Glucose Metabolism, Lactate, and Ammonia Production by the Human Placenta In Vitro

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Abstract
Summary: Fifteen human placentas were obtained at term. Placental fragments were incubated in a bicarbonate-buffered Earle's solution. Additions of glutamate (1 mM) or glutamine (1 or 2 mM) were made. All incubations showed a net utilization of glucose. There was a striking variability among placentas in the net glucose utilization rate (1.27 μmoles/g/hr-11.44 μmoles/g/hr, coefficient of variation = 62%). The intraplacental coefficient of variation in glucose utilization was only 14%. All placental incubations showed a net production of both lactate (mean = 7.5 μmoles/g placental wet weight/hr) and ammonia (mean = 3.5 μmoles/g placental wet weight/hr). There was no correlation between lactate or ammonia production and glucose utilization. The addition of sodium glutamate (1 μmole/ml) produced no change in glucose utilization or ammonia production. The addition of glutamine (1 and 2 μmoles/ml) produced a significant increase in ammonia production over that found in the controls, but no change in glucose utilization. Incubation with 2 μmoles glutamine/ml demonstrated an increase in lactate production. All incubations showed a striking increase in ammonia concentration after 45 min of incubation. Individual placentas may differ markedly in their ability to utilize glucose in an in vitro system. Ammonia production may be a normal metabolic endproduct in a tissue lacking an active urea cycle or a byproduct of the purine nucleotide cycle. Speculation: The marked variability in glucose utilization by the in vitro human placenta may be an important descriptive characteristic of the normal term placenta that is related to functional differences among apparently similar placentas. The human placenta produces ammonia in vitro at an appreciable rate. A high rate of ammonia and lactate production under aerobic conditions may be a general property of trophoblast.

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