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Analysis of subthreshold pace‐maker currents in chick embryonic heart cells.
Author(s) -
Clay J R,
Shrier A
Publication year - 1981
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1981.sp013639
Subject(s) - depolarization , membrane potential , chemistry , conductance , time constant , reversal potential , biophysics , voltage clamp , tetrodotoxin , subthreshold conduction , patch clamp , voltage , physics , biology , biochemistry , electrical engineering , condensed matter physics , receptor , engineering , transistor , quantum mechanics
1. Small re‐aggregates of cells dissociated from the ventricles of 7‐day‐old chick embryonic hearts beat spontaneously in low external potassium concentration (Ko = 1.3 mM) tissue culture medium. This activity was blocked by the addition of tetrodotoxin (TTX) or potassium ions to the external medium. 2. A two‐micro‐electrode voltage‐clamp technique was used to analyse the subthreshold currents responsible for the pace‐maker depolarization. 3. Voltage‐clamp steps 6‐10 sec in duration revealed a time‐dependent current having first order kinetics. Its membrane potential range of steady‐state activation was ‐90 to ‐60 mV. 4. The current kinetics were qualitatively similar to those of Hodgkin & Huxley (1952b) with a peak time constant of approximately 1 sec at V = ‐75 mV. The kinetics were independent of Ko. 5. The time‐dependent current was attributed to gated membrane channels. The fully activated current‐voltage (I‐V) relation of the channels was determined from the ratio of the amplitudes of the time‐dependent currents during and after voltage‐clamp steps following the procedure of Noble & Tsien (1968). 6. The fully activated I‐V relation displayed inward rectification with negative slope conductance at potentials more than 15 mV positive to its reversal potential. Changes of Ko shifted the I‐V curve along the voltage axis like a potassium electrode. 7. The time‐independent (background) current was obtained by subtracting the gated channel current from the steady‐state I‐V curve. This current also rectified in the inward direction. 8. The inwardly rectifying I‐V relations were theoretically described by a channel having a row of ion‐selective sites along which ions move in a single file (Hodgkin & Keynes, 1955), and a membrane‐bound particle which blocked the channel in a voltage‐dependent manner. 9. The relationship of the voltage‐clamp results to spontaneous activity is discussed and comparisons are made with measurements from whole embryonic heart and other cardiac tissues.