Effects of nutritional and hormonal factors on the metabolism of retinol-binding protein by primary cultures of rat hepatocytes.

Studies were conducted to explore hormonal and nutritional factors that might be involved in the regulation of retinol-binding protein (RBP) synthesis and secretion by the liver. The studies employed primary cultures of hepatocytes from normal rats. When cells were cultured in Dulbecco's modified Eagle's medium alone, a high rate of RBP secretion was observed initially, which declined and became quite low by 24 hr. Supplementing the medium with amino acids maintained RBP and albumin secretion at moderate (but less than initial) rates for at least 3 days. Further addition of dexamethasone maintained the production and secretion rates of RBP, transthyretin, and albumin close to the initial rates for up to 3-5 days in culture. The effects of dexamethasone were not rapid and were not specific for RBP; half-maximal effects were seen at 10(-9) to 10(-8) M levels. Hormonally treated hepatocytes produced and secreted RBP, transthyretin, and albumin at both absolute and relative rates similar to physiological values, as estimated from rates reported by others from studies in vivo (with both rats and humans) and with perfused livers. Glucagon addition partially maintained the secretion rates of these 3 proteins, but less effectively than did dexamethasone. A number of other hormones, added singly or in combination, did not affect RBP production or secretion. Addition of retinol to the cultured normal hepatocytes was without effect upon RBP secretion. These studies show that supplementing the culture medium of hepatocytes with amino acids and dexamethasone maintains RBP production and secretion for several days. In normal hepatocytes, with ample supply of retinol available within the cell, addition of exogenous retinol does not appear to influence RBP metabolism or secretion by the cells.