Changes in extracellular potassium activity in response to decreased pH in rabbit atrial muscle.

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Abstract
Extracellular and intracellular K+ activities of isolated superfused rabbit atrial muscle were measured using K+-sensitive liquid ion exchanger microelectrodes. When the pH of the bathing medium was decreased from 7.5 to 6.8, intracellular K+ activity fell and extracelular K+ activity rose from a mean control level of 3.6 mM to a new steady state level of 3.9 mM after 1 h. When the pH was further decreased to 6.1 extracellular K+ activity increased to a mean of 4.9 mM. Following the change in pH, the increase in extracellular K+ activity occurred over a period of 30-40 min when a stable value was reached and maintained for the next h. On return to normal pH the extracellular K+ activity returned to control with a time constant of 20 min or less. Measurements of intracellular K+ activity over 1 h showed a mean loss of 3 mM at pH 6.8, and a mean loss of 8 mM at pH 6.1. The loss was reversible within 20 min of return to control pH. The increase in extracellular K+ activity was accompanied by a decrease in resting membrane potential as well as decreases in maximum dV/dt and overshoot of the action potential. The action potential contour underwent complex changes consisting of decrease in the plateau and a prolongation of the time to full repolarization.