The adenosine inhibition of glutamate exocytosis in synaptosomes is removed by the collapse of the vesicle-cytosol deltapH plus the opening of farnesol-sensitive Ca(2+) channels.

Adenosine inhibits synaptosomal exocytosis of glutamate, triggered by KCl or by the K(+) channel inhibitor, 4-aminopyridine (4-AP), without affecting Ca(2+) influx. Its effect is removed by the activation of protein kinase C (PKC). We show that in the presence of the protein kinase inhibitor, staurosporine, the adenosine inhibition is removed also by collapsing deltapH between secretory vesicle and the cytosol with methylamine (MA), provided that exocytosis is triggered by KCl (which activates an initial transient spike of Ca(2+) influx) but not by 4-AP. If KCl is supplied prior to Ca(2+), the spike of Ca(2+) influx is absent and the adenosine inhibition is maintained. MA can remove the adenosine inhibition also with 4-AP, provided that tetraethylammonium (TEA), an inhibitor of a different class of K(+) channels, is supplied together with 4-AP. TEA promotes a further increase of cytosolic free Ca(2+) concentration ([Ca(2+)](i)), which adds to the 4-AP-induced Ca(2+) influx. Farnesol (5-10 microM), a physiological derivative of farnesyl pyrophosphate of the sterol biosynthetic pathway, specifically inhibits the Ca(2+) spike after KCl as well as the TEA-promoted Ca(2+) increase. At the same time, it prevents the removal of the adenosine inhibition by MA. We conclude that the adenosine inhibition is removed by the coincidence of two signals, the alkalinization of secretory vesicles and the opening of a particular class of Ca(2+) channels associated to the TEA-sensitive K(+) channels, equivalent to the Ca(2+) spike after KCl, and sensitive to farnesol.