Two roles for guanine nucleotides in the stimulus-secretion sequence of neutrophils

The term 'stimulus-secretion coupling' has, since first enunciated, been held to involve the mobilization of cytosol Ca2+, which in turn is sufficient to trigger exocytotic secretory processes in metabolically competent cells. However, recent studies on a wide range of secretory cell types indicate that a role for Ca2+ can be obviated: examples are stimulation with phorbol ester (phorbol myristate acetate, PMA) which causes the activation of protein kinase C or the stimulation of platelets with collagen. Ca2+-independent exocytosis also occurs when analogues of GTP are injected through the lumen of patch pipettes directly into the cytosol of mass cells. The results presented here suggest that GTP analogues can activate secretory processes by actions at two distinct locations: one may be at the level of the receptor involving the activation of polyphosphoinositide (PPI) phosphodiesterase with consequent liberation of diacylglycerol (DG); the other involves direct activation of the exocytotic mechanism. These conclusions are based on measurements of exocytotic secretion from permeabilized neutrophils into which we have been able to introduce, individually and in combination, Ca2+ chelators (EGTA and BAPTA), Ca2+ (buffered at micromolar concentrations with EGTA), analogues of GTP and GDP and the direct activator of protein kinase C, PMA.