2 ENDOTHELIN RECEPTORS (ET(A) AND ET(B)) EXPRESSED ON CIRCULAR SMOOTH-MUSCLE CELLS OF GUINEA-PIG CECUM

Background/Aims: The functional receptors for endothelin (ET) in colon ic smooth muscle are still unknown. This study investigated the expres sion of ET receptors in isolated circular smooth muscle cells of guine a pig cecum. Methods: Inhibition of I-125-ET-1 binding was examined us ing unlabeled ET-1, ET-2, ET-3, sarafotoxin 6c (S6c), and ET(A) antago nists. Expression of the ET-receptor message was investigated using re verse-transcription polymerase chain reaction. The contractile potency of the ET family and the inhibitory effect of ET(A) antagonists on ET -1-induced contraction were also investigated. Results: Unlabeled ET-1 , ET-2, and ET-3 inhibited the specific binding of I-125-ET-1 in a con centration-dependent manner, but the inhibitory effect of ET-3 was sma ller than those of ET-1 acid ET-2. At a 10(-6) mol/L concentration of S6c, the specific binding of I-125-ET-1 was 24.7%. S6c had clearly rea ched maximal inhibition. Abundant polymerase chain reaction products f or both the ET(A) and the ET(B) message were observed. ET-1 and ET-2 s howed similar contractile potency, but ET-3-induced and S6c-induced co ntractions were significantly less potent than the ET-1-induced contra ction. A significant response to S6c was obtained at a concentration a s low as 10-(10) mol/L. The ET(A) antagonists BQ-123 and FR 139317 sig nificantly inhibited ET-1-induced contraction. Conclusions: The result s show a direct contractile effect of ETs on circular smooth muscle of guinea pig cecum and the presence of both ET(A)- and ET(B)-receptor s ubtypes.