Plasma protein osmotic pressure equations and nomogram for sheep

The equations developed by Landis and Pappenheimer (Handbook of Physiology. Circulation, 1963, p. 961-1034) for calculating the protein osmotic pressure of human plasma proteins have been frequently used for other animal species without regard to the fractional albumin concentration or correction for protein-protein interaction. Using an electronic osmometer, we remeasured the protein osmotic pressure of purified sheep albumin and sheep plasma partially depleted of albumin. We measured protein osmotic pressures of serial dilutions over the concentration range 0-180 g/l for albumin and 0-100 g/l for the albumin-depleted proteins at room temperature (26 degrees C). Using a nonlinear least squares parameter-fitting computer program, we obtained the equation of best fit for purified albumin, and then we used that equation together with the measured albumin fraction to obtain the best-fit equation for the nonalbumin proteins. The equation for albumin is IIcmH2O,39 degrees C = 0.382C + 0.0028C2 + 0.000013C3, where C is albumin concentration in g/l. The equation for the nonalbumin fraction is IIcmH2O,39 degrees C = 0.119C + 0.0016C2. Up to 200- and 100-g/l protein concentration, respectively, these equations give the least standard error of the estimate for each of the virial coefficients. The computed number-average molecular weight for the nonalbumin proteins is 222,000. Using the new equations, we constructed a nomogram, based on the one of Nitta and co-workers (Tohoku J. Exp. Med. 135: 43-49, 1981). We tested the nomogram using 144 random samples of sheep plasma and lymph from 31 sheep. We obtained a correlation coefficient of 0.99 between the measured and nomogram estimates of protein osmotic pressure.