Protein kinase C and insulin regulation of red blood cell Na+/H+ exchange

Insulin activation of red blood cell (RBC) Na+/H+ (NKE) and Na+/Li+ (NLiE) exchanges is mimicked by okadaic acid, thus suggesting that it may change the state of phosphorylation of serine/threonine NHE residues. To investigate the role of the serine/threonine protein kinase C (PKC) in insulin regulation, we evaluated the effect of phorbol 12-myristate 13-acetate (PMA; 1 mu M) and insulin on PKC activity, membrane protein phosphorylation, and the activation kinetics of both exchangers. Our studies revealed that PMA decreased cytosolic PKC activity (4.1 +/- 0.6 to 2.3 +/- 0.5 pmol . mg protein(-1). min(-1), n = 9, P < 0.001), increased membrane PKC activity (42.3 +/- 5 to 132 +/- 12 pmol . mg . protein(-1). min(-1), n = 11, P < 0.001), and enhanced serine phosphorylation of bands 3, 4.1, and 4.9 membrane proteins. PIMA markedly reduced the Michaelis constant (K-m) for intracellular H+ (415 +/- 48 to 227 +/- 38 nM, n = 11, P < 0.01) but had no effect on the maximal transport rate (V-max) of NHE and the K-m for Na+ of NLiE. NHE activation and PKC activity were affected differently by insulin (100 mu U/ml) and PMA. Insulin increased the V-max of NHE and the K-m for Na+ of NLiE but had no effect on the K-m for intracellular H+ and membrane PKC activity. These findings lead us to conclude that in the human RBC, NHE is modulated by PKC and insulin through different biochemical mechanisms.