PROTEIN-KINASE AND G-PROTEIN REGULATION OF CA2-HT AND GABA( CURRENTS IN HERMISSENDA PHOTORECEPTORS BY 5)

The effects of serotonin (5-HT) and GABA on two Ca2+ currents, a trans ient low-voltage-activated current (tLVA) and a sustained high-voltage -activated current (sHVA) were examined in isolated photoreceptors of Hermissenda. The sHVA current was blocked by 5-HT and reduced by activ ation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate. The effects of 5-HT were transiently reversed by staurosporine and par tially blocked by the PKC inhibitor peptide [PKC(19-36)]. GABA enhance d both the tLVA and sHVA currents at low concentrations (5 nM to 5 mu M) and reduced the sHVA current at high concentrations (>10 mu M). The GABA-mediated enhancement of the Ca2+ current at low concentrations w as sensitive to block by picrotoxin. The protein kinase A (PKA) inhibi tor peptide [PKI(6-22)amide] blocked enhancement of both Ca2+ currents produced by cAMP analogs and GABA, suggesting that the effects at low concentrations may be PKA mediated. Caged GTP-gamma-S released by fla sh photolysis reduced the sHVA current, and pretreatment of the photor eceptors with pertussis toxin blocked the effects of higher concentrat ions of GABA, indicating that at higher concentrations, the effects ma y be G-protein mediated.