Protein-tyrosine kinases activate while protein-tyrosine phosphatases inhibit L-type calcium channel activity in pituitary GH3 cells.

The aim of this study was to evaluate the effect of protein-tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP) inhibitors on Ca2+ channels in GH3 cells. The activity of Ca2+ channels was monitored either by single-cell microfluorometry or by the whole-cell configuration of the patch-clamp technique. Genistein (20-200 micron) and herbimycin A (1-15 micron) inhibited [Ca2+]i rise induced either by 55 mM K+ or 10 micron Bay K 8644. In addition, genistein and lavendustin A inhibited whole-cell Ba2+ currents. By contrast, daidzein, a genistein analogue devoid of PTK inhibitory properties, did not modify Ca2+ channel activity. The inhibitory action of genistein on the [Ca2+]i increase was completely counteracted by the PTP inhibitor vanadate (100 micron). Furthermore, vanadate alone potentiated -Ca2+-i response to both 55 mM K+ and 10 micron Bay K 8644. The possibility that genistein could decrease the [Ca2+]i elevation by enhancing Ca2+ removal from the cytosol seems unlikely since genistein also reduced the increase in fura-2 fluorescence ratio induced by Ba2+, a cation that enters into the cells through Ca2+ channels but cannot be pumped out by Ca2+ extrusion mechanisms. Finally, in unstimulated GH3 cells, genistein caused a decline of [Ca2+]i and the disappearance of [Ca2+]i oscillations, whereas vanadate induced an increase of [Ca2+]i and the appearance of [Ca2+]i oscillations in otherwise non-oscillating cells. The present results suggest that in GH3 cells PTK activation causes an increase of L-type Ca2+ channel function, whereas PTPs exert an inhibitory role.