¹ From The Rockefeller University, New York 10021

Propagated action potentials can be obtained in canine cardiac Purkinje fibers exposed to Na-free solutions containing no inorganic cation other than Ca and K. Essentially similar action potentials are obtained if Na is replaced by tetraethylammonium (TEA), tetramethylammonium (TMA), or choline. In a solution containing 128 mM TEA and 16.2 mM Ca the characteristics of these electrical responses were: maximum diastolic potential, -59 ± 3.3 mV; overshoot, 20 ± 6.8 mV; maximum upstroke velocity, 3.7 ± 2.3 V/s; conduction velocity, 0.1 m/s; and action potential duration, 360 ± 45 ms. The magnitude of the overshoot varied with log Ca_o with a slope of about 30 mV/10-fold concentration change. The upstroke velocity was an approximately linear function of Ca_o. The active response was greatly diminished or abolished by Mn and D-600 but was unaffected by tetrodotoxin. These Ca-dependent responses appeared in a region of transmembrane potential (about -50 mV) at which the rapid Na-dependent upstroke is abolished even when Na is present.

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