ABSTRACT

The effects of Nomegestrol acetate (17α-acetoxy-6-methyl-19-nor-4,6-pregnadiene-3,20-dione), a new 19-nor-progesterone derivative, on renal Na⁺/K⁺-ATPase activity were assessed in normal and adrenalectomized rats, and compared with the stimulatory or inhibitory actions produced by other steroids. This compound displayed an inhibitory effect which was similar to, but smaller than, that induced by progesterone and quite distinct from the stimulation produced by 19-nor-progesterone and corticosteroids. In addition, unlike progesterone, it did not antagonize the effect of aldosterone in adrenalectomized rats. This result, together with previous in-vivo and in-vitro observations on this compound indicates that additional modifications introduced in the molecular structure of 19-nor-progesterone produces a potent progestagenic substance virtually devoid of effects on renal Na⁺/K⁺-ATPase activity and sodium loss in urine.

J. Endocr. (1986) 110, 37–41

Neurohypophysial hormone receptors and second messengers were studied in trout (Oncorhynchus mykiss) hepatocytes. Arginine vasotocin (AVT) and isotocin (IT) elicited a concentration-dependent inhibition of cAMP accumulation in the presence of 5x10(-8) M glucagon (maximal effect for 4.5x10(-7) M and 1.4x10(-7) M, half-maximal effect for 2.1x10(-8) M and 0.7x10(-8) M, AVT and IT respectively). The effect of glucagon was inhibited up to 90% by AVT and 80% by IT. While AVT inhibited (up to 50%) the basal cAMP production, IT had no such action. Specific V(1) or V(2) analogues (with reference to vasopressin in mammals) were used for pharmacological characterization of the type of neurohypophysial hormone receptor involved in this inhibition. The V(1) agonist [Phe(2), Orn(8)]-oxytocin inhibited the glucagon-stimulated cAMP production with a maximal effect for 6x10(-7) M and a half-maximal effect for 0.9x10(-8) M concentrations of the analogue. While the V(1) agonist reduced the glucagon-stimulated cAMP level by 70%, it showed only a tendency to reduce the basal level. The V(2) agonist [deamino(1), Val(4),d -Arg(8)]-vasopressin had no effect either on basal or on glucagon-stimulated cAMP production. The V(1) antagonist [d(CH(2))(5)(1), O-Me-Tyr(2), Arg(8)]-vasopressin totally reversed the 10(-8) M AVT-induced inhibition of 5x10(-8) M glucagon-stimulated cAMP production, whereas the V(2) antagonist [d(CH(2))(5)(1),d -Ile(2), Ile(4), Arg(8), Ala(9)]-vasopressin had no such effect. In this particular case, maximal and half-maximal effects of the V(1) antagonist were obtained for 2.3x10(-6) M and 1. 2x10(-6 )M respectively. Changes in intracellular calcium content were measured using the fluorescent probe FURA-2/AM. AVT and IT elicited a concentration-dependent increase in Ca(2+) accumulation. The comparison of the effect of 10(-8) M agonists versus AVT showed the following order of potency: AVT=IT>V(1) agonist>V(2) agonist. The V(1) antagonist reversed the AVT-induced Ca(2+) accumulation whereas the V(2) antagonist had no such effect. These results are taken as evidence for the presence in trout hepatocytes of neurohypophysial hormone receptors functionally close to the V(1a)-type linked to cAMP production and Ca(2+) mobilization.

ABSTRACT

Cyclic AMP levels were measured in freshly isolated hepatocytes of the rainbow trout. Compared with basal values, the average levels were increased up to 60 times in a dose-dependent manner either by mammalian glucagon (concentration range 1 nmol– 1 μmol/l; dose giving half maximum response (EC₅₀) 0· 18 μmol/l) or by forskolin (concentration range 0·1–100 μmol/l; EC₅₀ about 10 μmol/l). These stimulatory effects were partially inhibited by fish or mammalian neurohypophysial hormones used at relatively high concentrations (1–5 μmol/l). It is suggested that these results are evidence for the presence of V₁-type receptors in fish hepatocytes. Together with previous results obtained with gills on the hormonal inhibition of adenylate cyclase activity, they suggest that teleost fish may possess only V₁-type receptors (or two V₁-related types), while the V₂ receptors have evolved (or have become functional) in higher vertebrates.

J. Endocr. (1988) 119, 439–445

Abstract

We analysed the effects of specific neurohypophysial analogues for pharmacological characterization of the type of vasotocin receptor involved in the control of the adrenocorticotrophin hormone (ACTH) release from the perifused pituitary in the rainbow trout, Oncorhynchus mykiss.

Mammalian corticotrophin releasing factor (CRF) and teleostean neurohypophysial peptides (arginine vasotocin (AVT) and isotocin (IT)) stimulated ACTH release. Analysis of concentrations giving half-maximal effects (D₅₀) showed that these peptides affected ACTH release in the following order of potency: CRF (8 × 10⁻¹³ m) >AVT (2 × 10⁻¹⁰ m)>IT (10⁻⁷ m). Maximal responses (D_max) were obtained for hormonal concentrations of 10⁻¹⁰ m, 10⁻⁸ m and 10⁻⁶ m respectively. This suggests that AVT and IT have different roles in the control of ACTH release. The values obtained for AVT and IT were in agreement with the circulating levels we previously found for these peptides.

Specific V₁ or V₂ agonists or antagonists (with reference to vasopressin in mammals) were used to define the specificity of the neurohypophysial peptide receptor involved in this stimulation. The V₁ agonist, [Phe², Orn⁸]-oxytocin, stimulated ACTH release while the V₂ agonist, [deamino¹, Val⁴, d-Arg⁸]-vasopressin, had no such effect. Maximal and half-maximal responses were obtained in the presence of the V₁ agonist with 10⁻⁷ m and 7 × 10⁻⁹ m respectively, and were in the range of values obtained with natural peptides. The V₁ antagonist, [d(CH₂)₅ ¹, O-Me-Tyr², Arg⁸]-vasopressin, and the V₂ antagonist, [d(CH₂)₅ ¹, d-Ile², Ile⁴, Arg⁸, Ala⁹]-vasopressin, maximally reversed the 10⁻⁹ m AVT-stimulated ACTH release by 60% and 25% respectively, for a 5 × 10⁻¹⁰ m concentration of the analogues and a D₅₀ approximately 2 × 10⁻¹¹ m.

These results demonstrated the presence of only one V₁-type receptor in fish pituitary, with some of the structural and functional peculiarities typically displayed by the mammalian V_1a-type receptor, but distinct from it. In this sense, the fish pituitary vasotocin receptor may represent a novel type of neurohypophysial hormone receptor, more closely related to the mammalian V_1b-type.

Journal of Endocrinology (1996) 149, 109–115