SPONTANEOUS CONTRACTIONS OF CULTURED HEART CELLS IN HIGH K+ MEDIA

Cultured chick embryonic heart cells became partially depolarized and stopped beating when [K+]0 reached 50-70 mM by addition of KCl to the bathing medium. However, some cells beat spontaneously for many days when cultured chronically in media containing 65, 86, and 98 mM [K+]0 ([Na+]0 correspondingly reduced). In high [K+]0, neighboring cells contracted independently of each other, whereas in normal K+ (2.7 mM) they contracted synchronously. The cells contracted in response to mechanical prodding and to externally-applied stimulating current about tenfold greater than normal intensity. Intracellularly-recorded resting potentials (Em) were 1-11 mV and agree with values obtained from cells acutely exposed to high K+. Thus, [K+]i is not significantly changed when cells are chronically bathed in an elevated K+ solution. Large resting and action potentials rapidly returned following change to a low [K+]0 medium. Neither depolarizing nor hyperpolarizing current pulses elicited action potentials in high K+. Most cells stopped beating immediately after impalement. Of the few cells that continued to contract, some displayed small oscillations of Em (2-4 mV) synchronous with beating; however, no oscillations were observed in most cells. Hence, contraction of cells in high K+ need not be controlled by changes in Em. Ba2+ and Sr2+ (5-10 mM) enhanced excitability, allowing anodal-break action potentials to occur. Ba2+ caused no change in resting Em but increased input resistance; Sr2+ rapidly hyperpolarized by 8-25 mV and produced spontaneous action potentials. Since Em = EK in high [K+]0, the Sr2+-induced hyperpolarization may reflect an increased rate of ion pumping. In conclusion, the membrane properties of cultured heart cells chronically exposed to high K+ solutions are no different than those of cells acutely exposed. With either acute or chronic exposure to high K+ solutions, many K+-depolarized cells contract cyclically without accompanying changes in membrane potential. © 1969.