The role of growth hormone, dexamethasone and triiodothyronine in the regulation of glutamine synthetase in primary cultures of rat hepatocytes.

Abstract

Parenchymal cells from adult rat liver, maintained in monolayer culture, showed a rapid decay of the specific activity of glutamine synthetase between 12 and 20 h after plating, and a subsequent stabilization at about 30% of initial levels. Single hormone treatment with dexamethasone (2 μM) or human growth hormone (4 μg/ml) did not significantly alter the enzyme activity. However, simultaneous addition of both hormones during the second day of cultivation increased the level of glutamine synthetase 60–70% above control within 24 h, and more than 100%, if hormone addition was repeated after 10 h. This stimulation occurred after a lag phase of 6–8 h, and could be reinforced by the presence of triiodothyronine, which neither by itself nor in combination with dexamethasone or with growth hormone affected the enzyme activity. The concerted action of growth hormone and dexamethasone depended strongly on time, probably due to an insensitivity of the cultured hepatocytes against growth hormone during the first and after the third day of cultivation, while dexamethasone was found to act on the hepatocytes throughout the cultivation causing induction of tyrosine aminotransferase. The study of the dose-response of either dexamethasone or growth hormone revealed that the glucocorticoid had to be present in effective concentrations throughout to yield a continuous increase in enzyme activity. Human growth hormone was maximally effective in concentrations 50–100 times that of normal rat plasma. The effect of this hormone could be attributed to a somatogenic action on the hepatocytes, because its effect was mimicked by porcine growth hormone, whereas prolactin failed to stimulate glutamine synthetase. Preincubation of the hepatocytes with dexamethasone resulted in a shortening of the lag phase, while preincubation with growth hormone enhanced the rate of stimulation. Presence of cycloheximide or puromycin during the preincubation period completely abolished the effect of dexamethasone, whereas the increment in response to growth hormone was only partially affected, particularly during the first part of the preincubation period. It is suggested that dexamethasone exerts a permissive action, and growth hormone is responsible for the onset of the stimulatory process. Several presumptive characteristics of the hormonal action are outlined in the discussion.