PROTEIN-TYROSINE KINASES ACTIVATE WHILE PROTEIN-TYROSINE PHOSPHATASESINHIBIT L-TYPE CALCIUM-CHANNEL ACTIVITY IN PITUITARY GH(3) CELLS

The aim of this study was to evaluate the effect of protein-tyrosine k inase (PTK) and protein tyrosine phosphatase (PTP) inhibitors on Ca2channels in GH(3) cells, The activity of Ca2+ channels was monitored e ither by single-cell microfluorometry or by the whole cell configurati on of the patch-clamp technique, Genistein (20-200 mu M) and herbimyci n A (1-15 mu M) inhibited [Ca2+](i) rise induced either by 55 mM K+ or 10 mu M Bay K 8644, In addition, genistein and lavendustin A inhibite d whole-cell Ba2+ currents, By contrast, daidzein, a genistein analogu e devoid of PTK inhibitory properties, did not modify Ca2+ channel act ivity. The inhibitory action of genistein on the [Ca2+](i) increase wa s completely counteracted by the PTP inhibitor vanadate (100 mu M). Fu rthermore, vanadate alone potentiated [Ca2+](i) response to both 55 mM K+ and 10 mu M Bay K 8644. The possibility that genistein could decre ase the [Ca2+](i) elevation by enhancing Ca2+ removal from the cytosol seems unlikely since genistein also reduced the increase in fura-2 fl uorescence ratio induced by Ba2+, a cation that enters into the cells through Ca2+ channels but cannot be pumped out by Ca2+ extrusion mecha nisms, Finally, in unstimulated GH(3) cells, genistein caused a declin e of [Ca2+](i) and the disappearance of [Ca2+](i) oscillations, wherea s vanadate induced an increase of [Ca2+](i) and the appearance of [Ca2 +](i) oscillations in otherwise non-oscillating cells. The present res ults suggest that in GH(3) cells PTK activation causes an increase of L-type Ca2+ channel function, whereas PTPs exert an inhibitory role.