Volumetric properties of intracellular compartments in canine cardiac Purkinje cells.

Volumetric properties of canine cardiac Purkinje fibers were examined. Purkinje cells were superfused with anisosmolar solutions, and changes in extracellular space and relative cell volume were determined. The relationship between cellular volume and the osmolarity of the bathing medium was shown to be linear except in solutions of very low osmolarity. A linear regression line crossed the volume axis at 38%, suggesting an osmometric dead space of 38% and correspondingly an osmometric compartment comprising about 62% of the cell interior. To determine the volumetric properties of the "electrophysiological compartment," Purkinje cells were impaled with voltage-sensitive microelectrodes, and cellular resting potentials were recorded. When log K was plotted against resting potential (Em) in preparations bathed in normal and hyperosmotic solutions, it was shown that Em was increased in hyperosmotic solutions (13.5 and 21 mV in 600 and 850 mosM solutions, respectively). Calculations using the Nernst equation showed that the compartment containing the intracellular K involved in membrane electrical events behaves as a near-perfect osmometer in hyperosmotic solutions. Changes in the osmometric compartment were well correlated with K changes in the electrophysiological compartment, thus suggesting that the K is homogeneously distributed intracellularly.