PROTEIN-KINASE-C REGULATION OF P-GLYCOPROTEIN-MEDIATED XENOBIOTIC SECRETION IN RENAL PROXIMAL TUBULE

Fluorescence microscopy, fluorescent substrates [daunomycin and a fluo rescent cyclosporin A (CSA) derivative] and digital image analysis wer e used to examine the role of protein kinase C (PKC) in the control of p-glycoprotein in killifish renal proximal tubules. PKC activators, p horbol ester (phorbol la-myristate 13-acetate, PMA) and dioctylglycero l, reduced luminal drug accumulation, and protein kinase inhibitors, s taurosporine and l-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), increased luminal accumulation; a PMA analog that does not activate P KC was without effect. PMA effects were blocked by staurosporine. The increase in luminal fluorescence caused by staurosporine was blocked b y the p-glycoprotein substrate, CSA, indicating that this component of transport was indeed mediated by p-glycoprotein. Neither PMA, dioctyl glycerol, nor protein kinase inhibitors altered cellular drug accumula tion. Finally, in primary cultures of flounder proximal tubule cells, PMA decreased transepithelial [H-3]daunomycin secretion. This pharmaco logical approach demonstrates that in teleost renal proximal tubule, p -glycoprotein-mediated xenobiotic secretion is negatively correlated w ith changes in PKC activity, a finding that conflicts with results fro m studies using mammalian tumor cells that express p-glycoprotein.