Changes in NAD(P)H fluorescence and membrane current produced by glutamate uptake into salamander Müller cells

1. Glutamate uptake into isolated, whole-cell patch-clamped glial cells was studied by monitoring the increase of cell fluorescence generated as glutamate and NAD(P) were converted into alpha-ketoglutarate and NAD(P)H by glutamate dehydrogenase. The current generated by the glutamate uptake carrier was recorded simultaneously. 2. L-Glutamate evoked an increase of cell fluorescence and an inward uptake current. L- and D-aspartate generated an uptake current but no fluorescence response, consistent with the amino acid specificity of glutamate dehydrogenase. 3. In the absence of external sodium the glutamate-evoked fluorescence response and uptake current were abolished, showing that there is no sodium-independent glutamate uptake across the cell membrane. 4. Varying the glutamate concentration altered both the fluorescence response and the uptake current. The fluorescence response saturated at a lower glutamate concentration than the uptake current, and depended in a Michaelis-Menten fashion on the uptake current. 5. The fluorescence response and the uptake current were reduced by membrane depolarization, and also by removal of intracellular potassium. 6. The dependence of the fluorescence response on uptake current when membrane potential was altered or intracellular potassium was removed was the same as that seen when the external glutamate concentration was altered. 7. These fluorescence studies show that glutamate uptake is inhibited by depolarization and by removal of intracellular potassium, consistent with the conclusion of earlier work in which uptake was monitored solely as a membrane current. The data are consistent with high-affinity electrogenic sodium- and potassium-dependent glutamate uptake with fixed stoichiometry being the only significant influx route for glutamate. Other possible interpretations of the data are also discussed.

The mechanism of ion transport by the Na(+)-Ca2+,K+ exchange in rods isolated from the salamander retina

1. Membrane currents caused by the operation of electrogenic Na(+)-Ca2+,K+ exchange were recorded from isolated rod outer segments under voltage-clamp using a whole-cell electrode. 2. Reversed mode exchange currents (Na+i-Ca2+o,K+o) were recorded with a high internal [Na+] and when both Ca2+ and K+ were present in the external solution. Omission of either Ca2+ or K+ completely suppressed both the reversed exchange current and the entry of Ca2+. 3. The charge transferred by the exchange per Ca2+ ion transported was identical in both forward and reversed modes. 4. The reversed exchange current declined as Ca2+ accumulated inside the outer segment, and the form of this decline was consistent with a first-order inhibition by internal Ca2+. 5. The reversed exchange current was increased e-fold by a 230 mV depolarization over the range -51 to +29 mV. 6. The activation of reversed exchange by external Ca2+ was well described by first-order kinetics with a Michaelis constant, KappCao, of 34 microM in the presence of 20 mM external K+. KappCao was reduced by lowering external [K+], was increased by adding external Na+ and was unaffected by membrane potential. 7. External K+ also activated the exchange in a first-order manner with a Michaelis constant, KappKo, of 151 microM in the presence of 0.5 mM external Ca2+. KappKo was reduced by lowering external [Ca2+], increased by adding external Na+ and was unaffected by membrane potential. 8. When the level of internal Ca2+ was increased via reversed exchange, KappCao diminished in proportion to the reduction in the maximum current, but KappKo remained approximately constant. 9. These observations cannot be reconciled with simple models of the exchange in which ions bind simultaneously at opposite faces of the membrane before transport occurs. The results are broadly consistent with a consecutive model of the exchange in which unbinding of Na+ at either the external or the internal membrane surface is followed by binding of Ca2+ and then K+, and are fully reproduced by a model in which Ca2+ binds before all of the Na+ has dissociated from the exchange molecule.

Peripheral projections of nervus terminalis LHRH-containing neurons in the tiger salamander, Ambystoma tigrinum

The peripheral projections of the nervus terminalis (NT) have been difficult to examine due to the weak immunoreactivity of the processes to various antibodies. We performed two experimental manipulations in the tiger salamander in an attempt to increase the luteinizing hormone-releasing hormone-immunoreactive (LHRH-ir) labelling in the peripheral processes of the NT:1) the NT was sectioned centrally, or 2) a 100 mg melatonin pellet was embedded subcutaneously for 3 days prior to sacrifice. Following these manipulations, animals were sacrificed and tissue was processed with standard immunocytochemical techniques for the analysis of the distribution of LHRH-ir processes. In the nasal cavity, LHRH-ir fibers were observed projecting 1) into the rostral olfactory epithelium, 2) to Bowman's glands in the lamina propria of the rostromedial olfactory mucosa and ventrolateral mucosa between the main nasal cavity and Jacobson's organ, 3) into the naris constrictor muscle, and 4) along the palatine nerves and ganglia. These lesion and hormone manipulations have enabled the detection of peripheral projections of the NT not observed previously with immunocytochemical procedures alone. The wide distribution of LHRH-ir NT processes in the nasal cavity and cranium suggests that this nerve may influence many different cranial structures during appropriate pheromonal or neuroendocrine events.

D2-like dopamine receptors in amphibian retina: localization with fluorescent ligands

Dopamine induces several light adaptive changes in amphibian retina via receptors with D2-like pharmacology, but the identity of the primary target cells has not been determined. Using a fluorescent probe consisting of a selective D2 antagonist, N-(p-aminophenethyl)-spiperone (NAPS), derivatized with the fluorophore Bodipy (NAPS-Bodipy), we identified the distribution of dopamine binding sites in the retina of two amphibians, post-metamorphic Xenopus laevis and larval Ambystoma tigrinum. Specific labeling was defined as staining that was displaced by D2 selective ligands (eticlopride or sulpiride), but insensitive to D1 selective drugs (SCH 23390), adrenergic catecholamines (epinephrine or norepinephrine), or serotoninergic analogues (ketanserin). Both rod and cone cells showed specific dopamine D2-like binding sites arranged in clustered arrays on discrete membrane domains of the inner segment. Labeling of photoreceptor outer segments was continuous and was not displaced by competition with D2 selective ligands; this labeling was considered nonspecific. In addition, in both species, clustered binding of the D2-probe was found on Müller cells and on a subset of inner retinal cells with the morphology of amacrine/interplexiform cells. Our data provide direct evidence for D2 receptors on both rods and cones, and suggest that the receptors may be clustered into patches within a discrete cellular domain, the inner segment.

Metamorphic changes in glycolipids and myelin proteins and 2',3'-cyclic nucleotide 3'-phosphohydrolase in bullfrog and axolotl brains

The metamorphic changes in levels of glycolipids and myelin proteins and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) in the brains of bullfrog tadpoles, adult frogs, and axolotls were investigated, with particular emphasis on myelin maturation. The concentrations of cerebroside, sulfatide, and galactosyldiacylglycerol gradually increased from the onset of prometamorphosis throughout the active metamorphic period and then greatly increased after metamorphosis was completed. The ratio of glucocerebroside to galactocerebroside increased greatly in the prometamorphic period and then rapidly decreased to the frog level during the climax period. The fatty acid compositions of cerebroside and sulfatide showed a developmental change, with 24:1 being more predominant in the later metamorphic stage. The proportion of hydroxy fatty acids increased up to the onset of the prometamorphic stage and thereafter remained constant at approximately 50% of the total. The CNP activity remained unchanged throughout metamorphosis at 60% that in frog myelin and increased in the adult frog. The composition of tadpole myelin proteins remained constant during metamorphosis, with large basic protein being the most abundant, and in the frog, proteolipid protein and large basic protein were present in comparable amounts. The two adult forms of axolotl, i.e., the neotenous and metamorphosed forms, exhibited almost identical myelin constituents, and CNP activity in the neotenous form amounted to one-fifth that in the bullfrog. These results indicate that active biosynthesis of myelin marker components occurs as metamorphosis proceeds, but more pronounced changes of myelin components occur after metamorphosis is completed.

Characterization of the glutamate transporter in retinal cones of the tiger salamander

L-Glutamate elicits an inwardly rectifying current at hyperpolarized potentials in isolated retinal cones of the tiger salamander, as measured under whole-cell patch clamp. Evidence presented in this article supports the notion that cones possess a high-affinity glutamate transporter. This glutamate-elicited current shows no desensitization over a period of several minutes, and has an affinity (Km) of 10 microM. The inward current is mimicked by the amino acids L-aspartate, D-aspartate, L-cysteate, and to a lesser extent D-glutamate. It is neither blocked by the glutamate receptor antagonists kynurenic acid (1 mM), 6-cyano-7-nitroquinoxaline-2,3-dione (100 microM), or 2-amino-5-phosphonovalerate (100 microM), nor elicited by the glutamate receptor agonists (100 microM each) kainate, quisqualate, NMDA, or 2-amino-4-phosphonobutyrate. The glutamate-elicited current was reduced by the glutamate transport blockers dihydrokainate (DHKA), DL-threo-beta-hydroxyaspartate (beta HA), and L-trans-pyrrolidine-2,4-dicarboxylic acid. When glutamate was present on both sides of the membrane, the blockers reduced both uptake and release; the blocker-sensitive current as a function of membrane potential represents the transport current-voltage relation (I-V), and the reversal potential of the I-V represents the transporter equilibrium potential. This potential was a function of the equilibrium potential for glutamate. DHKA and beta HA depolarized horizontal cells in a retinal slice, and abolished their light responses, suggesting that in the absence of glutamate transport, glutamate concentrations in the cleft rise to a level that saturates the postsynaptic receptors. The high capacity of the cone glutamate transporter is well suited for the rapid removal of glutamate from the synaptic cleft required for the signaling of a light onset to postsynaptic cells.

Ionic selectivity of Ih channels of rod photoreceptors in tiger salamanders

Ionic selectivity of Ih channels of tiger salamander rod photoreceptors was investigated using whole-cell voltage clamp. Measured reversal potentials and the Goldman-Hodgkin-Katz voltage equation were used to calculate permeability ratios with 20 mM K+ as a reference. In the absence of external K+, Ih is small and hard to discern. Hence, we defined Ih as the current blocked by 2 mM external Cs+. Some small amines permeate Ih channels, with the following permeability ratios (PX/PK):NH4+, 0.17; methylammonium, 0.06; and hydrazine, 0.04. Other amines are tially impermeant: dimethylammonium (< 0.02), ethylammonium (< 0.01), and tetramethylammonium (< 0.01). When K+ is the only external permeant ion and its concentration is varied, the reversal potential of Ih follows the Nernst potential for a K+ electrode. Ih channels are also permeable to other alkali metal cations (PX/PK): T1+, > 1.55; K+, 1; Rb+, > 0.55; Na+, 0.33; Li+, 0.02. Except for Na+, the relative slope conductance had a similar sequence (GX/GK): T1+, 1.07; K+, 1; Rb+, 0.37; NH4+, 0.07; Na+, 0.02. Based on permeabilities to organic cations, the narrowest part of the pore has a diameter between 4.0 and 4.6 A. Some permeant cations have large effects on the gating kinetics of Ih channels; however, permeant cations appear to have little effect on the steady-state activation curve of Ih channels. Lowering K+ or replacing K+ with Na+ reduces the maximal conductance of Ih but does not shift or change the steepness of its voltage dependence. With ammonium or methylammonium replacing K+ a similar pattern is seen, except that there is a small positive shift of approximately 10 mV in the voltage dependence.

Monoamines and their metabolites in the amphibian (Ambystoma tigrinum) brain: Quantitative changes during metamorphosis and captivity

1. Monoamine neurotransmitters (epinephrine, norepinephrine, dopamine, serotonin and some of their metabolites (DOPEG, MHPG, DOPAC, 5-HIAA) were measured by HPLC in extracts from telencephalon (TEL) and diencephalon-midbrain (DM) before, during at the end of metamorphosis. 2. During metamorphosis MHPG increased and 5-HIAA decreased in TEL and DM while DOPEG decreased only in DM. 3. Monoamine levels were greater in the TEL and a larger increase in MHPG occurred there. 4. Captivity without metamorphosis also caused a significant depression of 5-HIAA in TEL and depression of DOPEG, MHPG and DOPAC in DM.

Leydig cells in the lingual epithelium of the axolotl, Ambystoma mexicanum, are immunoreactive for serotonin

The Leydig cells in the lingual epithelium of the axolotl were investigated by immunohistochemistry using serotonin antiserum. Serotonin-immunoreactivity was found in their secretory granules. The physiological role of serotonin in the Leydig cell, a type of exocrine cell, is unknown.

Primary structure of neutral and acidic oligosaccharide-alditols derived from the jelly coat of the Mexican axolotl. Occurrence of oligosaccharides with fucosyl(alpha 1-3)fucosyl(alpha 1-4)-3-deoxy-D-glycero-D-galacto-nonulosonic acid and galactosyl(alpha 1-4)[fucosyl(alpha 1-2)]galactosyl(beta 1-4)-N-acetylglucosamine sequences

Six major neutral and acidic oligosaccharide-alditols were prepared from the jelly coat of Mexican axolotl eggs. These compounds were demonstrated to contain 3-deoxy-D-glycero-D-galacto-nonulosonic acid (dNloA) and L-fucose (Fuc). The structures of the six major oligosaccharides were established as follows: [sequence: see text]

Biochemical plasticity in the Arizona tiger salamander (Ambystoma tigrinum nebulosum)

The Arizona tiger salamander, Ambystoma tigrinum nebulosum, is a developmentally polymorphic species. Some individuals become sexually mature while retaining some larval traits (paedomorphs), while other individuals mature as metamorphosed salamanders. In this study, relative enzyme activities of the products of two duplicate loci in each of three enzyme systems (aconitase, malate dehydrogenase, and isocitrate dehydrogenase) were measured in paedomorphs and in paedomorphs forced to metamorphose by treatment with thyroxine. We found that thyroxine and laboratory conditions affect enzyme activities of four of the six enzymes examined and that activities of products of duplicate loci are altered to different degrees.

Ultrastructural localization of sialylated glycoconjugates in cells of the salamander olfactory mucosa using lectin cytochemistry

An indirect gold-labeling method utilizing the lectin from Limax flavus was employed to characterize the subcellular distribution of sialic acid in glycoconjugates of the salamander olfactory mucosa. The highest density of lectin binding sites was in secretory vesicles of sustentacular cells. Significantly lower densities of lectin binding sites were found in secretory granules of acinar cells of both Bowman's and respiratory glands. Lectin binding in acinar cells of Bowman's glands was confined primarily to electron-lucent regions and membranes of secretory granules. In the olfactory mucus, the density of lectin binding sites was greater in the region of mucus closest to the nasal cavity than in that closest to the epithelial surface. At the epithelial surface, the density of lectin binding sites associated with olfactory cilia was 2.4-fold greater than that associated with microvilli of sustentacular cells or non-ciliary plasma membranes of olfactory receptor neurons, and 7.9-fold greater than non-microvillar sustentacular cell plasma membranes. Lectin binding sites were primarily associated with the glycocalyx of olfactory receptor cilia. The cilia on cells in the respiratory epithelium contained few lectin binding sites. Thus, sialylated glycoconjugates secreted by sustentacular cells are preferentially localized in the glycocalyx of the cilia of olfactory receptor neurons.

Amacrine cells in the tiger salamander retina: morphology, physiology, and neurotransmitter identification

Amacrine cells of the vertebrate retina comprise multiple neurochemical types. Yet details of their electrophysiological and morphology properties as they relate to neurotransmitter content are limited. This issue of relating light responsiveness, dendritic projection, and neurotransmitter content has been addressed in the retinal slice preparation of the tiger salamander. Amacrine cells were whole-cell clamped and stained with Lucifer yellow (LY), then processed to determine their immunoreactivity (IR) to GABA, glycine, dopamine or tyrosine hydroxylase (TOH), and glucagon antisera. Widefield, ON-OFF amacrine cells were glycine-IR. The processes of these cells extended laterally in the inner plexiform layer (IPL) from 250-600 microns. They were either multistratified in the IPL or monostratified near the IPL midline. Three multistratified ON-OFF narrowfield glycine-IR cells also were found. Four types of ON amacrine cells were found to be GABA-IR; all types had their processes concentrated in the proximal IPL (sublamina b). Type I cells were narrowfield (approximately 100 microns) with a compact projection. Type II cells were widefield (220-300 microns) with a sparse projection. Type III cells had an asymmetrical projection and varicose processes. Type IV cells were pyriform and monostratified in sublamina b. One narrowfield ON-OFF amacrine cell, with processes broadly distributed in the middle of the IPL, was GABA-IR. This cell appeared similar to an ON-OFF cell that was glycine-IR and may comprise a type in which GABA and glycine colocalize. Another class of amacrine cell, with processes forming a major plexus along the distal border of the IPL and a lesser plexus in the proximal IPL, produced slow responses at light ON and OFF; these cells were dopamine/TOH-IR. A narrowfield class of transient ON-OFF amacrine cell, with processes ramifying throughout both sublaminae a and b of the IPL, were glucagon-IR; these cells appeared to be dye-coupled at the soma. We have shown that, with respect to GABA, glycine, dopamine, and glucagon, salamander amacrine cells fall into rather discrete groups on the basis of ramification patterns in the IPL and responses to photic stimulation. The physiological, structural, and neurochemical diversity of amacrine cells is indicative of multiple and complex roles in retinal processing.

Reinvestigation of DNA ligase I in axolotl and Pleurodeles development

We have recently shown that the exclusion process causing the replacement of DNA ligases II by DNA ligase I in amphibian eggs after fertilization does not occur in the case of Xenopus laevis [Hardy, S., Aoufouchi, S., Thiebaud, P., and Prigent, C., (1991) Nucleic Acids Res. 19, 701-705]. Since this result is in contradiction with the situation reported in axolotl and Pleurodeles we decided to reinvestigate such results in both species. Three different approaches have been used: (1) the substrate specificity of DNA ligase I; (2) the DNA ligase-AMP adduct reaction and (3) the immunological detection using antibodies raised against the X.laevis DNA ligase I. Our results clearly demonstrate that DNA ligase I activity is associated with a single polypeptide which is present in oocyte, unfertilized egg and embryo of both amphibians. Therefore, the hypothesis of a change in DNA ligase forms, resulting from an expression of the DNA ligase I gene in axolotl and Pleurodeles early development must be rejected. We also show that, in contradiction with published data, the unfertilized sea urchin egg contains a DNA ligase activity able to join blunt ended DNA molecules.

Localization of substance P, CGRP, VIP, neuropeptide Y, and somatostatin immunoreactive nerve fibers in the carotid labyrinths of some amphibian species

Immunohistochemical localization of substance P (SP), CGRP, VIP, neuropeptide Y (NPY), and somatostatin (SOM) in the carotid labyrinth were compared in some species of amphibians using the peroxidase-antiperoxidase method. Immunoreactivity of SP, CGRP, VIP, and NPY was found in the nerve fibers distributed in the intervascular stroma of the carotid labyrinth. SP, CGRP, and VIP immunoreactive varicose fibers were densely distributed in the peripheral portion of the carotid labyrinth. Some SP-immunoreactive fibers were distributed similarly to CGRP-immunoreactive fibers. The density of NPY and SOM immunoreactive varicose fibers was low. No immunoreactivity of enkephalins was observed in the labyrinth. The intensities of these peptides were varied from species to species. No glomus cells showed immunoreactivity for any of the 7 peptides studied. These results suggest that the vascular regulatory function, which is one of the possible functions of the carotid labyrinth, is controlled by the peptidergic mechanisms in addition to regulation through intimate apposition of glomus and smooth muscle cells (g-s connection).

Localization of smooth-muscle markers in the ovaries of some ectothermic vertebrates

In the present study, we have localized desmin and alpha-smooth-muscle actin in the ovaries of the zebrafish, the axolotl, and the red-eared turtle, using the unlabelled antibody peroxidase-antiperoxidase technique. In the axolotl ovary both smooth-muscle markers were demonstrated in cord-like structures, extended along ovarian blood vessels, and in some inner ovarian epithelium cells. In the ovaries of the teleost, smooth-muscle-like cells are detected in a suspensory apparatus formed by venous cords, the tunica albuginea, and the coat around the ovarian artery. Also, in the turtle ovary, smooth-muscle-like cells were found in a suspensory apparatus formed by chordae, the tunica albuginea, and the theca externa of the ovarian follicles. At the present time, the prevailing hypothesis is that, in addition to a role in the mechanical support, the smooth-muscle-like cells in the ovaries of these vertebrates seem to be important with respect to ovarian contractile processes.

Heterogeneity in spinal radial glia demonstrated by intermediate filament expression and HRP labelling

Considerable evidence indicates that radial glial cells play an active role in guiding growing neurites during development of the vertebrate CNS. In this paper we describe subpopulations of radial glial in the spinal cord of the axolotl. Amphibians maintain radial glia throughout life, and subpopulations are described using anatomical criteria following filling of individual cells with horseradish peroxidase and immunocytochemical staining with a range of intermediate filament antibodies. Radial glial cells in specific regions of the spinal cord stain with a range of antibodies specific to human keratins 8 and 18, and to glial fibrillary acid protein (GFAP). Some of these antibodies show selective localized to specific regions of individual glial cell processes. Immunoblotting analysis indicates that two keratins are present in the axolotl CNS corresponding to the two earliest embryonic keratins of vertebrates, keratin 8 and 18. Comparisons of molecular weight indicate that these may correspond to keratins identified in Xenopus laevis, the genes of which have been cloned. Axolotl GFAP is also identified in Western blots and may be present in two forms of differing molecular weight. These results are discussed in terms of the likely role of radial glial cells, and comparisons are drawn between the keratin and GFAP types seen in the axolotl spinal cord and of those in other vertebrate groups.

Voltage-dependent ionic currents in taste receptor cells of the larval tiger salamander

Voltage-dependent membrane currents of cells dissociated from tongues of larval tiger salamanders (Ambystoma tigrinum) were studied using whole-cell and single-channel patch-clamp techniques. Nongustatory epithelial cells displayed only passive membrane properties. Cells dissociated from taste buds, presumed to be gustatory receptor cells, generated both inward and outward currents in response to depolarizing voltage steps from a holding potential of -60 or -80 mV. Almost all taste cells displayed a transient inward current that activated at -30 mV, reached a peak between 0 and +10 mV and rapidly inactivated. This inward current was blocked by tetrodotoxin (TTX) or by substitution of choline for Na+ in the bath solution, indicating that it was a Na+ current. Approximately 60% of the taste cells also displayed a sustained inward current which activated slowly at about -30 mV and reached a peak at 0 to +10 mV. The amplitude of the slow inward current was larger when Ca2+ was replaced by Ba2+ and it was blocked by bath applied CO2+, indicating it was a Ca2+ current. Delayed outward K+ currents were observed in all taste cells although in about 10% of the cells, they were small and activated only at voltages more depolarized than +10 mV. Normally, K+ currents activated at -40 mV and usually showed some inactivation during a 25-ms voltage step. The inactivating component of outward current was not observed at holding potentials more depolarized -40 mV. The outward currents were blocked by tetraethylammonium chloride (TEA) and BaCl2 in the bath or by substitution of Cs+ for K+ in the pipette solution. Both transient and noninactivating components of outward current were partially suppressed by CO2+, suggesting the presence of a Ca2(+)-activated K+ current component. Single-channel currents were recorded in cell-attached and outside-out patches of taste cell membranes. Two types of K+ channels were partially characterized, one having a mean unitary conductance of 21 pS, and the other, a conductance of 148 pS. These experiments demonstrate that tiger salamander taste cells have a variety of voltage- and ion-dependent currents including Na+ currents, Ca2+ currents and three types of K+ currents. One or more of these conductances may be modulated either directly by taste stimuli or indirectly by stimulus-regulated second messenger systems to give rise to stimulus-activated receptor potentials. Others may play a role in modulation of neurotransmitter release at synapses with taste nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of kainic and quisqualic acid receptors on inner retinal cells of the salamander Ambystoma mexicanum

The presence of kainic (KA) and quisqualic acid (QA) receptors on inner retinal neurones of the axolotl Ambystoma mexicanum has been studied using intracellular recording techniques. In the presence of CoCl2, which blocks neurotransmitter release, KA and QA depolarized the membrane. The minimum concentration of KA that induced a response was 1 microM and a maximum response was obtained with 10 microM (EC50: 3 microM). The operating range of QA was between 0.5 and 5 microM with an EC50 of 1 microM. These data show that inner retinal cells of the axolotl are sensitive to KA and QA. Cis-2,3-piperidine dicarboxylic acid (PDA, 3 mM) completely blocked responses to 5 microM KA, but not those induced by 2 microM QA. This suggests that the KA- and QA-sensitive receptors on inner retinal cells of the salamander are pharmacologically different and that PDA can be a valuable tool in distinguishing KA- and QA-sensitive receptors on these neurones.

Acid-base-electrolyte balance responses to catecholamine antagonists in Ambystoma tigrinum

Neotenic larval Ambystoma tigrinum were subjected to hypercapnia (3% CO2, 22 Torr) for 24 h under different conditions: alpha-adrenergic blockade using phentolamine, beta-adrenergic blockade using propranolol, and sham treatments. The sham animals were able to carry out a partial extracellular pH compensation that consisted of an increase in extracellular [HCO3-]. Animals treated with catecholamine antagonists did not compensate to the same extent. Analysis of plasma samples by high-performance liquid chromatography with electrochemical detection revealed a significant increase in circulating norepinephrine, but not epinephrine, during the high-CO2 exposure. Measurements of cutaneous ion transport showed that beta-antagonists block the increased Na+ influx associated with hypercapnia, whereas alpha-antagonists inhibited the decrease in cutaneous Cl- influx that is also associated with respiratory acidosis. Additionally, both alpha- and beta-blockers inhibited the increase in transcutaneous potential difference that accompanied the respiratory acidosis. The results are consistent with a role for circulating catecholamines in compensatory ion transport responses to respiratory acidosis in this species.

The effect of eel calcitonin on calcium influx and plasma ion levels in axolotls, Ambystoma mexicanum

Plasma total calcium and calcium influx, measured during 4-h 45Ca influx experiments, were lower in control axolotls, Ambystoma mexicanum, during August when compared to those in July. A single intraperitoneal injection of 10 micrograms of synthetic eel calcitonin significantly reduced the calcium influx rates during both months but the effect was greater in July (P less than 0.001) than in August (P less than 0.05). Plasma levels of calcium, sodium, potassium, and magnesium were not affected. In neotonous wholly aquatic amphibians, eel calcitonin may work in the same way as it does in fish by reducing the uptake of dissolved calcium from the ambient medium.

Immunocytochemical localization and immunochemical characterization of an insulin-related peptide in the pancreas of the urodele amphibian, Ambystoma mexicanum

The pancreas of the axolotl, Ambystoma mexicanum, was investigated by immunocytochemical methods for the presence of immunoreactivity to a number of antisera raised against mammalian insulins. All anti-insulin antisera tested revealed substantial amounts of reaction products confined solely to the aldehyde-fuchsinophilic B cells of the endocrine pancreas. The reactive cell population was detected by use of one polyclonal antiserum against bovine insulin and eight different monoclonal antibodies against insulins from various mammalian species. Six of these antibody clones have known specificity to sub-regions of the insulin molecule. Additionally, fractions of an ethanol-HCl extract of pancreatic tissue from Ambystoma was studied in both conventional dot-blot tests by means of the same panel of antibodies and a two-site sandwich time-resolved immunofluorometric assay for human insulin involving two of the monoclonal antibodies. These experiments support the immunocytochemical observations by demonstrating the existence of an insulin-related peptide with a great deal of structural resemblance to mammalian insulins and displaying antigenic determinants in common at least with the amino acid residues A8-10 and B26-30. In conclusion, we interpret the findings as indicating that the immunocytochemically revealed tissue bound antigen in the Ambystoma pancreatic B-cells may be a peptide related to human insulin.

Binding of adrenergic ligands to liver plasma membrane preparations from the axolotl, Ambystoma mexicanum; the toad, Xenopus laevis; and the Australian lungfish, Neoceratodus forsteri

The beta-adrenergic ligand iodocyanopindolol (ICP) bound specifically to hepatic plasma membrane preparations from the axolotl, Ambystoma mexicanum (Bmax, 40 fmol/mg protein (P) at free concentration above 140 pM; KD, 42 pM); the toad, Xenopus laevis (Bmax, 200 fmol/mg P at 1 nM; KD, 300 pM); and the Australian lungfish, Neoceratodus forsteri (Bmax, 100 fmol/mg P at 5 nM). For the lungfish, the Scatchard plot was curved showing two classes of binding site with KD's of 20 and 500 pM. Neither the alpha 1-adrenergic ligand prazosin nor the alpha 2-adrenergic ligand yohimbine bound specifically to hepatic membrane preparations from any of the three species. Several adrenergic ligands displaced ICP from hepatic membrane preparations of all three species with KD's of Axolotl--propranolol, 50 nM; isoprenaline, 600 nM; adrenaline, 10 microM; phenylephrine, 20 microM; noradrenaline, 40 microM; and phentolamine, greater than 100 microM; X. laevis--propranolol, 30 nM; isoprenaline, 100 microM; adrenaline, 200 microM; noradrenaline, 300 microM; phenylephrine, 1 mM; and phentolamine, greater than 1 mM; N. forsteri,--propranolol, 25 nM; isoprenaline, 1 microM; adrenaline, 20 microM; phenylephrine, 35 microM; noradrenaline, 600 microM; and phentolamine, 400 microM. These findings suggest that alpha-adrenergic receptors are not present in hepatic plasma membrane preparations from these three species and that the hepatic actions of catecholamines are mediated via beta-adrenergic receptors. The order of binding of the beta-adrenergic ligands suggests that the receptors are of the beta 2 type.

Hyaluronate accumulation and nerve-dependent growth during regeneration of larval Ambystoma limbs

Hyaluronate-mediated expansion of the extracellular matrix has been suggested as an important element of growth and morphogenesis in several developing systems. In vitro, various growth factors have been shown to stimulate hyaluronate synthesis as well as cell proliferation. A similar link between proliferation and hyaluronate production during in vivo growth is difficult to demonstrate, because in most systems the source of growth-promoting factors is either not known or not amenable to experimental manipulation. During amphibian limb regeneration, cell proliferation depends upon paracrine release of factors from axons in the limb stump, and the nerve supply can be eliminated or augmented experimentally for study of growth in this system. Denervated and amputated limbs of larval salamanders do not begin to regenerate until distal areas of the limb stumps are reinnervated. We have used such limbs to examine the effect exerted by the reappearance of nerves on the amount of hyaluronate in the tissue undergoing the growth response. Hyaluronate was demonstrated by the metachromatic dye Ethyl Stains-all, which stains hyaluronate blue while sulfated glycosaminoglycans (GAGs) and proteins in the extracellular matrix stain various shades of violet, and by microspectrophotometry of alcian-blue-stained GAGs in serial sections pretreated with buffer or with Streptomyces hyaluronidase (SH) to remove hyaluronate specifically. Both methods showed little hyaluronate in the distal region of limb stumps prior to reinnervation, while reinnervated stumps had amounts of hyaluronate similar to those of control blastemas. Autoradiography of 3H-glucosamine-labeled limbs indicated that hyaluronate in the blastemas of reinnervated limb stumps included material newly synthesized by cells throughout the growing tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of alpha-melanotropin from the pars intermedia of the larval amphibian, Ambystoma tigrinum

The effect of background adaptation on the steady-state levels of alpha-melanotropin in the pars intermedia of the larval amphibian. Ambystoma tigrinum, was investigated. Acid extracts of pars intermedia obtained from light-adapted and dark-adapted animals were analyzed by radioimmunoassay following Sephadex gel filtration chromatography, reverse-phase HPLC, and Sulfopropyl Sephadex cation-ion-exchange chromatography. For both background adaptation conditions similar results were obtained. The major form of alpha-melanotropin present in the pars intermedia has the following properties: (1) an apparent molecular mass of 1.5 kDa; (2) a net charge at pH 3.5 of +4; and (3) a retention time following reverse-phase HPLC similar to that of synthetic ACTH(1-13)amide. In dark-adapted animals a minor form of alpha-melanotropin which has a net charge of +3 at pH 3.5 was also detected. The latter form represented approximately 10% of the total alpha-melanotropin immunoreactivity in the pars intermedia of dark-adapted animals. These results strongly suggest that the predominant form of alpha-melanotropin in the pars intermedia of larval A. tigrinum is a nonacetylated ACTH(1-13)amide-like polypeptide.