Development of bilirubin metabolism and transport in the neonate.

Comprehensive physiologic study of the developmental processes of bilirubin metabolism and transport reveal a complex interaction of various steps. Phase I Physiologic Jaundice results from the simultaneous increase in bilirubin load presented to the liver and decrease in bilirubin conjugating capacity. Phase II appears to result from a mild decrease in hepatic uptake capacity, coupled with the continuing increase in bilirubin load. Since these results are based upon studies of newborn rhesus monkeys, confirmatory studies in human neonates are required. Perhaps the most challenging aspect of these observations relates to the concept of a developmentally determined delicate imbalance between two functions. It is unlikely that pharmacologic agents could radically alter a single function. Therefore, it is perhaps more realistic to think that drug treatments which only slightly alter two functions simultaneously but in the appropriate directions could more effectively reduce the risk of toxicity. Thus, a mild increase in bilirubin conjugation coupled with a small but significant decrease in bilirubin load could markedly alleviate the severity of physiologic jaundice.