Effect of prolactin on thyroxine-induced metamorphosis

An immunohistochemical and morphometric analysis of insulin, insulin-like growth factor I, glucagon, somatostatin, and PP in the development of the gastro-entero-pancreatic system of Xenopus laevis

The ontogeny of the classical islet hormones insulin (INS), glucagon (GLUC), somatostatin (SOM), and pancreatic polypeptide (PP) as well as insulin-like growth factor I (IGF-I) in the gastro-entero-pancreatic (GEP) system of Xenopus laevis (stages 41-66) was studied using double immunofluorescence and morphometric analysis. As early as stage 41, clustered INS-immunoreactive (-IR) and isolated GLUC-IR cells occurred in the pancreas. The first SOM-IR cells appeared at stage 43, followed by PP-IR cells at stage 46. About 79% of the PP immunoreactivity was confined to a subpopulation of the GLUC-IR cells. Both the GLUC/PP-IR cells and the PP-IR cells were located in a distinct area of the pancreas. The first islets occurred in premetamorphosis (around stage 50) and comprised mainly INS-IR and GLUC-IR cells. The majority of SOM-IR, PP-IR, and GLUC/PP-IR cells was dispersed. The numbers of hormone cells remained quite constant until the end of prometamorphosis (stage 58). Around stages 60-62, the islets were partly disintegrated and the numbers of islet cells slightly decreased. At stage 63, the cell number began to increase and reached the levels typical for the adult around stage 66. After metamorphic climax, the islets were reformed. In the gastrointestinal tract, transient INS-IR cells occurred prior to the adaptation of the gastrointestinal tract to feeding (stages 41-44) and during metamorphosis when there is remodeling of the gastrointestinal tract (stages 60-63). Therefore, INS released from the transient mucosal INS-IR cells may be involved in the temporary proliferation of mucosal epithelial cells. The first GLUC-IR and SOM-IR cells were seen at stage 41. PP-IR cells followed at stage 46. In contrast to the islets, GLUC-IR and PP-IR cells constituted different cell populations. Around stage 46, the first IGF-I immunoreactions appeared in the GEP-system. In pancreas, IGF-I immunoreactivity was found in the GLUC/PP-IR, cells (85-99%) but was absent from INS-IR, GLUC-IR, and SOM-IR cells. The IGF-I-IR gastro-entero-endocrine cells, however, seemed to contain none of the classical islet hormones.

Developmental and thyroid hormone-dependent regulation of pancreatic genes in Xenopus laevis

We have isolated and sequenced a cDNA encoding Xenopus laevis pancreatic trypsin, which has approximately 70% amino acid sequence identity to mammalian trypsinogen. Northern blotting analysis shows that the trypsin gene is activated just before the tadpole starts to feed, reaches peak activity in the swimming tadpole (premetamorphosis), and is then repressed during prometamorphosis, attaining its lowest activity at the climax of metamorphosis. The same gene is then activated again in frogs but to a much lower level. The pattern of the changes in trypsin gene expression is followed by at least two other pancreas-specific genes and marks the remodeling of the pancreas of the animal at metamorphosis. Thyroid hormone, which is the causative agent of metamorphosis, can down-regulate trypsin gene expression prematurely.

Skull development during anuran metamorphosis: III. Role of thyroid hormone in chondrogenesis

Metamorphosis of cranial cartilages in anuran amphibians constitutes one of the most dramatic and extensive ontogenetic transformations in vertebrates. We quantitatively examined the role of thyroid hormone (3,3',5-triiodo-L-thyronine; T3) in mediating gross aspects of this morphological repatterning in the skull of the Oriental fire-bellied toad, Bombina orientalis. T3 was administered via plastic (Elvax) micropellets in three treatment dosages (2.5, 0.25, and 0.025 microgram) and one control dosage (0 microgram) to tadpoles of three Gosner developmental stages--28/29, 30/31, and 32/33; tadpoles were recoved up to 8 d (treatment and control dosages) or 14 d (control dosage) later. Response of larval cartilages to exogenous T3 was dosage dependent but not implant-stage dependent; chondrogenic tissues that participate in metamorphic transformation are competent to respond to T3 well before they normally do. Metamorphic effects of T3 were visible within 2 d; in most treatment groups, the normal metamorphic sequence was two-thirds complete after 8 d. While T3 also induced precocious ossification, the normal temporal relation between bone formation and cartilage transformation was dissociated in experimental groups. Morphological integration between cartilage and bone during cranial metamorphosis is at least partly the result of each tissue responding independently to endocrine factors.

Investigations on the role of cAMP in regulating resorption of the tail fin from tadpoles of Rana catesbeiana

Conflicting reports have appeared regarding the role of cAMP in regulating resorption of the tadpole tail during anuran metamorphosis. That cyclic nucleotide has been suggested as a mediator of the effects of both the thyroid hormones and prolactin. We tested the effects of cAMP and its derivatives dibutyryl-cAMP and 8-bromo-cAMP on explants of tail fin from tadpoles of Rana catesbeiana maintained in tissue culture. Unmodified cAMP (0.1, 2 mM) did not influence resorption. Dibutyryl-cAMP (0.1, 1 mM) and 8-bromo-cAMP (1 mM) inhibited resorption of explants induced by thyroxine (T4). The phosphodiesterase inhibitor isobutylmethylxanthine similarly inhibited regression of explants cultured with T4. None of these agents affected the increase in specific activity of hexosaminidase brought about by T4. Although the effects of cAMP in antagonizing tail resorption were similar to those of prolactin, we found no direct effect of prolactin on levels of cAMP in cultured tail fin. Thus, the effects of prolactin appear not to be mediated by increased levels of cAMP. We conclude, however, that the elevation of cellular levels of cAMP does inhibit the resorptive action of T4.

Observations on the interaction of prolactin and thyroxine in the tail of the bullfrog tadpole

Explants of tail fin from tadpoles of Rana catesbeiana maintained in tissue culture underwent resorption when exposed to a medium containing 150 nM thyroxine (T4). Increases in the specific activity of hexosaminidase (Hex) and of acid phosphatase (AP) were associated with this response. Ovine prolactin inhibited resorption of explants in a dose-responsive manner; however, prolactin had no influence on the specific activity of either Hex or AP cultured explants. This result contrasted with the effect of prolactin in vivo, where it inhibited both the resorption of the tail and the increase in specific activity of hydrolytic enzymes induced by T4. The actions of both hormones, when applied in vivo, were more pronounced in the tail fin than in the muscular core of the tail at the stage of regression tested, indicating that the failure of prolactin to influence enzyme induction in vitro did not result from an incapacity of the fin to respond to prolactin in this fashion. It is suggested that the inhibition of T4-induced enzymic activity by prolactin which is observed in the intact animal may come about in response to some related but indirect systemic action of prolactin that is lacking when explants are cultured in vitro. It is concluded that inhibition of the induction of hydrolytic enzymes is not an essential factor in the inhibitory effect of prolactin on the resorption of the tail fin.

Antagonistic effects of prolactin and oxytocin on tail fin regression and acid phosphatase activity in metamorphosing Ambystoma tigrinum

We have found that the inhibition of thyroxine-induced tail fin regression by prolactin in larval tiger salamanders is antagonized by oxytocin. Other workers have shown that prolactin blocks the rise in activity of several hydrolytic enzymes that occurs in regressing tissue during metamorphosis. Here, we examine the effects of prolactin and oxytocin--given alone and in combination--on tail fin regression and acid phosphatase specific activity in this tissue. Both long-term (12-day) and short-term (48-hr) treatment paradigms using prolactin and oxytocin are investigated. The results show that long-term prolactin treatment of metamorphosing larvae blocks fin regression and the rise in acid phosphatase specific activity seen in metamorphosing controls; short-term prolactin treatment of metamorphosing larvae inhibits fin regression within 48 hr, but does not block the rise in acid phosphatase activity seen in controls; oxytocin antagonizes the effects of prolactin on tail fin regression; and oxytocin treatment (long-term or short-term) of metamorphosing larvae causes an elevation of acid phosphatase activity above that seen in metamorphosing controls. With long-term treatment, this effect of oxytocin is slightly antagonized by prolactin; with short-term treatment, no antagonism is observed even though an effect of prolactin on fin height is still evident. We have interpreted these results as suggesting that the effect of prolactin on hydrolase activity is not a prerequisite for its inhibitory effect on fin regression to occur.

Rana catesbeiana tail fin lactogenic hormone receptor: hydrodynamic characterization

The hydrodynamic properties of the 125I-hGH-receptor-detergent complex of the tadpole tail fin were determined. Based on the distribution coefficient from chromatography on Sepharose 6B the Stokes' radius of the complex was 5.03 nm. Using centrifugation through sucrose/H2O and sucrose/2H2O gradients the complex was determined to have a sedimentation coefficient of 4.05 S and a partial specific volume of 0.810 cm3/g. The 125I-hGH-receptor-detergent complex was calculated to have a mol wt of 121,000 and a frictional ratio of 1.38. Based on a calculated detergent content of 0.430 g/g complex the mol weight of the 125I-hGH-receptor was 69,100 and the mol wt of the receptor alone was 47,100.

Characterization of solubilized prolactin receptors from Rana catesbeiana tadpole tissues

Prolactin (PRL) receptors were solubilized from Rana catesbeiana tadpole liver and tail fin and female Sprague-Dawley rat-liver membranes by treatment with 1% Triton X-100. Binding of [125I]oPRL to tadpole-liver and tail-fin solubilized extracts reached equilibrium by 12 h at 19 degrees C. The binding was linear up to 50 micrograms of tadpole liver and tail-fin protein and 30-40 micrograms of rat-liver protein. Solubilization did not affect the dissociation constant for [125I]oPRL binding but did result in a greater number of sites. The binding was specific for oPRL and hGH, which has PRL-like activity. Neuraminidase pretreatment of membranes altered the binding affinity of oPRL to tadpole-liver membranes and solubilized extracts but not to tadpole-tail fin membranes or extracts. Pretreatment of membranes with neuraminidase did not affect the binding capacity of tadpole-liver or tail-fin membranes or solubilized extracts.

Changes in specific prolactin binding in Rana catesbeiana Tadpole tissues during metamorphosis and following prolactin and thyroid-hormone treatment

The prolactin-binding affinity (KD) and number of binding sites *N) in Rana catesbeiana tadpole liver, tail fin and kidney tissues were studied during metamorphosis and following administration of oPRL and L-T3 to premetamorphic tadpoles. With increasing developmental stage there was an increase in N; a maximum was found at stage XVIII followed by a gradual decrease in N through metamorphic climax for all 3 tissues. No change in KD was noted. L-T3 treatment of premetamorphic tadpoles for 7 days caused a significant decrease in tail length and height and body length and an increase in hindlimb length with a concurrent increase in N of approximately 3-fold while treatment for 1 or 3 days was without effect on tadpole morphology or oPRL binding. OPRL treatment for 7 days caused a significant increase in tail length and height and body length with no significant changes in hindlimb length and a 3-5-fold increase in N. Treatment with both L-T3 and oPRL for 7 days resulted in an inhibition of the T3-induced decrease in tail length and height and body length and no inhibition of the hindlimb length increase. N increased in all tissues similar to that found with eight treatment alone. No change in KD was noted in any of these studies. Therefore, oPRL and L-T3 are able to regulate the numbers of specific oPRL-binding sites in amphibian tissues. The change in N with development parallels the reported change in tadpole pituitary capacity to stimulate growth but occurs prior to the reported surge of endogenous T3 during metamorphosis. Thus, the variation in the number of oPRL-binding sites may be due to the changes in endogenous PRL levels during development.

Prolactin and tadpole metamorphosis. Evidence of prolactin receptors in premetamorphic Rana catesbeiana liver and tail fin

The binding characteristics of ovine prolactin (OPRL) to a particulate fraction from liver and tail fin of Rana catesbeiana tadpoles were studied. The specific binding of [125I]oPRL to both tissues was found to be a saturable process with a single class of binding sites in each tissue. Although the dissociation constants were similar for each tissue, the tail fin demonstrated a 10-fold higher binding capacity than the liver tissue. Pretreatment of the liver and tail fin particulate fractions with degradative enzymes revealed that trypsin and phospholipase C reduced the subsequent specific [125I]oPRL binding in both tissues. However, neuraminidase treatment decreased the prolactin binding in the liver while having no effect on the tail fin. The binding of prolactin to the amphibian tissues was found to be specific for prolactin and growth hormones. [125I]oPRL binding to both tissues was a reversible process although the dissociation rate was faster for the tail fin than for the liver. Therefore, prolactin receptors are associated with both a prolactin responsive tissue, the tail, and an unresponsive tissue, the liver, in the tadpole.

The effect of prolactin and thyroxine on tail resorption in R. pipiens: in vivo and in vitro

Thyroxine induces tail resorption in anurans both in vivo and in vitro. This event is accompanied by an increase in the activity of the hydrolytic enzymes acid phosphtase and B-N-acetylglucosaminidase. Prolactin treatment promotes tail growth and directly antagonizes the action of thyroxine on tail tissues. This is evident at the gross morphological level as well as at the biochemical level where prolactin inhibits the increase in activity on the hydrolytic enzymes that occurs during thyroid hormone induced resorption.

Prolactin-thyroxine antagonism and the metamorphosis of visual pigments in Rana catesbeiana tadpoles

The relationship between prolactin-thyroxin antagonism and the metamorphosis of visual pigments in larval amphibians has been investigated using bullfrog (Rana catesbeiana) tadpoles. Althought prolactin-thyroxine antagonism is demonstrable by morphological criteria, ovineprolactin does not appear to anatagonize thyroxine-induced rhodopsin synthesis. The hypothesisis offered that prolactin-thyroxine antagonism is the result of differential gene activities which are opposite in their physiological effects.