ENDOTHELIN RECEPTOR IN OSTEOBLASTIC CELLS IS COUPLED TO MULTIPLE MESSENGER SIGNALS

We analyzed the functional characteristics of endothelin (ET) peptides in the osteoblastic UMR-1O6 cells by studying receptor binding as well as dose-response curves for ET-1 and ET-3 on two biological responses: 1) induction of Ca2+ transients and 2) activation of the Na+-H+ exchanger. ET specifically binds to a single class of receptor with a rank order of affinity ET-1 >> ET-3. ET-1 and ET-3 dose dependently stimulated a rise in intracellular Ca2+ ([Ca2+](i)), with ET-1 being two orders of magnitude more potent than ET-3 [50% effective concentration (EC(50)) = 8 X 10(-10) and 9 x 10(-8) M for ET-1 and ET-3, respectively; P < 0.01]. The effect of ET-1 on [Ca2+](i) was 90% inhibitable by the ET(A) antagonist BQ-123. The activity of Na+-H+ exchange was studied by using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein as well as by Na-22(+) fluxes. ET-1 and ET-3 activated the exchanger in a concentration-dependent manner and with similar potencies (EC(50) similar to 10(-10) M). The action of ETs on Na+-H+ exchange was mimicked neither by phorbol esters nor by Ca2+ ionophores. It was, however, blocked by BQ-123 as well as by the protein tyrosine kinase inhibitor genistein. We conclude that in UMR-106 cells, a single ET receptor subtype is coupled to multiple effecters, a Ca2+ message system and a tyrosine-kinase system which, in turn, activates the Na+-H+ exchanger.