Determination of the charge of the plasma proteins and consequent Donnan equilibrium across the capillary barriers in the rat microvasculature.
OBJECTIVE
Due to the negatively charged proteins in plasma, a Donnan equilibrium will be formed between plasma and interstitium or, as in the glomerulus, between glomerular plasma and Bowman's space. The phenomenon is of great physiological significance in the sense that the electro-osmotic pressure offered by the small ions attracted to the proteins may account for an important part of the total colloid osmotic pressure and also as the electric potential consequent to the Donnan distribution will affect the transcapillary transport of all charged molecular compounds. The present study aimed at estimating the protein charge in rat plasma in order to validate its importance for colloid osmotic pressure and potential.


METHODS
The charge of the plasma proteins was determined in vitro from the concentration of sodium across a cellophane membrane separating a rat plasma sample from saline alone. However, in order to improve the sensitivity of the method, the studies were carried out at an ionic strength of 1/10 of physiological saline.


RESULTS
The average charge of plasma was estimated at 0.23 +/- 0.003 mEq g(-1) protein (mean +/- SE), and the standard variation at +/-0.01 mEq g(-1), i.e. about 5%. At the normal protein concentration in Wistar rats of 50 g L(-1), the charge of the proteins in systemic plasma was calculated to be 11.5 mEq L(-1), whereas in glomerular and peritubular capillary plasma, the larger protein concentration increases the protein charge to 14.4 mEq L(-1).


CONCLUSIONS
The results verify that the plasma protein charge accounts for about one-third of the total colloid osmotic pressure and that the obtained potential will constitute a major driving force for the transport of charged molecular compounds.
Rügheimer
L
L
Section of Integrative Physiology, Department of Medical Cell Biology, Uppsala University, Uppsala, Sweden. louise.rugheimer@mcb.uu.se
Hansell
P
P
Wolgast
M
M
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D001798:Blood Proteins / Q000502:physiology*; D002196:Capillaries / Q000378:metabolism*; D004560:Electricity; D007477:Ions / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008833:Microcirculation; D051381:Rats; D017207:Rats, Sprague-Dawley
D001798:Blood Proteins; D007477:Ions