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At least 955 records · Page 53Linked to original sources
PubMed1984

Intracellular chloride activity and membrane potential in stripped frog skin (Rana temporaria).

The regulation of cell chloride activity in frog skin was investigated using double barrelled Cl--microelectrodes to measure cell membrane potentials and chloride activity in the isolated frog epidermis. Experiments were done under short-circuit conditions, impaling cells from the serosal side. The basic electrophysiological parameters of the isolated skin were similar to those reported in the literature for whole preparations. Intracellular chloride activity was on average 21.9 mM and membrane potential was about 57 mV, implying that chloride was distributed away from its electrochemical equilibrium (i.e., concentrated inside the cells). Chloride activity decreased after removal of either Cl- or Na+ from the serosal bathing solution, with no change in membrane potential. The chloride permeability of the serosal membrane was calculated to be 2.6 X 10(-6) cm X s-1 which represents about 1/4 of the total conductance of the serosal membrane. We suggest that an electrically silent sodium-dependent uphill transport of chloride is present at the basolateral membrane of the frog skin, which accounts for the non-passive distribution of chloride.

Animals↗
PubMed1985

Lipid-protein interactions in frog rod outer segment disc membranes. Characterization by spin labels.

Freely-diffusing phospholipid spin labels have been employed to study rhodopsin-lipid interactions in frog rod outer segment disc membranes. Examination of the ESR spectra leads us to the conclusion that there are two motionally distinguishable populations of lipid existing in frog rod outer segment membranes over a wide physiological temperature range. Each of the spin probes used shows a two-component electron spin resonance (ESR) spectrum, one component of which is motionally restricted on the ESR timescale, and represents between 33 and 40% of the total integrated spectral intensity. The second spectral component which accounts for the remainder of the spectral intensity possesses a lineshape characteristic of anisotropic motion in a lipid bilayer, very similar in shape to that observed from the same spin labels in dispersions of whole extracted frog rod outer segment lipid. The motionally restricted spectral component is attributed to those spin labels in contact with the surface of rhodospin, while the major component is believed to originate from spin labels in the fluid lipid bilayer region of the membranes. Calculations indicate that the motionally restricted lipid is sufficient to cover the protein surface. This population of lipids is shown here and elsewhere (Watts, A., Volotovski, I.D. and Marsh, D. (1979) Biochemistry 18, 5006-5013) to be by no means rigidly immobilized, having motion in the 20 ns time regime as opposed to motions in the one nanosecond time regime found in the fluid bilayer. Little selectivity for the motionally restricted population is observed between the different spin-labelled phospholipid classes nor with a spin-labelled fatty acid or sterol.

Animals↗
PubMed1988

Benzodiazepines stimulate sodium ion transport in frog skin epithelium.

Benzodiazepine binding sites are present in a variety of non-neuronal tissues including the kidney where they are localized to distal nephron segments. It is postulated that renal binding sites are involved in modulating ion transport. This study examined the effects of two benzodiazepines on sodium transport in frog skin epithelium, a model system for sodium transport in renal collecting duct. Treatment of short-circuited frog skin with diazepam (a non-selective benzodiazepine agonist) stimulated amiloride-sensitive short-circuit current, reflecting stimulation of active sodium transport. The diazepam response was equally effective with either serosal or mucosal application of the drug. Maximal stimulation of the current (42 +/- 8%) was achieved with 10 microM diazepam (serosal). Short-circuit current was similarly augmented by serosal or mucosal addition of Ro5-4864, a benzodiazepine agonist with selective activity at peripheral (non-neuronal) receptors. The natriferic response to diazepam was additive to that of vasopressin or cyclic AMP suggesting that the mode of action of benzodiazepines is probably distinct from the cyclic AMP pathway. Thus, frog skin appears to be a useful model to examine the epithelial effects of benzodiazepines. Whether stimulation of sodium transport, however, involves peripheral-type benzodiazepine receptors in this tissue requires further studies.

Animals↗
PubMed1975

Effect of light stimulation on the synthesis and release of GABA in rat and frog retinae.

The effect of light stimulation on the incorporation of label from [14C]glucose and [3H]acetate into aspartate, glutamate, glutamine and GABA, both in vivo and in vitro, was studied in dark-adapted frog and rat retinae. With [14C]glucose as the precursor, but not with [3H]acetate, flashes of light appeared to increase the in vivo syntheses of both GABA and glutamine at the expense of glutamate in frog retina, while attenuating the labelling of aspartate and glutamate in isolated frog tissues. Amino acid synthesis from these radioactive substrates in rat retina was not altered under these conditions. Also, the turnover rates of amino acids derived from [14C]-glucose in rat retina in vivo were closely similar in light and darkness. Isolated retinae from both species accumulated [14C]GABA from the incubation medium. In each case the subsequent release of this compound was augmented by electric field stimulation, high K+ and by a high concentration of cold GABA in the medium, but not by a flashing light. Similarly, application of a light stimulus was unable to evoke the liberation of [14C]GABA derived from [14C]glucose in either species.

Aminobutyrates↗
PubMed1981

Effect of hypophysectomy and pituitary stalk transection on alpha-melanocyte-stimulating hormone-like immunoreactivity in the brain of the frog, Rana ridibunda Pallas.

The existence of an alpha-MSH-like molecule in the frog brain led us to investigate the role of the pituitary gland in the maintenance of the alpha-MSH content in 3 different regions of the brain. Acetic acid extracts of hypothalamus, rhombencephalon and telencephalon were analyzed by means of a highly specific radioimmunoassay for alpha-MSH in normal, sham-operated, pituitary disconnected and hypophysectomized frogs. Transection of the pituitary stalk gave rise to a significant decrease in alpha-MSH content in the intermediate lobe of the pituitary gland (-71% after 3 days), but did not affect alpha-MSH content in the distal lobe or in the brain. Eight days after total hypophysectomy, an alpha-MSH immunoreactive compound, co-eluting with synthetic alpha-MSH on Sephadex G-25, was found in the 3 brain regions studied. Removal of the whole pituitary gland did not significantly modify alpha-MSH content in the hypothalamus and the telencephalon. A slight increase in alpha-MSH was even observed in the rhombencephalon of hypophysectomized animals. Furthermore, no modification in alpha-MSH immunoreactivity occurred in any region of hypophysectomized animals. These results demonstrate the existence of alpha-MSH-like material in the brain of Rana ridibunda and establish that brain alpha-MSH in the frog is not of pituitary origin.

Animals↗
PubMed1982

Ultrastructure of frog cerebral and pial microvessels and their impermeability to lanthanum ions.

Permeabilities of single pial microvessels in the frog are now being estimated. Fine structural information on these vessels will therefore be needed in attempts to correlate structure and permeability. Microvessels from the dorsal part of the telencephalon and from the overlying pial layer in the common frog (Rana temporaria) and their permeability to lanthanum ions were studied in the electron microscope. Extensive structural similarities between the endothelial lining of cerebral and pial microvessels were demonstrated, indicating similarity in functional characteristics. Adjacent endothelial cells in both categories of vessels were joined by elaborate tight junctions. Lanthanum ions did not permeate the endothelium. Diffusion of the ion in the interendothelial clefts was stopped by the junctions. It is concluded that the frog has an endothelial blood-brain barrier which is tight to lanthanum ions.

Animals↗
PubMed1983

Peripheral organization of the vestibular efferent system in the frog: an electrophysiological study.

The distribution and the properties of efferent fibers in vestibular nerve were studied in the isolated frog labyrinth. Electrical stimulation of the central stump of any vestibular nerve branchlet elicited compound action potentials in all the other eighth nerve branchlets, indicating the existence of neural links between the various vestibular organs. The same experimental paradigm, when repeated in frogs with chronic section of the eighth nerve roots, demonstrated that these pathways are efferent collaterals extending to all vestibular organs. There are more collaterals linking the 3 semicircular canals than the otolith organs and the otoliths with the canal organs. Efferent connections in the eighth nerve were preserved in full after ablation of the ipsilateral hemi-cerebellum, suggesting that the efferent pathways probably originate in the brainstem. Intracellular recordings from single afferent fibers of both canal and otolith organs revealed that efferent fiber activation could elicit either inhibition or facilitation of the receptor discharge. It was concluded that the frog efferent vestibular system is endowed with non-selective control channels which allow single neurons to influence the receptor activity of different labyrinthine organs.

Animals↗
PubMed1986

Salt taste responses in the frog glossopharyngeal nerve: different receptor sites for Mg2+ and Na+.

The glossopharyngeal nerve of the frog responds to MgCl2 and NaCl applied to the tongue. To learn whether or not Mg2+ and Na+ react with different receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. Responses were recorded from the frog glossopharyngeal nerve during stimulation of the tongue with 100 mM MgCl2 and 500 mM NaCl. The magnitude of the MgCl2 response was selectively reduced after application of 0.1% pronase E to the dorsal tongue surface. The selective suppression of the MgCl2 response by the pronase treatment indicates that the receptor site responsible for the MgCl2 response differs from that for the NaCl response. The effect of pronase on the MgCl2 response was due to the proteolytic action. This suggests that the receptor site responsible for the MgCl2 response is composed of a protein or interacts secondarily with a protein affected by pronase treatment. It is suggested that there are multiple receptor sites for cations in salt taste reception of the frog.

Animals↗
PubMed1989

Effects of acromelic acid A on the binding of [3H]glutamic acid and [3H]kainic acid to synaptic membranes and on the depolarization at the frog spinal cord.

Triton treatment of synaptic membranes from the frog spinal cord enhanced the specific binding of [3H]glutamic acid compared with non-treated fresh and frozen ones, but not that of [3H]kainic acid. Acromelic acid A specifically inhibited the binding of [3H]kainic acid, and was approximately 100 times more potent than kainic acid. Acromelic acid A and excitatory amino acids caused a depolarization in the ventral root of the frog spinal cord in a dose-dependent manner, and the effect of acromelic acid A was much superior to that of kainic acid or domoic acid. Acromelic acid A is one of the most potent kainic acid agonist at the frog spinal cord.

Animals↗
PubMed1991

Involvement of non-selective cationic channels in the generation of pacemaker depolarizations and firing behaviour in cultured frog melanotrophs.

The firing patterns of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached mode, unitary currents were frequently observed as well as biphasic waveforms which were attributed to action potentials 'leaking' through the patch membrane. An inwardly rectifying single-unit current was observed with pipette solutions containing either 100 mM K+ or 100 mM Na+. Under both conditions, these channels displayed an identical I/V relationship, yielding a unitary conductance of 110 pS. The channel opening time was extremely long (50-3000 ms) and single-channel currents showed typical relaxations, which triggered bursts of action currents. In the whole-cell configuration large (2-12 mV) fluctuations in the membrane voltage of current-clamped cells frequently occurred. The deflections appeared to result from single-channel currents. Depolarizing 'events' often led to the discharge of action potentials. Taken together, our data provide evidence for the existence of high-conductance cationic channels in frog pars intermedia cells. These channels may, at least in some cases, be responsible for the generation of pacemaker depolarizations, thereby regulating firing behaviour. It is concluded, that the current traversing a single channel can seriously affect the membrane potential and excitability of frog melanotrophs.

Action Potentials↗
PubMed1992

Presumptive GABAergic feedback input to the frog olfactory bulb: a double labelling study with retrograde axonal tracing and GABA immunohistochemistry.

The localization of gamma-aminobutyric acid (GABA)-immunoreactive neurons that project to the main olfactory bulb in the frog was studied by combining GABA immunohistochemistry with retrograde axonal tracing. Double-labelled neurons were observed in the dorsal, medial and lateral pallial areas involved in reciprocal connections with the main olfactory bulb. GABA-immunoreactive cell bodies constituted only a part of the population of bulbopetal neurons in each pallial region. This suggests that bulbopetal neurons in the frog pallium belong to neuronal populations which differ in their neurotransmitter specificities. The bulbopetal GABA-immunoreactive pallial neurons may be the source of GABAergic feedback input to the frog main olfactory bulb.

Animals↗
PubMed1976

Neuromuscular blocking action of cadmium and manganese in isolated frog striated muscles.

In isolated frog sciatic nerve--gastrocnemius muscle preparations, exposure for 60 min to Cd2+ (0.1-2 mM) caused a marked attenuation of the twitch tension developed by stimulation of nerves but no or only a slight inhibition of the tension produced by direct stimulation at maximum twitch height. The inhibitory effect was partially reversed by 1 mM cysteine. In frog sartorius muscles, the addition of Cd2+ (0.1 mM) alos abolished the end-plate potential evoked by nerve stimulation but did not suppress potential changes induced by iontophoretically applied acetylcholine. The addition of Mn2+ (2 and 5 mM) attenuated the response of muscles to both direct and indirect stimulation; a greater attenuation of the latter was observed. The inhibition was not reserved by cysteine but was partially reversed by excess Ca2+. Contractile responses of frog rectus abdominis muscles to acetylcholine were not significantly affected by Cd2+ and Mn2+ but were attenuated by d-tubocurarine in a dose-dependent manner. Impulse conduction along sciatic nerves was not impaired by Cd2+ and Mn2+ but was by procaine. It appears that Cd2+ interferes with the release of acetylcholine from motor nerve terminals by reducing the transmembrane influxes of Ca2+, the influx possibly relating to SH groups of membrane constituents.

Acetylcholine↗
PubMed1981

Atypical characteristics of frog and chick erythrocyte beta-adrenoceptors.

The characteristics of beta-adrenoceptors on frog and chick erythrocytes have been compared to those present on rat erythrocyte, lung and cerebral cortical membranes. The receptors have been examined using the specific ligand [3H]dihydroalprenolol ([3H]DHA) with conventional filtration techniques. The equilibrium dissociation constant (KD) of [3H]DHA and the affinities of the non-selective beta-adrenoceptor antagonists propranolol, timolol and alprenolol, were very similar in all preparations examined. The relative order of potency of the catecholamines, isoprenaline greater than adrenaline greater than noradrenaline in rat lung, rat erythrocyte and frog erythrocyte; isoprenaline greater than noradrenaline greater than or equal to adrenaline in chick erythrocyte and rat cortex, suggest an overall beta 2-classification for the former and beta 1-classification of the latter tissues. However, the use of highly selective beta 1- or beta 2-agents and computer-assisted iterative curve fitting revealed that whereas rat cortex and lung possess both receptor subtypes (lung 80% beta 2, 20% beta 1: cortex 65% beta 1, 35% beta 2) only beta 2-sites were observed in rat erythrocytes. On the other hand, chick and frog erythrocytes possess a homogeneous population of receptors that do not strictly correspond either to beta 1- or beta 2-adrenoceptors. The atypical nature of these non-mammalian erythrocyte beta-adrenoceptors is discussed in relation to their previous extensive use as model beta-adrenoceptor systems.

Animals↗
PubMed1987

[3H]desipramine labels with high affinity the neuronal transporter for adrenaline in the frog heart.

Desipramine, a tricyclic antidepressant, inhibits the neuronal uptake of adrenaline and noradrenaline and, as a radioligand, labels the noradrenaline transporter in central and peripheral tissues of the rat. To study whether [3H]desipramine also labels the neuronal adrenaline transporter in vitro, its binding was evaluated in the frog heart, a tissue with a rich adrenergic innervation but virtually devoid of noradrenergic innervation. [3H]Desipramine binding to membranes from the frog heart was of high affinity (Kd = 1.94 nM) and was potently inhibited by nisoxetine and (+)oxaprotiline. Unexpectedly, [3H]desipramine binding to the transporter for adrenaline in the frog heart was also sensitive to inhibition by imipramine and the atypical antidepressants mianserin and iprindol. This is the first study to demonstrate radioligand binding to the neuronal transporter for adrenaline. The results indicate that the pharmacological profile of the transporter for adrenaline may be different from that of the noradrenergic transporter, if species differences can be excluded. It remains to be established if the affinity of imipramine, mianserin and iprindol for the adrenaline transporter contributes to their therapeutic efficacy in depression.

Animals↗
PubMed1992

Adenine nucleotide analogues, including gamma-phosphate-substituted analogues, are metabolised extracellularly in innervated frog sartorius muscle.

The metabolism of adenine nucleotides and of their analogues by ecto-enzymes in the innervated frog sartorius muscle was investigated with HPLC. The breakdown of beta, gamma-methylene-ATP was also evaluated by studying the ability of the adenosine uptake inhibitor, dipyridamole, and of the adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), to modify the effect of beta, gamma-methylene-ATP on nerve-evoked twitches. ATP-gamma-S at low (10 microM) but not at high (> or = 100 microM) concentrations was quickly metabolised into a compound with a higher negative charge. L-ATP, homo-ATP and 2-methylthio-ATP were metabolised into compounds with a lower negative charge. Beta-gamma-Imido-ATP and gamma-anilino-ATP were only metabolised slightly. As determined by HPLC, beta, gamma-methylene-ATP was not metabolised. In contrast, this ATP analogue inhibited nerve-evoked twitch responses, an effect which was potentiated by dipyridamole and antagonised by DPCPX. Alpha, beta-Methylene-ATP was dephosphorylated into alpha, beta-methylene-ADP, which was virtually resistant to metabolism in the absence of ATP. In the presence of ATP, alpha, beta-methylene-ADP was transiently phosphorylated into alpha, beta-methylene-ATP. Formation of ATP from ADP was observed even in the absence of an exogenous phosphate donor, and was prevented by the adenylate kinase inhibitor, P1P5-di-(adenosine-5')pentaphosphate (AP5A). AP5A caused only partial inhibition of AMP formation from ADP. The results suggest that some ATP analogues with substitutions in the gamma-phosphate, such as ATP-gamma-S and beta, gamma-methylene-ATP, are metabolised in the innervated frog sartorius muscle. The ADP analogue, alpha, beta-methylene-ADP, might be a substrate for an ecto-nucleoside diphosphate kinase. ADP, besides being dephosphorylated, is also a substrate for an ecto-adenylate kinase in innervated frog sartorius muscle.

Adenine Nucleotides↗
PubMed1983

Phospholipid molecular species of frog rod outer segment membranes.

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were isolated from frog retinal rod outer segment (ROS) membranes and their major molecular species determined. Saturated fatty acids are predominantly located on position-1 and polyunsaturates on position-2, although this assignment is not absolute. One-fifth of the species of PC are of the saturated and monoenoic type, compared to only trace amounts of these species in PE. On the other hand, PE contains at least 50% dipolyunsaturated species, while PC has 6%. The predominant molecular species of frog PC are: 18:0-22:6 omega 3 (39%), 16:0-22:6 omega 3 (17%), 16:0-16:1 (6%), 16:0-18:1 (5%) and 16:0-16:0 (4%). The major molecular species of frog PE are: 22:6 omega 3-22:6 omega 3 (21%), 18:0-22:6 omega 3 (18%), 18:1-22:6 omega 3 (14%), 22:4 omega 6-22:6 omega 3 (10%), 22:5 omega (3 + 6)-22:6 omega 3 (10%), 16:0-22:6 omega 3 (7%) and 20:4 omega 6-22:6 omega 3 (5%). The specific molecular species are discussed in relation to their effect on fluidity and asymmetry of ROS disc membranes.

Animals↗
PubMed1983

The electrical coupling of epithelium and fibers in the frog lens.

Electrical coupling has been measured between central epithelial cells and fiber cells in the frog lens following the removal of the lens capsule with collagenase. Current passed from a microelectrode inserted in a lens fiber cell was found to induce a potential difference in epithelial cells which was indistinguishable from that measured in nearby fiber cells. The measurements were made under circumstances where the current and voltage microelectrodes could be shown to be in different cells and an intraepithelial location of the voltage-measuring microelectrode could be verified. The electrical coupling appears to be direct from superficial fiber cells to epithelial cells, rather than indirect through equatorial epithelial cells. The epithelial cells and surface fiber cells each had resting potentials of -61.8 +/- 2.1 mV in this collagenase-treated preparation. Scanning electron microscopy (SEM) of mechanically decapsulated frog lenses showed specialized junction-like plaques on the expanded anterior ends of fiber cells which made contact with epithelial cells. Such specialized plaques were not found on the expanded posterior ends of these fibers where they contracted the posterior capsule. The studies provide direct evidence for cell-to-cell communication between frog lens epithelial cells and fibers.

Animals↗
PubMed1985

Adenine nucleotide and P-creatine levels in layers of frog retina as a function of dark and light adaptation.

Eight layers of frog retina were analyzed for ATP, P-creatine, ATP + ADP, and AMP under conditions of dark, 2 sec, 2 min, and in the case of AMP, 2 hr of light adaptation. Samples of each layer, usually ranging between 5 and 50 ng, were dissected from lyophilized frozen sections. After brief light exposure, ATP dropped while ADP rose sharply in the pigment epithelium, outer segments, and inner segments; ADP was too low to be measured accurately in the inner retina. The profile of ATP, P-creatine, and ATP + ADP concentrations showed peaks in the inner nuclear and ganglion layers. AMP, by contrast, was highest in the two plexiform layers. Levels in the inner retina dropped after only 2 sec of light but rose after 2 hr to levels that were higher than dark values in all retinal layers. AMP was often characterized by a non-uniform distribution: adjacent areas of a layer agreed very closely in value to each other but could vary several-fold from a different section of the same frog or from another frog exposed to the same conditions. This distribution produced clusters of values, particularly prominent in dark-adapted animals, something not observed with the other metabolites measured. The peaks of AMP in the plexiform layers suggest that AMP may be a by-product of dopamine-stimulated adenylate cyclase which also has peaks in these same layers.

Adenine Nucleotides↗