Perturbations in the membrane potential of cultured heart cells: role of calcium.

Abstract

Intracellular recordings were obtained from spheroidal aggregates of 7-day embryonic chick heart cells after 3 days in gyratory culture. Three types of perturbations in the membrane potential were observed under experimental conditions expected to increase intracellular calcium: 1) multiple oscillations (of 5-20 mV peak-to-peak amplitude) during diastole in aggregates exposed to 10-15 mM Ca, 5 microM strophanthidin, or K-free solutions; 2) less periodic spontaneous voltage fluctuations (of less than 1.5 mV peak-to-peak amplitude) in aggregates exposed to solutions containing 22% of the normal [Na], and 3) depolarizing afterpotentials (DAPs), following repolarization of the action potential, in aggregates treated with 20-50 microM A23187, a Ca ionophore, or 5-10 mM caffeine. The oscillations were reduced markedly by 0.03-3.0 microM tetrodotoxin (TTX) and were blocked by 5-10 mM caffeine. Spontaneous voltage fluctuations were increased by raising external Ca, were unaffected by 30 microM TTX, and were blocked by 5 mM caffeine. DAPs were not blocked by 5 mM caffeine or by 0.1 microM TTX and 1 microgram/ml D 600, concentrations that greatly reduced the action potential upstroke velocity and plateau, respectively. Two intracellular electrodes were employed to test for electrotonic coupling between cells within an aggregate. An electrotonic response in one cell could be recorded when current was injected into another cell during recordings of each of the perturbations but was somewhat less during spontaneous voltage fluctuations. Possible ionic mechanisms for the perturbations and for concomitant changes in the configuration of the action potential are discussed.