The whole-cell patch-clamp technique was used to measure cell membrane capacitance (C m) to monitor exocytosis in single-cultured bovine prolactin-secreting cells (lactotrophs) of the anterior pituitary. The cells were dialyzed with solutions containing different concentrations of ionised Ca and non-hydrolyzable GTP analogues (GTP-γ-S and GMP-PNP) to activate G-proteins. We have identified two distinct effects of G-protein activation on Ca-induced exocytosis: (i) the maximum C m increase due to intracellular Ca-dependent exocytosis was diminished, suggesting an inhibitory role of G-proteins close to the site of granule fusion, while (ii) the rate of C m increase (ΔC m/Δt) was facilitated, revealing conversely a stimulatory role of G-proteins in the translocation of secretory granules to the fusion sites.
