Tracer priming in human protein turnover studies with [15N]glycine.

Sixty-three studies in healthy normal volunteers (n = 29), malnourished cancer (n = 8) or non-cancer patients (n = 9), and postoperative radical cystectomy patients (n = 17) were conducted to evaluate the primed constant infusion labeling technique for the estimation of whole-body protein turnover under a variety of dietary conditions. [15N]Glycine was used as the tracer with a prime to infusion ratio of 1300 to 3300 min and a continuous-infusion rate of 0.11 to 0.33 micrograms 15N . kg-1 . min-1 for 24 to 36 hr. The isotopic steady-state enrichment was reached in all subjects both in urinary urea and ammonia between 10 and 26 hr (mean 18 +/- 2). During protein calorie fasting the attainment of isotopic steady state is much quicker (10 to 18 hr) with a primed constant infusion than with a constant infusion alone (approximately 38 hr). A P/I ratio greater or less than 1800 (min) usually resulted in a delay of plateau attainment without affecting the protein turnover values. Reliable estimates of protein kinetics in humans can be made in clinical conditions with a 26-hr infusion of glycine at the rate of 0.28 microgram 15N . kg-1 . min-1 with a P/I ratio of 1800 min, collecting six urine samples every 2 hr from 16 hr and analyzing for both urinary urea and ammonia enrichments.

Effects of nutritional depletion and repletion on plasma methotrexate pharmacokinetics.

Clinically, a relationship exists among nutritional status, drug responsiveness, and host toxicity, but the effects of nutrition on chemotherapy drug metabolism are relatively unknown. This article evaluates the effect of malnutrition and nutritional repletion on plasma methotrexate (MTX) pharmacokinetics. In Study I, 55 non-tumor-bearing rats were fed regular diet (RD) (N = 27) or protein-free diet (PFD) (N = 28) for 10 days. On day 10, MTX 20 mg/kg was injected intraperitoneally and four to six animals in each group were killed at 30 minutes, 1, 2, 6, 24, and 48 hours following MTX administration. Plasma MTX was measured by dihydrofolate reductase enzyme inhibition assay and was found to be significantly elevated (P less than 0.01) in PFD animals as compared with RD animals at 1 and 2 hours following MTX injection. In Study II, 87 tumor-bearing rats were fed RD (N = 29) or PFD (N = 58) for 10 days. At this time, 27 PFD rats were switched to RD (PFD----RD), while 31 rats remained on PFD. MTX 20 mg/kg was injected intraperitoneally and six to eight rats in each group were killed at 1, 2, 6, and 24 hours post-MTX injection. Mean levels were found to be significantly elevated at 2, 6, and 24 hours in PFD (P less than 0.01) and PFD----RD (P less than 0.05) compared with RD rats. Several investigators have shown that increased plasma MTX levels at 24 and 48 hours after drug administration correlates with host toxicity in patients receiving chemotherapy. In this experimental study, malnutrition delayed plasma MTX clearance, and nutritional repletion for 2 days was insufficient to return MTX pharmacokinetics to normal. These results in experimental animals suggest an explanation for increased MTX toxicity in malnourished individuals.

Effects of postoperative infusion of branched chain amino acids on nitrogen balance and forearm muscle substrate flux.

A branched chain amino acid (45% BCAA)--enriched solution was compared with a standard amino acid (25% BCAA) solution in a randomized blinded study in eight patients undergoing cystectomy and ileal loop diversion. Solution administration rates were designed to provide 20 kcal/kg/day and 0.26 gm nitrogen/kg/day postoperatively from days 1 to 8. Daily urine creatinine and nitrogen levels were measured. On day 4 during infusion, forearm blood flow was measured by capacitance plethysmography. Arterial and antecubital deep venous blood samples were drawn simultaneously for determinations of glucose, lactate, pyruvate, and amino acids; substrate flux was calculated (mumol/ml/min, uptake = +, release = -). The mean total amino acid flux was significantly improved In the 45% BCAA group (+1.7 +/- 0.4) versus the 25% BCAA group (-0.03 +/- 0.2, P less than 0.05). The mean total BCAA flux was significantly improved in the 45% BCAA group (+1.1 +/- 0.4) compared with the 25% BCAA group (-0.03 +/- 0.2) (P less than 0.02). There were no significant differences between groups in alanine, glutamine, glucose, lactate, or pyruvate forearm flux. The cumulative adjusted 7-day nitrogen balance was -1.4 +/- 7 gm in the 45% BCAA group and -14.2 +/- 4 gm in the 25% BCAA group (NS). the 45% BCAA solution resulted in significant uptake of total amino acid and BCAA into forearm muscle, which was not significantly demonstrated in whole body nitrogen balance studies. Sixty-six percent of the increase in total amino acid forearm flux with use of the 45% BCAA solution was due to the uptake of BCAA even though less leucine was given compared with the 25% BCAA solution.