Les hormones gonadotropes de l'hypophyse : biochimie et biologie comparées ; spécificité et évolution
1980; Elsevier BV; Volume: 20; Issue: 2 Linguagem: Francês
10.1051/rnd
ISSN1297-9708
Autores Tópico(s)Growth Hormone and Insulin-like Growth Factors
ResumoThe pituitary gonadotropic hormones : comparative biochemistry and biology ; specificity and evolution.This review summarizes recent data on the nature of pituitary gonadotropins (GTH) and their biosynthesis, secretion, control and biological effects in vertebrates.While the existence of LH-and FSH-like hormones has been demonstrated in a num- ber of tetrapods, it is still controversial in fish.With the possible exception of the stimula- tion of yolk protein uptake in oocytes, one GTH may assume the various gonadotropic functions of the pituitary.Like mammalian GTH's, the fish GTH's purified so far have two subunits.Using carp gonadotropin (c-GTH), the partial sequences of these subunits were determined to be homologous to the mammalian a and types.c-GTH(3 was also shown to be closer to mammalian LH! than to FSH P .Immunological studies have confirmed these findings and demonstrated that, from fish to mammals, the antigenic determinants were better conserved for LH! than for the rx subunits.An hybrid molecule, formed by associat- ing bovine LHa and c-GTHp, was more active in frogs than either of the mother-hormones, while it was inactive on adenylate cyclase in homogenates of rat or goldfish ovaries.This hybrid, however, was as active as c-GTH in increasing c-AMP concentration in pieces of eel ovary.Interestingly, sturgeon gonadotropin (aci-GTH) had somewhat similar biological properties.Aci-GTH subunits were partially purified, and an hybrid (bovine LHa-aci- modifications at functional sites like those governing a and [3 association and cell hormone binding.Several hypotheses have been proposed for this molecular evolution.The (3 subunit would play a major role in the determinism of functional and zoological specificities of action, i.e. hormone recognition by the target cells.The cellular structures recognizing the hormones and permitting them to act also evidence modifications during vertebrate evolution.It is difficult to speculate on these changes since the exact nature of the receptors is unknown.However, it is suggested that each strategy (hormonal change, receptor modification) prevailed at different stages of vertebrate phylogenesis.Some concepts in comparative biology, such as the idea of receptor variability, are discussed : (i) even within one species, the specificity of the different receptors for the same hormone may vary (ii) there are probably several types of receptors with different speci- ficities (iii) specificity in a same type may also vary with the physiological conditions.It is finally pointed out that some species of lower vertebrates may be useful experimental models for studying general problems in reproductive biology.Abréviations GTH gonadotropine.LH lutropine.FSH follitropine.TSH thyréotropine.CG gonadotropine chorionique.La ou les lettres minuscules placées avant les sigles des hormones désignent l'origine zoologique de ces dernières (initiale ou initiales du nom latin du genre) : c (de carpe), aci (d'esturgeon), het (de poisson-chat indien), onc (de saumon du Pacifique), ang (d'anguille), ti (de Tilapie), scy (de roussette), h (d'homme), o (de mouton), b (de boeuf), su (de porc).PMSG gonadotropine de sérum de jument gravide.LHRH hormone hypothalamique de Mammifères entraînant une décharge de LH et FSH.AMPc 3'5' adénosine monophosphate cyclique.GMPc 3'5' guanosine monophosphate cyclique.AC adénylate cyclase.PK protéine kinase.PAGE électrophorèse en gel de polyacrylamide.K D coefficient d'exclusion en filtration sur gel.AS antisérum.RIA dosage radioimmunologique.La biologie comparée des hormones gonadotropes hypophysaires constitue, pour plusieurs raisons, un thème de recherche en plein développement.La diversité des Vertébrés, liée à leur phylogénie et aux adaptations à des environnements très variés, se reflète dans les modalités du contrôle des gonades ; en plus de leur intérêt fonda- mental, les recherches en physiologie comparée de la reproduction sont donc suscep- tibles de fournir des modèles originaux utiles pour la solution de problèmes de biologie générale.D'un autre côté, la connaissance des propriétés des GTH de certaines espèces dont l'élevage présente un intérêt économique est nécessaire : elle permettrait une application fructueuse des traitements hypophysaires qui maintiennent ou accélè- rent la reproduction, souvent difficile à obtenir chez des animaux en captivité.Un troi- sième aspect, enfin, concerne l'évolution biochimique et physiologique.Une circons- tance où ce thème peut être abordé, avec prudence certes, mais avec quelques chances * Les Squamates groupent les lézards et les serpents.
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