Influence of Extracellular K⁺ Concentration on Cable Properties and Excitability of Sheep Cardiac Purkinje Fibers

Abstract

Conduction velocity was increased from 3.5 m/sec to 4.1 m/sec in sheep cardiac Purkinje fibers when the superfusing Tyrode solution was changed from 2.7 mM K⁺ to 4.0 mM K⁺. Further increase to 7.0 mM K⁺ resulted in a fall in conduction velocity to 3.3 m/sec. To understand the nature of the cellular change responsible for this effect, cable analyses were made. With increases in extracellular K⁺ concentration membrane resistance decreased and membrane capacitance did not change. Resistance of the myoplasm tended to fall in 7.0 mM [K]_o. Since these effects did not explain the change in conduction velocity, excitability was studied by strength-duration curves using intracellular micropipettes for current passage and recording. Increase in K⁺ from 2.7 to 4.0 mM resulted in a shift of the entire curve to the left, with a fall in rheobasic current from 121 to 104 nanoamperes and a fall in the time constant from 2.79 to 2.4 msec. Normalized plotting of stimulating current over rheobasic current (I/I_Rh) against duration of stimulating current over the time constant (t/τ) suggested that the curves were not basically different. The increase in K⁺ from 2.7 to 4.0 mM was associated with a depolarization of the resting membrane, consistent with alteration in the potassium equilibrium potential, but no change in the absolute membrane potential for threshold. In this way, the changes in membrane voltage and charge necessary for excitation were reduced in 4.0 mM [K]_o.