Sealing mechanism of electrically ruptured membrane by La<SUP>3+ </SUP>and polyethyleneglycol in ventricular myocytes

Increase of membrane potential (hyperpolarization) ruptures membrane as has been demonstrated by irregular inward currents (I_hi) and nuclear ethidium fluorescence increase (EB_fluo) in cardiac myocytes. We found that La³⁺ depresses I_hi. Polyethyleneglycol (PEG) is known to plug some large ion channels. Here, we applied La³⁺ and PEG (MW, 4,000) together or independently to isolated rabbit ventricular myocytes to examine their closing effects on electrically produced pores. The time integral of whole-cell I_hi during 40 s pulses (Q_hi) and EB_fluo during these pulses were estimated at membrane potentials between –80 and –160 mV. Q_hi was depressed by La³⁺ (30, 100, 1,000 nM) to about 15% at each potential and this depression was not affected by PEG. In contrast to La³⁺, Q_hi was little affected by 2 and 5% PEG in control. However, Q_hi was suppressed by 2 and 5% PEG in the presence of 1 mM-La³⁺. EB_fluo was depressed by La³⁺ maximally at 30 nM among above concentrations to 61% in control and to 42%in the presence of 5%-PEG. PEG (5%) voltage-independently depressed EB_fluo to 50% in control and the depression was increased by 0.1 mM- La³⁺ to 26%. It was suggested that La³⁺ mainly closes pores with small diameter, while PEG selectively closes those with larger diameter. La³⁺ could seal the pores by binding to their outer end and PEG could cooperatively enhance closing action of La³⁺ in the presence of high concentrations of La⁺. In addition, the results indicate that hyperpolarizations produce small pores with additional small number of ethidium⁺-permeable large pores. [Jpn J Physiol 54 Suppl:S98 (2004)]