HETEROGENEITY OF FOREARM METABOLISM WITH SPECIAL REFERENCE TO FREE FATTY ACIDS*

Abstract

Under basal conditions in blood draining chiefly superficial tissues of the forearm, concentrations of glucose and K (in plasma) are lower and concentrations of lactate, free fatty acids (FFA) and oxygen are higher than in blood draining chiefly deep tissues of the forearm. The fraction of O2 uptake accounted for by glucose uptake, corrected for lactate production, and complete oxidation of glucose assumed, is much greater in superficial than in deep tissues. Whereas there is net leak of K from deep forearm tissues, there is no significant difference between arterial and superficial venous plasma K concentration. Superficial tissues, presumably adipose tissues, discharge FFA into venous blood. FFA in deep venous plasma is not significantly different from arterial FFA. Arterial K concentration is greater in males, who also show a greater difference in potassium concentration between deep and superficial veins. Females show a much higher arterial FFA concentration, a tendency for FFA concentration to fall with time, and a decreasing output of FFA from superficial tissues. The changes in arterial concentration of FFA with time are related to the height of the initial values: low concentrations tend to rise and high ones to fall. Owing to anatomical considerations, the Fick principle is not entirely applicable to assessment of FFA metabolism by forearm muscles. It can occasionally be shown that FFA''s act as major substrates for resting muscle under basal conditions.