Mechanism Underlying the Changes in Plasma Potassium Concentration during Infusion of Isosmotic Nonelectrolyte Solution

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Abstract
Generally, during infusion of an isosmotic nonelectrolyte solution that permeates the cell membrane, plasma K+ concentration ([K+]pl) either does not change or it increases slightly. The mechanism underlying this [K+]pl change has not been clarified. We continuously monitored the [K+]pl and plasma Na+ concentration ([Na+]pl) for 10 min during isosmotic mannitol infusion of 1.6 ml/100 g body weight in rats with intact kidney function (intact mannitol group). In addition, in nephrectomized rats, we compared the [K+]pl change during infusion with isosmotic mannitol (which permeates the cell membrane; mannitol nephrectomized group) with that during infusion with isosmotic sucrose (which does not permeate the cell membrane; sucrose nephrectomized group) to evaluate the effect of cell volume regulation. In the intact mannitol group, [Na+]pl decreased with dilution, and [K+]pl remained relatively constant. In the sucrose nephrectomized group, [K+]pl decreased by the same percentage as [Na+]pl and gradually increased to greater than the control level. In the mannitol nephrectomized group, however, [K+]pl increased immediately after the beginning of the infusion and reached the same level as that in the sucrose nephrectomized group. To confirm that the difference in [K+]pl between the mannitol and sucrose nephrectomized groups was dependent on cell volume regulation, we investigated the changes in mean corpuscular volume of red blood cells, using a Coulter counter. This value remained constant during isosmotic sucrose infusion but increased during isosmotic mannitol infusion, returning to the original volume after the infusion. We kept [HCO3-] and pH constant throughout the experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

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