UP-REGULATION OF L-TYPE AND NON-L,NON-N-TYPE CA2-KINASE-C ACTIVATION IN INSULIN-SECRETING RINM5F CELLS( CHANNELS BY BASAL AND STIMULATED PROTEIN)

We studied the effect of protein kinase C (PKC) inhibition and activat ion on voltage-dependent Ca2+ channels in rat insulinoma RINm5F cells, PKC down-regulation by chronic (24 h) treatment with the PKC activato r phorbol 12-myristate 13-acetate (PMA) reduced by about 60% the Ba2currents through L- and non-L, non-N-type high-voltage-activated Ca2channels, indicating that PKC tonically up-regulates the two main Ca2 channel subtypes of RINm5F cells under basal conditions, Consistently , PKC activation by acute PMA application caused only a modest increas e (average 23%) of Ba2+ currents in a minority of cells (24%), L- and non-L, non-N-type channels were differentially up-regulated by either basal or stimulated PKC activation, Acute up-regulation was predominan t on L-type channels and caused an I/V shift of the Ba2+ currents in t he hyperpolarizing direction, Non-L, non-N-type channels were less aff ected by acute PMA application, possibly reflecting a more effective t onic PKC up-regulatory action, Unexpectedly, the increase of Ba2+ curr ents during acute PMA application was followed by a progressive curren t decrease, which was also observed in isolation in another 24% of the cells and could be ascribed to PKC-induced ATP depletion, rather than to a direct effect of PKC on Ca2+ channels, We also provide evidence that PKC-mediated phosphorylation is not involved in the G-protein-med iated noradrenergic modulation of Ca2+ channels in RINm5F cells.