Biphasic change in plasma potassium concentration by mannitol infusion in rats

Changes in plasma Na+([Na+]pl) and K+([K+]pl) concentrations were continuously measured in nephrectomized rats consequent to intravenous infusion of isosmotic mannitol (M group) or sucrose (S group) at 1.6 ml/100 g body wt. for 10 min, and for the following 30 min. The effect of a Ca2+ channel blocker (diltiazem, 0.1 mg/100 g body wt.) was also evaluated. In the S group, [K+]pl decreased during the infusion and rose above the control level after the discontinuance of infusion. In the M group, [K+]pl gradually increased even during the infusion and reached the same level as the S group at the end of the experiment. In both the S and M groups, [Na+]pl decreased, while PaCO2, pH, and HCO3- did not change. The increase in [K+]pl in the M group had two components. The first was a transient component as presented by the difference of [K+]pl between the M and S group. It might be attributable to K+ release from the intracellular space during a regulatory volume decrease consequent to cell swelling induced by isosmotic mannitol infusion. The second component of the increase in [K+]pl after the infusion was observed in both groups, and this increase might be related to the dilution of ions induced by the large amount of infused nonelectrolyte solution. Diltiazem suppressed both of these changes in [K+]pl, suggesting that they are related to Ca2+ channels. The results suggest that increase in [K]pl by isosmotic mannitol infusion is related to the effects of regulatory volume decrease and dilutional effect.