title: Activation of protein kinase C by phorbol ester induces downregulation of the Na+/K(+)-ATPase in oocytes of Xenopus laevis.
abstract: Full-grown prophase-arrested oocytes of Xenopus laevis were treated with 50 nM phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, or with 50 nM 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD) that does not activate protein kinase C. The effect on membrane currents and capacitance, inulin uptake and ouabain binding, and on membrane morphology were analyzed. (i) During application of PMA, current generated by the Na+/K+ pump decreases; in addition, Cl- and K+ channels become inhibited. This general decrease in membrane conductance reaches steady state after about 60 min. 4 alpha PDD was ineffective. (ii) Ouabain binding experiments demonstrate that PMA (K1/2 = 7 nM), but not 4 alpha PPD, induces a reduction of the number of pump molecules in the surface membrane. Permeabilization of oocytes by digitonin plus 0.02% SDS renders all binding sites present prior to PMA treatment again accessible for ouabain. The KD value for ouabain binding is not influenced. 4 alpha PDD was ineffective. (iii) Exposure of oocytes to PMA reduces membrane capacitance and stimulates uptake of inulin suggesting an increase in endocytosis. Electron micrographs show that PMA reduces the number and length of microvilli, leading finally to a smooth membrane surface with a reduced surface area. From these results we conclude that stimulation of protein kinase C leads to downregulation of the sodium pump. A major portion of this inhibition is brought about by reduction in area of surface membrane with a concomitant internalization of pump molecules. In addition to this mode of downregulation, a direct effect of stimulation of protein kinase C on the pump molecule cannot be excluded.
authors.affiliation: Max-Planck-Institut für Biophysik, Frankfurt/M., Federal Republic of Germany.
authors.forename: L A
authors.initials: LA
authors.lastname: Vasilets
authors.forename: G
authors.initials: G
authors.lastname: Schmalzing
authors.forename: K
authors.initials: K
authors.lastname: Mädefessel
authors.forename: W
authors.initials: W
authors.lastname: Haase
authors.forename: W
authors.initials: W
authors.lastname: Schwarz
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000502:physiology / Q000648:ultrastructure; D002463:Cell Membrane Permeability; D015536:Down-Regulation; D004553:Electric Conductivity; D004789:Enzyme Activation; D007444:Inulin / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials; D008854:Microscopy, Electron; D009865:Oocytes / Q000201:enzymology*; D010042:Ouabain / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014982:Xenopus laevis
chemical_list: D010042:Ouabain; D007444:Inulin; D011493:Protein Kinase C; D000254:Sodium-Potassium-Exchanging ATPase; D013755:Tetradecanoylphorbol Acetate