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PubMed1985

Purification of somatostatin from frog brain: coisolation with retinal somatostatin-like immunoreactivity.

Somatostatin-like immunoreactivity (SLI) was purified from frog brain and retina, and the structure of the brain peptide was determined. Frog brain (101 g) and retinal (45 g) tissues were extracted with 3% acetic acid, yielding 9.6 and 0.44 nmol of SLI, respectively. SLI was further purified by chromatography on a somatostatin immunoaffinity column followed by sequential application to reverse-phase C-18 HPLC columns. The brain and retinal peptides, purified roughly 100,000-fold with net yields of 7.5 and 2.3%, respectively, appeared identical in the final steps of purification. The amino acid sequence of brain SLI, as determined by a gas-phase automated Edman degradation technique, was as follows: Ala-Gly-(Cys)-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-(Cys). Our data indicate that despite structural variations in somatostatins of other lower vertebrates, the amino acid sequence of frog brain and, by deduction, retinal SLI is identical to that of somatostatin tetradecapeptide. These findings support the physiological relevance of studies directed at elucidating the neurotransmitter function of somatostatin using the well-established models of frog brain and retina.

Amino Acid Sequence↗
PubMed1976

A comparative study of the effects of glutamate and kainate on the lobster muscle fibre and the frog spinal cord.

1 The depolarizing actions of glutamate and its conformationally restricted analogue kainate were investigated on the lobster muscle fibre and the frog spinal cord using intracellular and extracellular recordings, respectively. 2 Bath-applied kainate was less potent than glutamate on the lobster fibre but more potent on the frog cord. From the log-log transformation of dose-response curves it was proposed that more than one glutamate molecule was necessary to activate both the lobster and the frog receptor sites. In the frog, at least three kainate molecules were thought to be required for receptor activation. 3 The ionic dependence of glutamate and kainate responses appeared different for the two tissues. 4 Some possible explanations of the differential tissue sensitivity to kainate are discussed.

Animals↗
PubMed1976

Structure-activity relations of excitatory amino acids on frog and rat spinal neurones.

1 A series of compounds structurally related to glutamic acid has been tested on frog and rat spinal neurones. The substances were added to procaine-containing medium bathing the isolated hemiscected spinal cord of the frog, and their potencies in depolarizing motoneurones were assessed by the magnitude of the potential produced in the ventral root. The electrophoretic technique was used to administer the substances around single interneurones of the rat spinal cord and the relative potencies of the compounds as excitants assessed by the magnitude of the currents required to produce similar rates of neuronal firing. 2 Parallel structure-activity relations were observed in the two series of experiments, suggesting that the receptors for excitatory amino acids on frog and rat spinal neurones are similar. 3 Quisqualate, domoate and kainate were the strongest excitants in both animals, with potencies around two orders of magnitude higher than that of L-glutamate. 4 2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) was a stronger excitant and L-3,4-dihydroxyphenylalanine (L-dopa) a weaker excotamt than L-glutamate on frog spinal motoneurones. The former compounds was also a more potent convulsant than L-glutamate on intraventricular injection into mouse brain. The lack of activity of 6-OH-DOPA on electrophoretic administration was attributed to oxidation. 5 Unlike the majority of amino acid excitants, several of the compounds shown in the present work to have moderate excitatory activity are not anionic at physiological pH. This indicates either that two negatively charged groups are not essential for interaction with a common excitatory receptor, or that more than one type of receptor is involved in the actions demonstrated.

Amino Acids↗
PubMed1991

Effects of aluminium on electrical and mechanical properties of frog atrial muscle.

1. The effects of aluminium on membrane ionic currents were studied in single cardiac myocytes. Most of the work was done on frog atrial cells, but some experiments were also carried out on single cells isolated from rabbit ventricles and atria. 2. The effects of aluminium on the force of contraction of frog atrial trabeculae were also investigated. 3. Aluminium was prepared from AlCl3 as a stock 0.5 M solution which has a pH of 3.5. Before each experiment, this solution was added to the control solution, to give a final concentration of 20-100 micrograms ml-1 aluminium (0.75-3.75 mM AlCl3). The solutions were brought to a pH of 7.4 or 7.6. at which they consist of a mixture of amorphous aluminium hydroxides and a very small amount of soluble ionic aluminium complexes: free aluminium cations (less than 10 pM), aluminohydroxide anions (less than 8 microM). The addition of this suspension reduced the peak inward calcium currents in single rabbit atrial and ventricular cells and in frog atrial cells. In the latter, the peak current was reduced (at + 10 mV) to 45% of control (mean of 9 cells). This effect was reversible upon washout, and was obtained at all membrane potentials, with no shift of the calcium current voltage relationship along the voltage axis. 4. Aluminium also reduced the time-dependent potassium current IK. This reduction was observed at all membrane potentials. For example, at + 10 mV, the mean reduction of IK (n = 9) was to 69% of the control amplitude. This effect, which was very difficult to reverse, was not due to IK rundown. The fully activated current-voltage relationships (obtained by standard 'tail' analysis) showed that the effect of aluminium was due mainly to a decrease in conductance and not to a shift in the activation range of IK. The mean voltage of half activation was shifted by 8 mV in the depolarizing direction (n = 5). 5. The background potassium current IK1 was also slightly but consistently changed in a complex fashion, with an outward shift at membrane potentials positive to -60 mV. For example, at a membrane potential of -40mV, the mean shift was by 22 + 4pA. At more negative potentials, there was an inward shift in the current amplitudes. For example, for steps to -I00 mV the current elicited was larger (more inward) by 53 pA (mean value, n = 10). The reversal potential was slightly shifted (<10 mV) in the hyperpolarizing direction. 6. The force of contraction of frog atrial trabeculae was altered by aluminium in a complex manner, which showed marked seasonal variation. During most of the year, 50-100,ug ml-1 aluminium caused a biphasic change, with an early small and consistent decrease, followed by a large increase in twitch amplitude. For a short period corresponding to the (local) winter months the sensitivity to aluminium was greatly enhanced. Aluminium lOOupgml-1 totally abolished contraction (n = 5), while a lower concentration (20,ug ml- 1) produced a sustained reduction in the force of contraction. Similar biphasic and seasonal responses have been reported to be induced by lanthanum. 7. The biphasic changes in twitch amplitude were independent of the transmembrane sodium gradient. Aluminium produced the same effects when 90% of the extracellular sodium was replaced by lithium. Caffeine (5 mM) attenuated or even inverted the positive inotropic effect of aluminium. These results imply that aluminium alters the release of calcium from intracellular, caffeine-sensitive stores. This could be effected either by augmenting the amount released during each activation, and/or by increasing the loading of stores prior to release.

Aluminum↗
PubMed1993

On the high affinity binding site for [3H]-1,3-dipropyl-8-cyclopentylxanthine in frog brain membranes.

1. Radioligand binding properties of the adenosine receptor ligands, [3H]-1,3-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), and [3H]-R-phenylisopropyladenosine ([3H]-R-PIA) were investigated in frog brain membranes. 2. The specific binding of the adenosine antagonist, [3H]-DPCPX to frog brain membranes showed one binding site with Kd and Bmax values of 43.8 nM and 0.238 +/- 0.016 pmol mg-1 protein, respectively. Guanosine 5'-triphosphate (GTP, 100 microM) decreased to 72 +/- 7% and Mg2+ (8 mM) increased to 121 +/- 3% [3H]-DPCPX (40 nM) binding to frog brain membranes. 3. [3H]-DPCPX saturation binding experiments performed in the presence of Mg2+ (8 mM), or in the presence of GTP showed that Mg2+ ions decreased the Kd value of [3H]-DPCPX to 14 nM, and GTP increased this value to 65.6 nM. Bmax values were not significantly (P > 0.05) modified (0.261 +/- 0.018 pmol mg-1 protein, with Mg2+, and 0.266 +/- 0.026 pmol mg-1 protein, in presence of GTP) by the presence of Mg2+ or GTP. 4. The specific binding of [3H]-R-PIA (15 nM) was decreased to 37 +/- 6% by GTP (100 microM) and increased to 123 +/- 4% by Mg2+ (8 mM). [3H]-R-PIA saturation binding experiments performed in the presence of Mg2+ (8 mM) showed one binding site with Kd and Bmax values of 0.9 nM and 0.229 +/- 0.008 pmol mg-1 of protein, respectively. 5. The concentration-inhibition curves of adenosine agonists and antagonists versus [3H]-DPCPX binding showed the following order of potencies: CPA> R-PIA~ NECA> S-PIA> > CGS 21680, for the agonists, and XAC ~-DPCPX> > XCC> PACPX, for the antagonists.6. The present results suggest that the adenosine binding site in the frog brain membranes is G-protein coupled, but that the antagonist affinities and the pharmacological profile is different from the Al or A2 adenosine receptors.

Adenosine↗
PubMed1982

Dose-response analysis of phenytoin on electrically induced seizures and spontaneous activity of cerebellar purkinje cells in the frog.

The hypothesis that phenytoin exerts its anticonvulsant effect by increasing the spontaneous firing rate of cerebellar Purkinje cells was tested in frogs (Rana pipiens). Time-dose-effect relationships were first established for the anticonvulsant effect of phenytoin in intact frogs. Maximal seizures were induced by corneal electroshock (MES), and phenytoin was injected into the ventral lymph sac. At the time of peak effect (3 h), 20-40 mg/kg phenytoin protected 60-80% of frogs against tonic hindlimb extension (THE). With this functional data base, experiments were then undertaken to test the effect of phenytoin on Purkinje cell firing rates. Phenytoin was injected into the ventral lymph sac 30-45 min prior to anesthesia with tricaine. The cranium was opened, the dura mater overlying the cerebellum was removed, and the frog was then curarized. Single-unit extracellular recordings from Purkinje cells were made with NaCl-filled glass micropipettes 2-6 h after phenytoin injection, the expected time of maximum anticonvulsant effect. Effective anticonvulsant doses (20-40 mg/kg) of phenytoin produced no alteration in the spontaneous firing rates of cerebellar Purkinje cells compared to the rates in solvent-injected controls. Consequently, the hypothesis that the anticonvulsant effect of phenytoin is mediated by an action on Purkinje cell firing rates is not supported by the results of this study.

Animals↗
PubMedDate not supplied

Ultrastructural features of cystic and merogonic stages of Hepatozoon sipedon (Apicomplexa: Adeleorina) in northern leopard frogs (Rana pipiens) and northern water snakes (Nerodia sipedon) from Ontario, Canada.

The cystic and merogonic stages of the haemogregarine Hepatozoon sipedon, infecting Northern water snakes (Nerodia sipedon sipedon) and Northern leopard frogs (Rana pipiens), respectively, in Ontario, Canada, were investigated by transmission electron microscopy. Cysts, which were observed in the liver of Northern leopard frogs (Rana pipiens) after these anurans ingested mosquitoes (Culex pipiens) containing oocysts of the parasite, harboured two cystozoites, each of which contained a large crystalloid inclusion anterior to the nucleus. Two types of meronts were observed in snakes that were fed the liver of infected frogs. Macromeronts, which matured in endothelial cells of the liver approximately 16 d after snakes ingested infected frogs, contained about 50 large macromerozoites. Macromerozoites emerged from macromeronts, entered the bloodstream of the snake, and reinfected endothelial cells. Micromeronts, which matured about 34 d post-inoculation, contained about 150 micromerozoites that infected erythrocytes and transformed into gamonts. The ultrastructural features of micromeronts and macromeronts differed only slightly: immature macromeronts and macromerozoites contained numerous amylopectin and lipid inclusions, whereas immature micromeronts and micromerozoites did not contain amylopectin inclusions and featured fewer, smaller lipid inclusions. A comparison of cystic stages among Hepatozoon species in different groups of vertebrates is presented with respect to their structure and evolutionary significance.

Animals↗
PubMed1997

Antibodies against a furosemide binding protein from Ehrlich ascites tumour cells inhibit Na+, K+, Cl- co-transport in frog retinal pigment epithelium.

PURPOSE: To investigate whether antibodies against a 100 kDa protein purified by furosemide affinity chromatography from Ehrlich ascites tumour cells could inhibit Na+, K+, Cl- co-transport in the isolated frog retinal pigment epithelium. METHODS: The rate of Na+, K+, Cl- co-transport across the retinal membrane in the isolated frog RPE preparation was measured as the rate of decrease in the intracellular Cl- activity observed after administration of furosemide in the apical bath. The intracellular Cl- activity was measured with double barrelled Cl- sensitive microelectrodes. RESULTS: Incubation of frog retinal pigment epithelium for 30 min with antibodies reduced the rate of Na+, K+, Cl- co-transport by 43%, while leaving all other measured electrophysiological parameters intact. CONCLUSION: The antibodies inhibit Na+,K+,Cl- co-transport in the frog retinal pigment epithelium. This could be due to binding of the antibodies to the co-transporter itself or to a regulatory protein.

Animals↗
PubMed2000

Genetic diversity and fitness in peripheral and central populations of the European tree frog Hyla arborea.

Genetic diversity is expected to decrease in small and isolated populations as a consequence of founder effects, bottlenecks, inbreeding and genetic drift. In this study we analyse temporal and spatial effects on genetic variation and progeny viability of the European tree frog (Hyla arborea) at two scales. First, the Swedish distribution has been isolated from the continental distribution for more than 8000 thousand years, and secondly, within Sweden, recent habitat alterations that have taken place during this century have increased isolation between local populations. Genetic variation and progeny survival in relation to isolation was studied within the entire Swedish distribution of the tree frog. Allozyme electrophoresis analysis of froglets, sampled across the Swedish distribution, revealed a low overall genetic variation (1.06 alleles/locus) at the protein level in comparison with continental populations (1.54-1.68 alleles/locus). However, egg hatchability (97%) and early larval survival (95%) were not lower than in other parts of the tree frog distribution or in other anuran species. Within the Swedish distribution, early larval survival was lower in isolated breeding ponds than in more central ones. However, no differences in genetic variation were found in relation to isolation. Polymorphism was detected only at a single locus, and was restricted geographically to the eastern part of the Swedish distribution. Bottlenecks due to climatic changes and fragmentation of suitable habitat (primarily natural pastures with ponds) are suggested as possible causes of the low genetic diversity of the Swedish tree frog population.

Animals↗
PubMed1979

Temperature compensation of sodium transport and ATPase activity in frog skin.

Na+ transport across frog skin, measured as short-circuit current (SCC) shows perfect temperature compensation in frogs acclimated to 6 degrees, 12 degrees, and 23 degrees C as SCC values observed at the acclimation temperatures are equal (about 13 muA/cm2). Reacclimation experiments show that this is not a starvation effect. While very little temperature compensation is seen in the activity of Na+, K+-ATPase in epidermal homogenates from frog skins, the activity of Mg2+-ATPase shows inverse compensation at assay temperatures from 4 degrees to 48 degrees C. This ATPase is apparently activated either by Mg2+ or by Ca2+ and it probably controls the passive permeability of epidermal cells. It is suggested that the inverse temperature compensation in the activity of this enzyme is the main mechanism by which the observed perfect temperature compensation of Na+ transport across frog skin occurs.

Acclimatization↗
PubMed1982

Shortening induced deactivation of skinned fibres of frog and mouse striated muscle.

The depressant effect of active shortening, previously established in intact muscle fibres, was studied during calcium induced contractures of chemically skinned fibres from the semitendinosus muscle of Rana temporaria and the psoas muscle of the mouse. The decrease in contractile activity was determined by comparing the rate of force redevelopment (at a given tension level) after a large (test) and a small (control) release step. Under standard experimental conditions (ionic strength: frog 135 mM, mouse 190 mM; Ca2+ 3.0 microM; Mg2+: frog 25 microM, mouse 100 microM; MgATP2-: frog 1.0 mM, mouse 2.0 mM) active shortening of 0.15 microns per sarcomere (in excess of control release) reduced the contractile activity by approximately 50% of the control in both frog and mouse muscle fibres. Full contractile activity was regained within less than 4 s during isometric activity after the shortening phase. The depressant effect of shortening was steadily reduced, to almost complete disappearance of the effect, by increasing the free calcium concentration within the range 1.5-12.0 microM. Similarly, an increase in ionic strength from 105 to 235 mM reduced the depressant effect by approximately 40%. In contrast, there was a progressive enhancement of the shortening effect as the magnesium ion concentration was increased from 25 to 590 microM. It is proposed that interaction between the myosin cross-bridges and the thin filament during sarcomere shortening leads to a decrease in troponin-calcium binding resulting in a temporary deactivation of the contractile system.

Adenosine Triphosphate↗
PubMed1989

The relationship of frog pressure to heel expansion.

A study was carried out on five horses and four ponies to assess the effect of altering frog pressure on hoof expansion. Measurements of hoof expansion were made using strain gauges. It was concluded that whereas frog pressure affects hoof expansion, it is only one of several factors. Increasing frog pressure may result in increased expansion, or contraction, or may have virtually no affect. The variable results of changing frog pressure should be taken into account when considering therapeutic shoeing.

Animals↗
PubMed2001

In vivo and in vitro heel expansion in relation to shoeing and frog pressure.

The objective of this study was to validate a simple method to measure the mediolateral expansion of the heels and to apply this method in an in vivo experiment. It was also the aim to quantify the mediolateral expansion in different areas of the heel using an in vitro model. In the in vitro study, 5 right and 5 left distal forelimb specimens from 5 Standardbreds were mounted vertically in a custom-made compressive test machine. The heel expansion was measured using optical kinematic analysis and a potentiometer system. Specimens were tested unshod, with frog pressure and with the weightbearing rim raised. In the in vivo study, a potentiometer system was fitted to the left forelimb of 5 Standardbred trotters. Measurements were performed with the horses at hand, in walk and in trot. They were tested unshod, shod with a standard 8 mm iron shoe and finally with a shoe combined with an inflatable hoof-cushion. The results showed a good correlation between the 2 measurement systems (r = 0.98). This study describes the effect of different frog pressures on heel movement. Increased pressure on the frog and the sole increased heel expansion, but some heel expansion still occurred when the frog and sole were unsupported. The study supports, therefore, both the depression and the pressure theory. The significance of these results is the manipulation of heel movement as affected by farrier techniques and its possible relationship to health of the hoof. Further studies are needed to apply this knowledge correctly to farrier practice.

Animals↗
PubMed1968

The distribution of the T-system along the sarcomeres of frog and toad sartorius muscles.

1. Frog and toad sartorius muscles were soaked in ferritin suspensions and then fixed and prepared for electron microscopy. Ferritin particles were counted in micrographs of regions where fibril striations were in good register, and the number of particles plotted according to position along the sarcomeres.2. The distribution of ferritin particles in both frog and toad muscle sarcomeres could be accounted for by a single peak centred at the Z-line, with a total width of about 0.4 mu.3. The peak in toad fibres was slightly broader than that in frog fibres, and this was due to more frequent branching of the T-system in the toad. In both frog and toad sartorius muscles branching increases the total quantity of T-system by around 30%.4. No peak was found in the ferritin distribution which could account for the peak of albumin distribution observed near the A-I boundary of toad sartorius muscles by Hill (1964).

Albumins↗
PubMed1974

Tension responses to quick length changes of glycerinated skeletal muscle fibres from the frog and tortoise.

1. Twitch fibres isolated from the sartorius muscle of the frog were glycerinated (cf. Heinl, 1972) and thin fibre bundles dissected from the m. ileofibularis of the tortoise were briefly glycerinated as described by Julian (1971).2. The glycerinated fibres (length 0.3-0.5 cm) were fixed to an apparatus which performed length changes within 5 msec and recorded the time course of tension changes in the fibres.3. The fibres were suspended in a relaxing medium, containing ATP and 4 mM-EGTA. Contraction was induced by raising the calcium concentration to 4 mM-CaEGTA.4. The tension time course of activated fibres following quick length changes (0.1-1% L(0)) was studied. The tension records produced by quick releases and stretches could be resolved into four phases similar to the kind shown in Fig. 1 a.5. The phase of quick tension recovery was found to take place more rapidly in frog than in tortoise fibres: it was completed in approximately 30 msec (after stretch) and in approximately 20 msec (after release) in frog fibres (3 degrees C). The corresponding values obtained for tortoise fibres were approximately 300 and approximately 400 msec (3 degrees C).6. In tortoise fibres the size of the elastic and quick recovery phase increased with rising isometric tension (induced by raising the calcium concentration (pCa 8 to 5)), and decreased with increasing sarcomere length (2.5-4.2 mum). In fibres, in which the rigor state was induced by withdrawal of ATP, no quick tension recovery was recorded.7. It is suggested that the rotational movement of the crossbridge head on the actin filament, postulated by Huxley & Simmons (1971 b) is taking place more slowly in the tortoise than in the frog muscle. Furthermore, it is suggested that this rotational movement does not occur in the rigor state, as no quick tension recovery was recorded there.

Adenosine Triphosphate↗
PubMed1974

Effect of calcium and vasopressin on the response of frog skin to prostaglandin E1.

1. Prostaglandin E(1) increases sodium transport as measured by short circuit current (SCC) across isolated frog skin whereas calcium, added to the external Ringer fluid, decreases sodium transport. To help establish the site of action of prostaglandin the possible interaction of these two agents on sodium transport has been examined.2. The effect of a standard dose of prostaglandin (0.5 x 10(-6)M) on the short circuit current was tested on paired skins with either zero or high calcium (22.4 mM) in the external Ringer fluid. In ten experiments the responses to prostaglandin (expressed in muA/cm(2)) were not significantly affected by external calcium.3. In another series of experiments the chelating agent, EGTA, was included in calcium-free external Ringer in order to promote greater depletion of skin calcium. The response of these skins to the standard dose of prostaglandin was of the same order of magnitude as that of control skins. The response was not sustained in contrast to that of normal skins and skins in high-calcium fluids.4. In a further series of experiments the reverse procedure was adopted whereby the response of the skin to low and high doses of calcium in the external Ringer was recorded in control conditions and when the skin had responded fully to twice the standard dose of prostaglandin. In addition, the calcium-sensitive current was calculated for each skin in both circumstances. The latter was unchanged on addition of prostaglandin, and graded doses of calcium caused the same degree of inhibition of the short circuit current.5. The results show no interaction between external calcium and prostaglandin and also no need for external calcium in prostaglandin stimulation of sodium transport.6. The findings do not support the concept of chelation by prostaglandin of calcium from critical sites on the skin as the primary mechanism of its action on sodium transport. The results closely parallel those of a similar type of study into the relationship between vasopressin and external calcium on frog skin also.7. When frog skin has responded fully to either prostaglandin E(1) or vasopressin, it shows no response to the other, although removal of calcium from the external Ringer fluid causes a further increase in short circuit current.8. Vasopressin causes a further increase in short circuit current in skins treated with prostaglandin F(1alpha). Prostaglandin F(1alpha) may be a weaker agonist on frog skin than either vasopressin or prostaglandin E(1).

Animals↗
PubMed1977

The effects of manganese ions on the contraction of the frog's heart.

1. The effects of Mn(2+) in particular but also of Ni(2+), Co(2+), Cd(2+), verapamil and D600 on the contraction of isolated frog atrial trabeculae have been investigated. Contraction was initiated either by electrical stimulation, or by raising the [K](o) or by lowering the [Na](o).2. Mn ions like Ca ions cause a hyperpolarization of the cell membrane and a rise in the threshold for the action potential and twitch. Mn ions, particularly at low concentrations, reduce the overshoot of the ventricular action potential.3. Mn ions reduce the strength of the regularly evoked heart beats. Prolonged exposure, in beating hearts, results in a rise in resting tension and often a small recovery of the heart beat.4. In normal Ringer solution and in Ringer free of Na ions, the addition of Mn causes the tension-depolarization curve to be displaced by an amount equivalent to an 18 mV hyperpolarization of the membrane potential for a tenfold increase in the divalent cation concentration.5. Mn, Co, Ni and Cd ions all cause a marked reduction in the tension generated by exclusion of Na ions from the bathing fluid. In the presence of these divalent cations the contracture divides into an initial phasic and a later tonic contraction. This inhibition is reversed by raising the [Ca](o), while the tension developed during the initial phasic contraction varies with the [Ca](o)/[Mn](o) quotient.6. A similar tonic contracture is initiated after exposure to Na-free fluid containing a high [Mn](o) by the addition of a small concentration of Na, Li, hydrazinium or hydroxylammonium ions.7. The organic ;Ca antagonists' verapamil and D600 have little effect on the contracture induced by lowering [Na](o) even after prolonged exposure at relatively high concentrations but they do inhibit the twitch contraction and the K contractures.8. The effects of Mn on the Na-withdrawal contracture of frog heart can be interpreted in terms of an exclusively extracellular effect where Mn ions resemble Na ions in their action, and both antagonize the movement of Ca across the cell membrane.9. The experimental evidence suggests that the K contracture in frog heart is initiated by a mechanism which is, in some ways different to that underlying the Na-withdrawal contracture, and may involve two different sources of activator Ca.10. The several different effects of Mn on the frog heart probably reflects the ability of this cation to interfere with many processes involving Ca, and that there are a number of such processes involved in the results described in this work. The effects of Mn are more complex than might be generally supposed.

Animals↗
PubMed1983

Slow changes in currents through sodium channels in frog muscle membrane.

We used a patch clamp to measure Na currents across 10-15 micron diameter circular patches of frog and rat skeletal muscle membrane. We tested for electrophoretic mobility of Na channels, by applying steady lateral fields (of the order of 10 mV micron-1) across the wall of the patch pipette. Application of steady negative potentials to the inside of the pipette resulted in a fall in the number of functional Na channels in the patch. This fall took several minutes to complete and was reversible. It was assayed by applying suitable depolarizations at approximately 11 sec intervals. When a steady lateral field was applied in the absence of changes in membrane potential of the patch, the loss of Na current was virtually abolished. Thus it was not due to electrophoretic movement of channels, but instead to depolarization of the sarcolemma. Evidently, a very slow inactivation of Na conductance operates in skeletal muscle. In frog muscle, the rate constants for loss and recovery of Na current were about 0.1 min-1 (17 degrees C) at resting potential. Rate constants were higher at more positive and at more negative membrane potentials. Current amplitude was reduced to 0.5 at about -76 mV. Roughly similar results were found in rat omohyoid muscle. A further inactivation mechanism, whose rate was intermediate between conventional fast inactivation and the very slow process described here, was present also in both rat and frog muscle. In frog muscle, lateral fields do not alter the potential dependence of fast inactivation. Either the surface charge due to membrane lipids does not influence inactivation or the lipids immediately surrounding the Na channel are restricted in their mobility.

Animals↗