Activation of phospholipase C via adenosine receptors provides synergistic signals for secretion in antigen-stimulated RBL-2H3 cells. Evidence for a novel adenosine receptor

Abstract

5'-(N-Ethyl)carboxamidoadenosine (NECA), an analog of adenosine, transiently stimulated a rat tumor mast cell (RBL-2H3 cells) to cause a release of inositol phosphates and an increase in levels of Ca2+ in the cytosol. It failed, however, to stimulate a sustained uptake of 45Ca2+ or secretion. The effects of other agents that act on P1- or P2-purinergic receptors suggested that NECA and other adenosine agonists acted via a novel subtype of adenosine membrane receptor. Although the order of potency of agonists was characteristic of A2-adenosine receptors, there was no indication of the involvement of adenylate cyclase, and antagonists such as isobutylmethylxanthine, 8-phenyltheophylline, and 8-p-sulfophenyltheophylline inhibited the responses to either NECA or antigen. The fact that stimulation of inositol phospholipid hydrolysis by NECA in washed, permeabilized RBL-2H3 cells was blocked by pertussis toxin as well as by cholera toxin suggested instead that the NECA-sensitive receptor activated phospholipase C via a G-protein. In contrast to NECA, antigen stimulation resulted in a pertussis toxin-resistant, sustained hydrolysis of inositol phospholipids, increases in free intracellular Ca2+, accelerated influx of 45Ca2+, and secretion from RBL-2H3 cells. In combination with NECA, all responses to antigen were markedly enhanced, and the enhancement was selectively blocked by pertussis toxin. The ability of antigen, but not NECA, to provoke secretion may be dependent primarily on the sustained activation of a cholera toxin-sensitive Ca2+ influx pathway that serves to amplify stimulatory signals for secretion. These studies also suggested that phospholipase C could be activated through different G-proteins via different receptors within the same cell.