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The induction of tyrosine aminotransferase (TAT) in the HTC hepatoma cell line is observed after a single administration of an active steroid. A few minutes of contact between the cells and dexamethasone or corticosterone is sufficient to induce TAT synthesis to its maximal level. When radiolabeled hormones are used, no radioactivity is found in the cell one hour after removal of the hormone from the culture medium, whereas TAT activity remains optimal. Thus, the hormone behaves like a start signal for the optimal synthesis of the enzyme and its continuous presence in the medium is not necessary during the whole induction cycle.

The vasoconstrictive effect of alpha-adrenoceptor agonists was studied on isolated perfused kidney from normal Wistar rats. Methoxamine (ED50 = 1.61 +/- 0.16 X 10(-6) M; m +/- SEM), phenylephrine (2.33 +/- 0.16 X 10(-6) M) and alpha-methylnoradrenaline (1.60 +/- 0.40 X 10(-6) M) acted as full agonists. Dopamine (9.5 +/- 1.08 X 10(-5) M) had a partial agonistic effect. Clonidine and B-HT 920 were inactive up to the concentration of 10(-4) M. Prazosin, but not yohimbine (10(-6) M) antagonized the response to phenylephrine (pA2 = 8.48 +/- 0.12) and to alpha-methylnoradrenaline (8.02 +/- 0.10). Our results suggest that the postsynaptic alpha-adrenoceptors which when stimulated increases vascular resistance of isolated rat kidney, belong exclusively to the alpha 1-subtype.