Development of aldosterone-stimulation of short-circuit current across larval frog skin

Localization of two nitric oxide synthase isoforms, eNOS and iNOS, in the skin of Triturus italicus (Amphibia, Urodela) during development

We analyzed the skin of Triturus italicus (Amphibia, Urodela) histologically during the larval, pre-metamorphic and adult phases in parallel with the immunohistochemical evaluation of the expression of two nitric oxide synthase (NOS) isoforms, i.e. the inducible NOS (iNOS) and the endothelial NOS (eNOS). Our results indicate that, during the larval and adult phases, substantial changes in the intensity and localization of both iNOS and eNOS are present. In contrast, the pre-metamorphic newts show a labelling pattern similar to that found in the skin of adult individuals. These data suggest an involvement of the NOS system in the larval epidermis during functional maturation phase starting at the climax and preceding structural rearrangements during metamorphosis, emphasizing the putative functional importance of differential isoform expression related to a distinct phase of the biological cycle.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Possible role of aldosterone and T(3) in development of amiloride-blockable SCC across frog skin in vivo

There are inconsistencies between the in vitro and in vivo effects of thyroid hormone and aldosterone (Aldo) on the development of an amiloride-blockable short-circuit current (SCC) across bullfrog skin [Takada, M., H. Yai, and K. Takayama-Arita. Am. J. Physiol. 268 (Cell Physiol. 37): C218-C226, 1995]. To address this issue, tadpoles were raised in Aldo + T(3). An amiloride-blockable SCC developed across the skin before forelimbs appeared. Noise analysis of the characteristics (single-channel current, blocking and unblocking rate coefficients, and apparent dissociation constant) of this amiloride-blockable Na(+) channel showed that it really was of the adult type. A similar SCC developed at stage XIX in the skin of tadpoles raised with Aldo alone. These results strongly support our hypothesis that the crucial hormone in the development of this SCC is Aldo but that a suppression mechanism attenuates its effect on SCC development until it is removed by the increase in the serum concentration of thyroid hormone (which starts at stages XVIII-XIX in vivo).

Prolactin enables normal development of ACh-stimulated current in cultured larval bullfrog skin

The response to acetylcholine (ACh) can be used as a marker for larval-type bullfrog skin because apically applied ACh induces an increase in short-circuit current (SCC) in larval-type but not adult-type skin. EDTA-treated larval skin, which contains only basal cells and does not respond to ACh, was used as the starting material for our culture. ACh, carbamylcholine, and choline stimulated SCC in skin that had been cultured with aldosterone (5 x 10(-7) M) supplemented with prolactin (PRL; 2 micrograms/ml). Atropine and d-tubocurarine each inhibited the ACh-induced stimulation of SCC in skin so cultured. Eserine, an inhibitor of acetylcholinesterase, also inhibited the ACh response. Amiloride stimulated SCC itself, but it reduced the ACh response. All of these results are quite similar to those seen in intact larval skin, suggesting that a larval-skin had differentiated from the basal cells used as the starting point for our culture. This is the first physiological report that PRL induces differentiation in vitro into a true larval-type bullfrog skin.

Corticoid-induced differentiation of amiloride-blockable active Na+ transport across larval bullfrog skin in vitro

The hormone-induced differentiation of an active Na+ transport across larval bullfrog skin during metamorphosis was investigated in vitro and in vivo. In in vitro experiments, EDTA-treated larval dorsal skin from which apical cells were removed was used. Even in the absence of thyroid hormone, corticoids induced the differentiation. Although aldosterone was the most potent hormone, hydrocortisone or corticosterone was also effective. Prolactin inhibited the corticoid-induced differentiation. The differentiation of the transport system coincided almost exactly with the appearance of adult features of the epidermis, namely, the epidermis at 7 days carried the human blood group antigen A, a specific molecular marker of adult-type bullfrog epidermis. The transport system appeared to develop in cells that had been newly generated from basal cells. On the contrary, in in vivo experiments, the effect of amiloride on the short-circuit current of the skin of tadpoles raised in the presence of aldosterone was very small, suggesting that a mechanism exists to inhibit the ability of aldosterone to induce the differentiation of the transport system in vivo.

Invertebrate epithelial Na+ channels: amiloride-induced current-noise in crab gill

Epithelial sheets (including cuticle) from posterior gills of the freshwater-adapted euryhaline crab Eriocheir sinensis were obtained according to the method of Schwarz and Graszynski ((1989) Comp. Biochem. Physiol. 92A, 601-604; (1989) Verh. Dtsch. Zool. Ges. 82, 211 and (1989) Arch. Int. Physiol. Biochim. 97, C45). With external NaCl-saline, the outward-directed short-circuit current (Isc) could hardly be influenced by external amiloride up to 100 mumol/l but was, on the contrary, strictly dependent on apical Cl- (Onken, Graszynski and Zeiske (1991) J. Comp. Physiol. B 161, 293-301). In absence of external chloride an inward-directed, amiloride-inhibitable Isc was observed which depended on external Na+ (thus, Isc approximately INa) in a two-step, saturating mode. The Isc-block by amiloride obeyed saturation kinetics (half-maximal at less than or equal to 1 mumol/l, suggesting apical Na(+)-channels). Only for Na+ concentrations below 100 mmol/l we found an indication for a competitive interaction between Na+ and amiloride at the channel. Current fluctuation analysis revealed the presence of an amiloride-induced relaxation (Lorentzian) component in the Isc-noise (so-called 'blocker-noise'). The Lorentzian parameter-shifts with increasing amiloride concentration indicate first-order kinetics of the blocker with its apical receptor. Using a 'two-state' blocking model we calculated, for amiloride concentrations between 2 and 5 mumol/l, a mean single-channel current of 0.46 pA and a mean channel density of 250.10(6) cm-2.