Gi-3 protein mediates the increase in voltage-gated K+ currents by somatostatin on cultured ovine somatotrophs

Voltage-gated K+ currents in rat somatotrophs are increased by somatostatin (SRIF) through unidentified G protein. In this experiment, somatotroph-enriched cells (up to 85%) were obtained from ovine pituitary glands and further identified by the increase in K+ currents by SRIF. The whole cell recording was employed to study the voltage-gated K+ currents. A reversible increase in K+ currents (up to 150% of control) was obtained in response to local application of SRIF (10 nM) but not vehicle. When the guanosine 5'-O-(3-thiotriphosphate) was included in the pipette solution (200 mu M), the recovery phase of K+ current response to SRIF was abolished. Inclusion of guanosine 5'-O-(2-thiodiphosphate) (200 mu M) in pipette solution blocked the K+ current response to SRIF. Intracellular dialysis of antibodies against alpha(o)-, alpha(i)-, alpha(i-1-2)-, or alpha(i-3)-subunits of G proteins via patch pipettes was confirmed by immunofluorescent staining of the antibodies. Antibody dialysis alone did not modify voltage-gated K+ currents. Dialysis of anti-alpha(i) or anti-alpha(i-3) antibodies significantly attenuated the increase in K+ currents that was obtained after application of 10 or 100 nM SRIF. Dialysis with anti-alpha(o), anti-alpha(i-1-2), or heat-inactivated (60 degrees C for 10 min) anti-alpha(i) antibodies did not diminish the effect of SRIF on K+ currents. We conclude that the G(i-3) protein mediates the effect of SRIF on voltage-gated K+ currents in ovine somatotrophs.