Metabolism of tritium- and 14C-labeled alanine in rats

[3-3H]- and [U-14C]alanine were administered to starved rats by bolus injection and by continuous infusion. The specific activities of alanine, glucose, and lactate in blood were followed. The tracer kinetics of alanine depended on the site of tracer administration and sampling. Tracer was either administered into the aorta and blood sampled from the vena cava (A-VC mode) or tracer was administered into the vena cava and arterial blood sampled (V-A mode) (Katz, J. F. Okajima, and A. Dunn. Biochem J. 194: 513-524, 1981). When tracer was infused in the A-VC mode the plateau specific activity of alanine was about half that obtained in the V-A mode. The parameters of alanine turnover were calculated from the specific activities obtained in the A-VC mode. The calculated apparent replacement rate averaged 1.9 mg.min-1.kg-1 for [U-14C]- and 3.9 mg.min-1.kg-1 for [3-3H]alanine, indicating a carbon recycling of about 50%. The apparent contribution of alanine carbon to that of glucose is 15%. The maximal activity in plasma water is attained at about 5 min after bolus injection of [3-3H]alanine and that of [14C]glucose in blood is attained about 10 min after the injection of [U-14C]alanine. Maximal specific activity of [3H]- and [14C]lactate is attained within about 1 min after injection. The apparent mean transit time and alanine mass were calculated from the areas of washout curves after the continuous infusion was terminated. The mean transit time for [3H]alanine was 10 min and apparent total body mass of alanine of the order of 40 mg/kg. The apparent means transit time for [U-14C]alanine ranged from 33 to 66 min corresponding to a mass of the order of 100 mg/kg of alanine or 40 mg/kg of alanine carbon.