The functional modulation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors by protein kinase C (PKC) was investigated in cultures enriched in retinal amacrine-like cells. The kainate-evoked [Ca2+]i increase is due to Ca2+ entry through open AMPA receptor channels, because it was blocked by the active isomer of a 2,3-benzodiazepine (LY 303070), an AMPA receptor antagonist. The AMPA receptor response to kainate was potentiated by phorbol 12-myristate 13-acetate, which specifically stimulates PKC, and it was decreased by bisindolylmaleimide I, a selective inhibitor of PKC, as well as by PKC down-regulation. The results indicate not only that the AMPA receptor activation has a PKC requirement, but also that PKC amplifies maximal receptor activation by 100 microM kainate. The effect of PKC activation or inhibition on voltage-gated Ca2+-channel activity was also investigated. Activation of PKC caused inhibition of Ca2+ channels, and the same effect was produced by inhibition of PKC, whereas the inactive analogue of the phorbol ester did not affect channel activity. Our results show an important role for PKC in regulating the function of both AMPA receptors and Ca2+ channels in cultured retina cells.