REGULATION OF ION-TRANSPORT BY ENDOTHELINS IN RAT COLONIC MUCOSA - EFFECTS OF AN ET(A) ANTAGONIST (FR139317) AND AN ET(B) AGONIST (IRL1620)

Rat colonic mucosa contains ET(A) and ET(B) receptors with Kd values f or endothelin (ET)-1 of 32 and 11 pM and maximal binding capacities of 277 and 181 fmol/mg protein, respectively. In muscle-stripped rat col on without tonic nerve activity in Ussing chambers, the serosal additi on of ET-1, ET-3 and IRL1620 inhibited amiloride-sensitive noncoupled Na+ entry and enhanced diphenylamine-2-carboxylate-sensitive Cl- secre tion, producing a sustained decrease and a transient increase in the s hort-circuit current (Isc) and the transepithelial conductance, respec tively. EC(50) values of ET-1, ET-3 and IRL1620 and the maximal change s in Isc were 2.0, 10.2 and 10.9 nM and -12.7, -7.0 and -7.1 mu A/cm(2 ), respectively for the Na+ entry; these values were 50, 220 and 225 n M and +57.3, +47.3 and +21.3 mu A/cm(2), respectively, for the Cl- sec retion. FR139317 (100 nM) inhibited ET-1-induced Na+ and Cl- movements , shifting the concentration-response curves to the right (EC(50) = 25 nM and 1 mu M, respectively), and inhibited ET-3 (>100 nM)-induced Cl - movement, decreasing the maximal response to 35%, but it did not inh ibit either ET-3-induced Na+ movement nor IRL1620-induced Na+ and Cl- movements. The removal of serosal Ca++ reduced 100 nM ET-1- and IRL162 0-evoked changes in Isc by 50% and 70% for the Na+ entry and by 80% an d 100% for the Cl- secretion, respectively. Indomethacin (1 mu M) also reduced changes in Isc by 30% and 70% for the Cl- secretion but did n ot affect the Na+ entry. Our results show that ET(A) and ET(B) recepto rs regulate Na+ and Cl- transport by different mechanisms.