NONSELECTIVE EFFECTS OF THE PUTATIVE PHOSPHOLIPASE-C INHIBITOR, U73122, ON ADENOSINE A(1) RECEPTOR-MEDIATED SIGNAL-TRANSDUCTION EVENTS IN CHINESE-HAMSTER OVARY CELLS

Adenosine A(1) receptors can signal, through G(i/o) proteins, to inhib it adenylyl cyclase activity and also to stimulate phosphoinositide hy drolysis and the subsequent release of intracellular Ca2+ stores. The aminosteroid U73122 (1-[6-[[17 10)-trien-17-yl]amino]hexyl]-1H-pyrrole -2,5-dione) has been widely used as an inhibitor of phospholipase C, t he enzyme mediating phosphoinositide hydrolysis. Using U73122, we soug ht to selectively block signalling through the phospholipase C pathway , in Chinese hamster ovary (CHO-K1) cells heterologously expressing hu man adenosine Al receptors. U73122 inhibited A(1) receptor-mediated ph osphoinositide hydrolysis, as measured by total inositol phosphate acc umulation, over the concentration range 1-15 mu M. However, over the s ame concentration range, it also appeared to inhibit A(1) receptor-med iated inhibition of forskolin-stimulated cyclic AMP accumulation, A(1) receptor agonist-promoted [S-35]GTP gamma S binding, and at the highe r concentrations (10-15 mu M) produced marked morphological changes, l eading to cytolysis. The structural analogue of U73122, U73343 (1-[6-[ [17 0-trien-17-yl]amino]hexyl]-2,5-pyrrolidone-dione), typically used as an inactive control compound, had little effect on these events. Th e data suggest that U73122 is not a selective inhibitor of phospholipa se C activity, interfering with adenosine A(1) receptor signalling gen erally, either at the pre-effector level involving G(i/o) proteins, or as a consequence of the morphological changes it induces. BIOCHEM PHA RMACOL 56;11:1455-1462, 1998. (C) 1998 Elsevier Science Inc.