CONTRACTILE EFFECTS OF ENDOTHELIN-1 AND LOCALIZATION OF ENDOTHELIN BINDING-SITES IN RABBIT LOWER URINARY-TRACT SMOOTH-MUSCLE

In isolated rabbit bladder and urethral smooth muscle, endothelin-1 caused concentration-related, slowly developing contractions that were difficult to wash out. Relative to contractions induced by K+ (124 mM), contractions in bladder preparations reached a higher amplitude than in urethral preparations. There was a marked tachyphylaxis to the effects of the peptide. The endothelin-1-induced contractions were not significantly affected by phentolamine or indomethacin in the urethra, or by scopolamine or indomethacin in the bladder. Incubation for 30 min in a Ca2+-free solution abolished the endothelin-1-induced contractions. Nifedipine did not affect the actions of endothelin-1 in the urethra but had a marked inhibitory action on its effects in the bladder. In the presence of endothelin-1, Ca2+-induced contractions were significantly blocked by nifedipine in the bladder but not in the urethra. Urethral preparations at resting tension responded to electrical stimulation by tetrodotoxin-sensitive, frequency-dependent contractions sensitive to α-adrenoceptor blockade. Pretreatment with endothelin-1 (10-9 M) produced a significant increase in the nerve-induced contractions but had no significant effect on contractions induced by exogenous noradrenaline. Endothelin-1 did not affect spontaneous or stimulation-induced efflux of 3H-labelled noradrenaline in urethral smooth muscle. Preparations contracted by endothelin-1 were frequency-dependently relaxed by electrical stimulation. The peptide had no significant effect on the responses induced by electrical stimulation in thebladder preparations. In both bladder and urethra, [125I]endothelin-1 binding sites were found mainly in the outer longitudinal muscle layer, in vessels and in the submucosa. The highest density of binding sites appeared to be in vessels and the outer muscle layer in both types of muscle. The results suggest that in the rabbit both bladder and urethral smooth muscle contain binding sites for endothelin. The peptide has contractant effects dependent on extracellular calcium in both types of tissue, but voltage-operated calcium channels seem to be involved in activation only of bladder smooth muscle. The functional importance of endothelin-1 in the rabbit lower urinary tract remains to be elucidated.