Effect of Na+- or Ca2+-filled liposomes on electrical activity of cultured heart cells

Phosphatidylcholine (PC) liposomes were used to deliver their entrapped ions into spontaneously contracting cultured heart cells (reaggregates) prepared from 15-day embryonic chick ventricles (superfused at 8 ml/min). With slowly rising action potentials (AP) (+Vmax less than 30 V/s), when Na+ liposomes were added (13.2% vol/vol), there was a progressive decrease in slope of the pacemaker potential and in firing frequency. Recovery on washout was rapid. When vesicles containing Ca2+ (6.6% vol/vol) were added, there was an immediate decrease in AP duration. Within 10 min, +Vmax, overshoot, maximum diastolic potential, and frequency decreased, and all spontaneous activity stopped within 30 min. Electrical stimulation could not elicit AP responses. Partial recovery of APs occurred after 60 min of washout, but the recovering APs initially were abnormal. Lower liposome concentration (3.3%) had a similar but slower effect. The rapidity of the effects depended on the Ca2+ concentration inside the vesicles (0.15 and 1 M). K+-containing liposomes had no effect. This difference between liposomes containing Na+, Ca2+, and K+ suggests that the effects observed were due to the ions and not to the phosphatidylcholine. Injection of Ca2+ could cause depolarization by the Ca2+-activation of a nonspecific Na+-K+ channel (i.e., gNa, K, (Ca]; inhibition of automaticity by injection of Na+ or Ca2+ could be due to diminution of the electrochemical gradient for inward background depolarizing current. Thus PC liposomes provide a good means for delivering substances inside the cells without altering the electrical properties of the membrane by the lipid vehicle.