Continuous measurement of plasma protein content in the alert rat during and after 3 to 5 minutes of moderate activity.

A refractometric method was used for the continuous registration of plasma protein concentration in rats during and after 3 and 5 minutes of provoked activity. Simultaneous conductometric measurements of hematocrit (hct) showed that, although both invariably changing in the same direction, the relative change of protein concentration is always less than that of hct: plasma volume changes calculated from the former fall short of those calculated from hct by 34 +/- 23% during the hemoconcentrative period during activity and by 52 +/- 11% during the hemodilutory period after activity. The difference between these figures was significant, thus implying that fluid leaving the circulation during the filtration phase is less rich in protein than that entering it during the absorptive phase of microvascular adjustments. A kinetic analysis of the period after activity was made. The rate constants of fluid- and protein-flux were closely correlated. Both plasma volume and intravascular protein mass increased asymptotically to a new equilibrium 6% above control within 30 min after activity. It is suggested that the excess protein is mobilized from large parenchymatous organs, mainly the liver.