Abstract
1. The administration of triamcinolone (19–190μg/animal) to postnatal rats increased the arginine synthetase system activity 1.2–2.5-fold above control values 24h after exposure to the hormone. Cortisol (hydrocortisone), however, increased the arginine synthetase system activity only when larger (190μg/animal) or repeated daily doses were given. Glucagon (100μg/animal) stimulated arginine synthetase system activity only after the second postnatal day. None of these agents increased the activity in 19.5–21.5-day foetuses after intrauterine administration. 2. The viability of foetal rat liver explants maintained in organ culture for up to 54h was validated both by ultramicroscopic examination and by incorporation of radioactive leucine and orotic acid. 3. In organ cultures of foetal rat liver explants (18.5 days to term), triamcinolone (20μg/ml of medium) evoked a 2.8–4.3-fold increase after 24h of incubation. This increase was completely inhibited by actinomycin D (25μg/ml) or cycloheximide (10μg/ml). Cortisol (5–50μg/ml) or glucagon (0.067–67μg/ml) also increased the arginine synthetase system activity above the respective control values, but there was no increase in activity with insulin (0.05–0.25i.u./ml). 4. Maximum concentrations of glucagon (67μg/ml), dibutyryl cyclic AMP (6-N,2′-O-dibutyryladenosine 3′:5′-cyclic monophosphate) (0.1mm) and triamcinolone (20μg/ml) incubated for 24h with foetal rat liver explants each produced between a two-and three-fold increase in the activity of the arginine synthetase system. Combinations of maximum amounts of glucagon and the cyclic nucleotide did not produce a greater effect than either agent alone. However, the combination of dibutyryl cyclic AMP with triamcinolone appeared to produce somewhat less than additive effects. 5. The effects of the cyclic nucleotide and triamcinolone were evident after 12h of incubation and increased steadily throughout the 24h of observation. This time-course of increased enzyme activity is very much slower than that reported for the induction of other enzymes in explant cultures of foetal rat liver.
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