Effect of a phospholipase A2 with cardiotoxin-like properties, from Bungarus fasciatus snake venom, on calcium-modulated potassium currents

Q.H. Gong, S.J. Wieland, J.E. Fletcher, G.E. Conner and M.S. Jiang. Effect of a phospholipase A₂ with cardiotoxin-like properties, from Bungarus fasciatus snake venom, on calcium-modulated potassium currents. Toxicon27, 1339–1349, 1989.—The action of a 16,300mol. wt phospholipase A₂ with cardiotoxin-like properties from Bungarus fasciatus venom on membrane electrical properties of two human cell types was examined in vitro by using tight-seal whole-cell recording methods. Epithelial cells exhibited a voltage-and Ca²⁺-activated K⁺current; the sensitivity for voltage activation of the K⁺ current was enhanced by increasing free Ca²⁺ in the recording pipette from 10⁻⁸ M to 2 × 10⁻⁶ M. In contrast, peripheral blood lymphocytes possessed voltage-activated K⁺ currents that were inhibited by increasing intracellular Ca²⁺.Exposure of either preparation to B.fasciatus toxin (0.2–5 × 10⁻⁶ M) for up to 30 min in the bath did not alter membrane leakage current, as judged by the maintenance of low pre-treatment values over the range of − 140mV to − 40mV. However, the sensitivity for voltage activation of the K⁺ current was enhanced in the epithelial cells even at the lowest concentrations tested. In contrast to the results with epithelial cells, toxin exposure inhibited the activation of voltage-activated K⁺ currents in human lymphocytes, suggesting a specific increase in intracellular Ca²⁺ levels in both cell types.The fluorescent probe indo-1/AM was used to monitor cytoplasmic Ca²⁺ levels. Exposure of either lymphocytes or epithelial cells to toxin (10⁻⁶ M) resulted in a transient increase in Ca²⁺. However, while the Ca²⁺ response to toxin was transient, K-channel modulation by the toxin appeared to be irreversible over the experimental time course. The longer-lasting modulation of Ca²⁺-regulated K⁺ channels may reflect an irreversible action of the B.fasciatus phospholipase A₂ on a Ca²⁺-dependent regulatory process.