AJP - Heart and Circulatory Physiology, Vol 247, Issue 2 273-H282, Copyright © 1984 by American Physiological Society
Perturbations in the membrane potential of cultured heart cells: role of calcium
R. D. Nathan and M. L. Bhattacharyya
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.
