The short-term effect of prolactin on the active Na transport system of the tadpole skin during metamorphosis

Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na(+) from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na(+) channel (ENaC), Na(+)/K(+)-pump, and basolateral K(+) channels, which regulate Na(+) transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 microg/ml, PRL had no effect on the SCC. PRL (1 microg/ml) was sufficient to stimulate the SCC since 1 and 10 microg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 microg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na(+)/K(+)-pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers' solution on each side. PRL (10 microg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na(+)/K(+)-pump [although PRL (1 microg/ml) had no effect on this SCC]. The effect of PRL on basolateral K(+) channels was investigated using apically nystatin-permeabilized skin with high-K Ringer's solution on the apical side. PRL (10 microg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K(+) channels. Thus, although PRL stimulates the Na(+)/K(+)-pump, this effect probably contributes less than that on ENaC to the regulation of Na(+) transport across adult bullfrog skin.

Temperature and humidity alter prolactin receptor expression in the skin of toad (Bufo bankorensis and Bufo melanostictus)

Bufo bankorensis and Bufo melanostictus, the only two species of Bufonidae genus in Taiwan, live in habitats that differ in altitude and humidity. This study tested the hypothesis that prolactin receptor (PRLR) expression responds to environmental change. Western blot analysis showed that the PRLR protein was widely distributed in brain, lung, liver, kidney, dorsal skin and ventral skin of toads. The level PRLR protein was elevated in the dorsal skin of the two toad species treated with dry or wet conditions for 14 days. The increase in PRLR of dorsal skin in B. bankorensis was higher than that in B. melanostictus. This experimental result suggests that B. bankorensis secretes more mucus to reduce water evaporation from its thinner cuticle than B. melanostictus. The expression of PRLR protein was increased in the lung of B. bankorensis and decreased in the lung of B. melanostictus. Moreover, PRLR protein levels were increased in the kidneys in the two species toad, likely due to reduction in water lost through lung and urine. The two toad species were subjected to varying temperatures (25 degrees C, 15 degrees C and 10 degrees C) for 14 days. The lowest PRLR protein expression was observed at 10 degrees C. Comparison of the decreasing trend in PRLR protein levels demonstrated that the variation in B. bankorensis was significantly higher than that in B. melanostictus. Comparisons of variation in PRLR protein expression in the two species under different environments suggest that B. bankorensis is more adaptable to different environments than B. melanostictus.

Prolactin increases open-channel density of epithelial Na+ channel in adult frog skin

The short-term effect of prolactin on the skin of the adult tree frog Hyla arborea japonica was investigated using current-fluctuation analysis. Basolateral application of ovine prolactin (10 microg ml(-1)) (1) increased the amiloride-blockable short-circuit current (SCC) across the skin 2.6+/-0.4-fold and (2) increased the open-channel density (M) of the epithelial Na(+) channel 6.1+/-1.2-fold but decreased the single-channel current i to 0.4+/-0.1 times the control value (N=9). The increase in SCC induced by prolactin was thus due to an increase in M, not i. Apparently, in amphibians prolactin has not only a counteracting effect on metamorphosis but also a stimulatory effect on the development of adult-type features, such as this amiloride-blockable SCC.

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Prolactin inhibits corticoid-induced differentiation of active Na+ transport across cultured frog tadpole skin

Active Na+ transport differentiates in larval bullfrog skin cultured with corticoids. After 2 wk in culture, the epidermis became positive against human blood group antigen A, the marker for the adult-type cells of the epidermis, but was negative to the antibody against the acetylcholine receptor, the marker for the larval-type epidermis. Amiloride (10(-5) M) did not inhibit the differentiation of active Na+ transport. On the other hand, in skin cultured with prolactin (2 micrograms/ml), the epidermis remained negative against antigen A and positive against acetylcholine receptor, and the differentiation of active Na+ transport was inhibited. Thyroid hormone did not antagonize the inhibitory action of prolactin on this transport differentiation. Prolactin affected the basal cells of the larval epidermis and inhibited development of corticoid-induced adult features in the epidermis.

Hormonal effect on the osmotic, electrolyte and nitrogen balance in terrestrial Amphibia

Two main hormones regulate water balance in amphibian. First, mesotocin (MT) acting as a diuretic agent, and second arginine vasotocin (AVT) being an anti-diuretic hormone. In addition, prolactin (PRL), aldosterone, corticosterone, angiotensin II and atriunatriuretic hormones, play a role too in regulating water and ion balance. The hormones affect the epidermis and bladder permeability to water and ions as well as the kidney through the control of the glomerular filtration rate (GFR). The main questions concern the presence and action of these hormones during the amphibian's life history. Are they present in both larval and adult stages? Are these hormones being synthesized in both aquatic and terrestrial adult phases? Under what circumstances are they being stored or released? Would the target organs (epidermis, bladder, kidney) respond in a similar way during all periods? The problem is the fact that under most circumstances an amphibian while in an aquatic environment responds physiologically differently than when on land. Only partial information concerning hormone presence, release and control of water balance is available at the moment, and even that is fragmentary and based on only a very small number of amphibian species.

Seasonal variations in the effect of prolactin on the short-circuit current across bullfrog skin

The effect of serosal application of ovine prolactin (PRL) on the short-circuit current (SCC) across the abdominal skin of larval or adult Rana catesbeiana was compared between two seasons, spring-summer and autumn-winter. Prolactin induced a transient increase in SCC in Cl-Ringer's solution, to a greater extent in spring-summer than in autumn-winter. The skin of stage XXV frogs was more sensitive to PRL than the skin of adult frogs. On the other hand, PRL had no effect on SCC in SO4-Ringer's solution either in summer or in autumn.

Effect of prolactin on the active Na transport across Rana catesbeiana skin during metamorphosis

The effect of long-term application of prolactin (PRL) on active Na transport across the abdominal skin of Rana catesbeiana tadpoles during metamorphosis was investigated. At Taylor-Kollros stage XX, the potential difference (PD) and short-circuit current (SCC) across the skin were absent and resistance to an active Na current (RNa) was infinite. At stage XXI, the PD and SCC clearly appeared and increased thereafter. The RNa after stage XXI remained at a relatively constant value of about 5-10 k omega.cm2, whereas the electromotive force of the active Na current (ENa) greatly increased in stages XXI-XXII. It thus appears that Na channels are first formed at stage XXI, and that the Na pump rapidly develops during stages XXI-XXII. Long-term application of PRL was started at both stage XXI (when PD, SCC, and ENa are still low) and stage XXV (when PD, SCC, and ENa are high). Prolactin (20 micrograms/g body wt) was injected every other day for 2 weeks. Since the PD, SCC, and ENa remained low after the injection was started at stage XXI, early treatment with PRL apparently inhibits the differentiation of the Na pump (i.e., ENa). By contrast, treatment with PRL starting at stage XXV decreased the PD and SCC and increased the RNa, but was without effect of the ENa of this group. It appears that PRL has no effect on the Na pump already differentiated, although it inhibits the maintenance of Na channels formed in earlier stages.

Effect of prolactin on transcutaneous Na transport in the Japanese newt, Cynops pyrrhogaster

The effect of ovine prolactin (PRL) on the electric parameters related to active Na transport across the skin was investigated in the Japanese newt, Cynops pyrrhogaster. Spring newts living in a pond or stream for breeding and autumn newts migrating from water to land for hibernation were collected and used for the experiments. Autumn newt skin was found to have a higher transepithelial potential difference (TEP) and higher resistances to the active Na current (RNa) and to the shunt pathway (RS) than spring newt skin. Injection of PRL, 20 micrograms/g body wt, into autumn newts every other day for 2 weeks induced an increase in RNa, resulting in decreases in TEP and the short-circuit current (SCC), but barely changed the skin resistance (RM), the electromotive force of the active Na current (ENa), or RS. On the other hand, such injections of PRL into spring newts induced decreases in TEP and SCC, but had no effect on RM, ENa, RNa, or RS. Autumn newts appear to be more sensitive to PRL than spring newts. Insofar as autumn newts are concerned, it is concluded that the effect of PRL is probably due to blockade of the Na channel rather than inhibition of Na pump activity.