INVOLVEMENT OF PROTEIN KINASE C IN THE REGULATION OF Na+/Ca2+ EXCHANGER IN BOVINE ADRENAL CHROMAFFIN CELLS

• 1 The Na⁺/Ca²⁺ exchanger (NCX) exchanges Na+ and Ca²⁺ bidirectionally through the forward mode (Ca²⁺ extrusion) or the reverse mode (Ca²⁺ influx). The present study was undertaken to clarify the role of protein kinase C (PKC) in the regulation of NCX in bovine adrenal chromaffin cells. The Na⁺‐loaded cells were prepared by treatment with 100 µmol/L ouabain and 50 µmol/L veratridine. Incubation of Na⁺‐loaded cells with Na⁺‐free solution in the presence of the Ca²⁺ channel blockers nicardipine (3 µmol/L) and ω‐conotoxin MVIIC (0.3 µmol/L) caused Ca²⁺ uptake and catecholamine release.
• 2 The Na⁺‐dependent Ca²⁺ uptake and catecholamine release were inhibited by 2‐4‐(2,5‐difluorophenyl)methoxy]phenoxy]‐5‐ethoxyaniline (SEA0400; 1 µmol/L) and 2‐2‐4‐(4‐nitrobenzyloxy)phenyl]isothiourea (KB‐R7943; 10 µmol/L), both NCX inhibitors. These results indicate that the Na⁺‐dependent responses are mostly due to activation of the NCX working in the reverse mode.
• 3 In addition, we examined the effects of PKC inhibitors and an activator on the NCX‐mediated Ca²⁺ uptake and catecholamine release. Bisindolylmaleimide I (0.3–10 µmol/L) and chelerythrine (3–100 µmol/L), both PKC inhibitors, inhibited NCX‐mediated responses. In contrast, phorbol 12,13‐dibutyrate (0.1–10 µmol/L), a PKC activator, enhanced the responses. Bisindolylmaleimide I and chelerythrine, at effective concentrations for inhibition of Na⁺‐dependent catecholamine release, had a little or no effect on high K⁺‐induced catecholamine release in intact cells or on Ca²⁺‐induced catecholamine release in β‐escin‐permeabilized cells.
• 4 These results suggest that PKC is involved in the activation of NCX in bovine adrenal chromaffin cells.