes-related alterations in ET-1-induced contractions. For statistical analysis, the patient population was divided into 2 age groups, A (less or equal to 59 years of age; = 11 patients) and B (greater or equal to 60 years of age; n = 7 patients), and further subdivided into 2 diagnostic categories, diabetic (n = 7 patients) and nondiabetic (n = 11 patients). Construction of cumulative concentration response curves (CRCs) for ET-1-induced contractions revealed characteristically slow onset and long-lasting responses. Endothelin-1 CRC data were computer fit to the logistic equation to derive E_max (calculated maximal response), pEC₅₀ (negative logarithm of the concentration that elicits one-half of the calculated maximal response) and slope factor (n) values. Two-factor analysis of variance revealed no detectable age- or diabetes-related alterations, nor any age-diabetes interaction in any of the logistic parameters. Furthermore, logistic analysis of ET-1 CRC data on 14 isolated corporal tissue strips derived from 3 potent patients with documented spontaneous erections revealed no differences in ET-1 contractility from that observed for patients with organic erectile dysfunction. Importantly, despite an apparent absence of age- or diabetes-related alterations in ET-1-induced steady-state contractions, preliminary studies demonstrated that coadministration of the alpha₁ -adrenergic agonist phenylephrine and ET-1 produce much greater contractile responses that those observed in the presence of phenylephrine (PE) alone. Moreover, the magnitude of the augmentation was precisely that predicted by a model for simple additivity of agonist effects. Such observations suggest that the physiological relevance of ET-1 to corporal physiology may be related to its ability to augment the contractile responses of other vasomodulators present in the human corpora, in particular, perhaps modulating the contractile responses to sympathetic activity.