Hyperpolarization and lysophosphatidylcholine induce inward currents and ethidium fluorescence in rabbit ventricular myocytes.

Abstract

Strong electric pulses produce reversible or irreversible membrane breakdown (electroporation). We analysed the permeation properties of minute pores caused by hyperpolarization or lysophosphatidylcholine (LPC) by comparing the amount of charge carried by irregular inward currents (I(hi)) with changes in ethidium bromide (EB) fluorescence in isolated rabbit ventricular myocytes. Forty-second negative pulses from a holding potential of -20 mV induced I(hi) whose conductance increased with hyperpolarization; the mean conductance (G(hi)) was 63.6 +/- 9.9 pS pF(-1) (mean +/- S.E.M., n = 9) at -160 mV. EB fluorescence increased during voltage pulses in parallel with the time integral of I(hi) (Q(hi)), with the magnitude of the increases in nuclear EB fluorescence being 5.3 times greater than in the cytoplasm at -160 mV. Similar hyperpolarization-induced parallel increases in I(hi) and EB fluorescence were also obtained in Na(+)-free, N-methyl-D-glucamine (NMDG) solution. LPC (10 microM) induced large (101.2 +/- 21.2 pS pF(-1), n = 16), rapid (rise times, 1-10 ms) I(hi) with slow relaxation rates at -80 mV that reflected increases in G(hi) to 94.3 +/- 24.8 pS pF(-1) (n = 8) at 6 min. Plots of EB fluorescence vs. Q(hi) were well fitted by a common Hill's equation with a Hill coefficient of 0.97. Taken together, our findings indicate that hyperpolarization and LPC produced pores having the same filter properties for the permeation of small ions, including ethidium(+), and that I(hi) (carried in part by Ca(2+)) generated by membrane breakdown are capable of supplying sufficient ions to evoke abnormal excitation and contraction in cardiac myocytes.