Regional differences in the functional and biochemical properties of endothelin receptor subtypes in the rabbit prostatic urethra

OBJECTIVE To examine the regional differences in the functional (pharmacological) and biochemical properties of endothelin (ET) receptors in the rabbit prostatic urethra. MATERIALS AND METHODS The properties of ET receptors in 6‐month‐old male rabbit prostatic urethras were examined using isolated muscle‐bath and radioligand receptor‐binding techniques. Using plasma membrane suspensions, saturation and inhibition experiments with [125I]ET‐1 and unlabelled agonists and antagonists (ETA‐selective antagonist BQ123, and ETB‐selective agonist sarafotoxin 6c, STX6c) were done to determine the ET receptor densities and their subtype specificities in the different regions of the urethra. RESULTS The ETs (ET‐1 and ET‐3) produced significant concentration‐dependent contractile responses in the smooth muscle strips from the different regions of the urethra. Although the maximum contractile responses induced by ET‐1 were similar in the different regions, the maximum contractile responses induced by ET‐3 were greater in the distal region than in the proximal or middle regions, suggesting that the contractile response to ET‐1 is more potent than that to ET‐3 in all regions, and that there are region‐specific differences in the responses to ET‐3 but not ET‐1. Moreover, the ET‐3‐induced contractile response was suppressed by BQ788 (a selective antagonist of the ETB receptor) suggesting that the ETB receptor subtype contributes to the contractile responses mediated by ET‐3. The ET receptors were expressed in higher concentrations in the distal than in the proximal or middle regions. BQ123 and STX6c inhibited [125I]ET‐1 binding in all regions with high and low affinity constants, indicating the presence of both ETA and ETB receptor subtypes. The proportions of high‐affinity binding sites for BQ123, representing ETA receptors, were ≈ 68%, 63% and 42% in the proximal, middle and distal regions, respectively. By contrast, the proportions of high‐affinity binding sites for STX6c, representing ETB receptors, were ≈ 27%, 35% and 52% in the proximal, middle, and distal regions, respectively. These data indicate the presence of regional differences in the densities and subtype specificities of ET receptor subtypes, and the existence of regional differences in the rabbit prostatic urethra. CONCLUSION The results suggest regional differences in ETB receptor subtypes that mediate contractile responses to ET‐3, reflecting differences in the densities and specificities of the ET recepto