Effects of phorbol 12-myristate 13-acetate on potassium transport in the red blood cells of frog Rana temporaria

Phorbol 12-myristate 13-acetate (PMA), a stimulator of PKC, was examined for its influence on K⁺ (⁸⁶Rb) influx in the frog erythrocyte. PMA, 0.1 μM, was found to accelerate ouabain-sensitive K⁺ influx, which was suppressed by 73% with 1 mM amiloride, indicating secondary activation of the Na⁺–K⁺-pump due to stimulation of Na⁺ /H⁺ exchange. PMA-induced stimulation of the sodium pump was completely inhibited with 1 μM staurosporine and by ~50% with 20 μM chelerythrine. In contrast to Na⁺–K⁺-pump, an activity of Cl⁻-dependent K⁺ transport (K–Cl cotransport, KCC), calculated as the difference between K⁺ influxes in Cl⁻ and NO₃ ⁻-media, was substantially decreased under the influence of PMA. Staurosporine fully restored the PMA-induced inhibition of KCC, whereas chelerythrine did not exert any influence. Osmotic swelling of the frog erythrocytes was accompanied by approximately twofold stimulation of KCC. Swelling-activated KCC was inhibited by ~50 and ~83% in the presence of PMA and genistein, respectively, but not chelerythrine. Exposure of the frog erythrocytes to 5 mM fluoride (F⁻) also reduced the KCC activity in isotonic and hypotonic media, with maximal suppression of K⁺ influx in both media being observed upon simultaneous addition of PMA and F⁻. Furosemide and [(dihydronindenyl)oxy] alkanoic acid inhibited the K⁺ influx in both the media by ~50–60%. The results obtained show both the direct and indirect effects of PMA on the K⁺ transport in frog erythrocytes and a complicated picture of KCC regulation in frog erythrocytes with involvement of PKC, tyrosine kinase and protein phosphatase.