STIMULATION OF CA-2+-INDEPENDENT CATECHOLAMINE SECRETION FROM DIGITONIN-PERMEABILIZED BOVINE ADRENAL CHROMAFFIN CELLS BY GUANINE-NUCLEOTIDE ANALOGS - RELATIONSHIP TO ARACHIDONATE RELEASE

The effect of GTP analogues on catecholamine secretion and [3H]arachidonic acid release from digitonin-permeabilized adrenal chromaffin cells was examined. Several GTP analogues stimulated Ca2+-independent exocytosis, with the order of efficacy being XTP > ITP > guanosine 5'-[βγ-imido]triphosphate (p[NH]ppG) > guanosine 5'-[γ-thio]triphosphate (GTP[S]). The stimulatory effect of the GTP analogues appeared to be due to activation of a conventional GTP-binding protein, as it was inhibited by guanosine 5'-[β-thio]diphosphate (GDP[S]). In contrast, Ca2+-dependent exocytosis was only partially inhibited by high doses of GDP[S]. GTP did not stimulate Ca2+-independent exocytosis, but instead was found to inhibit secretion caused by micromolar Ca2+. Arachidonic acid (100 μM) also stimulated Ca2+-independent catecholamine secretion. Determination of the effect of GTP analogues on release of free [3H]arachidonic acid into the medium showed that it was stimulated by GTP[S] but inhibited by GTP, p[NH]ppG, ITP and XTP. The inhibition of [3H]arachidonic acid release by XTP was not prevented by GDP[S]. These results demonstrate that activation of a GTP-binding protein by certain GTP analogues can induce Ca2+-independent secretion in adrenal chromaffin cells and that the effect of GTP analogues on Ca2+-independent secretion can be dissociated from generation of arachidonic acid.