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PubMed1987

In vivo phosphorylation of titin and nebulin in frog skeletal muscle.

Titin and nebulin are two major protein components of the sarcomere matrix in striated muscles. Purified titin and nebulin from frog (Xenopus laevis) skeletal muscle are similar in size and in amino acid composition to their mammalian or avian counterparts. Both proteins contain substantial amounts of protein-bound phosphate: about 5 to 6 per titin subunit and 8 to 9 per nebulin subunit. Injection of radioactive inorganic phosphate into the dorsal lymph sacs of Xenopus laevis resulted in the significant incorporation of radioactivity into titin and nebulin within three days of incubation. Purified titin from in vivo labeled frog gastrocnemius muscle contains one mole of radioactive phosphoserine per mole of titin subunit. These data indicate that phosphorylation of frog titin and nebulin occurs in vivo.

Amino Acids↗
PubMed1977

Diffusion of 133Xe through frog skins, toad bladders, and water boundary layers.

We have measured the total permeability coefficients P as a function of stirring frequency omega for 133Xe through frog skins and toad bladders. The permeability coefficients for the frog skins and toad bladders proper are, respectively, Pm = (3.9 +/- 0.8) X 10(-4) cm/s and (7.4 +/- 4.2) X 10(-4) cm/s. "Unstirred" water layer thickness delta is determined concurrently, from the frequency dependence of P(omega); the result for frog skin is delta = (0.060 +/- 0.016) square root of omega(rad/s) cm. The stirring frequency range is from omega = 7.5 rad/s (72 rpm) to 55 rad/s (530 rpm). The results support the conclusions that the principal barrier to Xe diffusion in these epithelia is inter- and intracellular water, and that the diffusion is passive and rapid. The experimental method may be straightforwardly adapted to the measurement of diffusion or counterdiffusion of any gamma-radioactive soluble or partly soluble solute through any flat membrane or through a solvent. We estimate the amount of total body-absorbed radioactivity due to environmental 133Xe to be 50 fCi for an ambient concentration of 2.6 pCi/m3 of air.

Animals↗
PubMed1986

External [K+] and the block of the K+ inward rectifier by external Cs+ in frog skeletal muscle.

Frog skeletal muscle has a K+ channel called the inward rectifier, which passes inward current more readily than outward current. Gay and Stanfield (1977) described a voltage-dependent block of inward K+ currents through the inward rectifier by external Cs+ in frog muscle. Here, frog single muscle fibers were voltage clamped using the vaseline-gap voltage-clamp technique to study the effect of external [K+] on the voltage-dependent block of inward K+ currents through the inward rectifier by external Cs+. The block of inward K+ currents through the channel by external Cs+ was found to depend on external [K+], such that increasing the external concentration of the permeant ion K+ potentiated the block produced by the impermeant external Cs+. These findings are not consistent with a one-ion channel model for the inward rectifier. The Eyring rate theory formalism for channels, viewed as single-file multi-ion pores (Hille and Schwarz, 1978), was used to develop a two-site multi-ion model for the inward rectifier. This model successfully reproduced the experimentally observed potentiation of the Cs+ block of the channel by external K+, thus lending further support to the view of the inward rectifier as a multi-ion channel.

Animals↗
PubMed1993

Resting myoplasmic free calcium in frog skeletal muscle fibers estimated with fluo-3.

Fluo-3 is an unusual tetracarboxylate Ca2+ indicator. For recent lots supplied by Molecular Probes Inc. (Eugene, OR), FMAX, the fluorescence intensity of the indicator in its Ca(2+)-bound form, is approximately 200 times that of FMIN, the fluorescence intensity of the indicator in its Ca(2+)-free form. (For earlier lots, impurities may account for the smaller reported values of FMAX/FMIN, 36-40). We have injected fluo-3 from a high-purity lot into intact single fibers from frog muscle and measured the indicator's absorbance and fluorescence signals at rest (A and F, respectively) and changes in absorbance and fluorescence following action potential stimulation (delta A and delta F signals substantially lagged behind that of the myoplasmic free Ca2+ transient. Our analysis of fluo-3's signals from myoplasm therefore focused on information about the level of resting myoplasmic free [Ca2+] ([Ca2+]r). From A, delta A, and in vitro estimates of fluo-3's molar extinction coefficients, the change in the fraction of fluo-3 in the Ca(2+)-bound form during activity (delta f) was estimated. From delta f, delta F, and F, the fraction of the indicator in the Ca(2+)-bound form in the resting fiber (fr) was estimated by fr = (delta f x F/delta F) + (1-FMAX/FMIN)-1. Since FMAX/FMIN is large, the contribution of the second term to the estimate of fr is small. At 16 degrees C, the mean value (mean +/- S.E.) of fr was 0.086 +/- 0.004 (N = 15). From two estimates of the apparent dissociation constant of fluo-3 for Ca2+ in the myoplasm, 1.09 and 2.57 microM, the average value of [Ca2+]r is calculated to be 0.10 and 0.24 microM, respectively. The smaller of these estimates lies near the upper end of the range of values for [Ca2+]r in frog fibers (0.02-0.12 microM) estimated by others with aequorin and Ca(2+)-selective electrodes. The larger of the estimates lies within the range of values (0.2-0.3 microM) previously estimated in this laboratory with fura red. We conclude that [Ca2+]r in frog fibers is at least 0.1 microM and possibly as large as 0.3 microM.

Action Potentials↗
PubMed1995

Approximating the isometric force-calcium relation of intact frog muscle using skinned fibers.

In previous papers we used estimates of the composition of frog muscle and calculations involving the likely fixed charge density in myofibrils to propose bathing solutions for skinned fibers, which best mimic the normal intracellular milieu of intact muscle fibers. We tested predictions of this calculation using measurements of the potential across the boundary of skinned frog muscle fibers bathed in this solution. The average potential was -3.1 mV, close to that predicted from a simple Donnan equilibrium. The contribution of ATP hydrolysis to a diffusion potential was probably small because addition of 1 mM vanadate to the solution decreased the fiber actomyosin ATPase rate (measured by high-performance liquid chromatography) by at least 73% but had little effect on the measured potential. Using these solutions, we obtained force-pCa curves from mechanically skinned fibers at three different temperatures, allowing the solution pH to change with temperature in the same fashion as the intracellular pH of intact fibers varies with temperature. The bath concentration of Ca2+ required for half-maximal activation of isometric force was 1.45 microM (22 degrees C, pH 7.18), 2.58 microM (16 degrees C, pH 7.25), and 3.36 microM (5 degrees C, pH 7.59). The [Ca2+] at the threshold of activation at 16 degrees C was approximately 1 microM, in good agreement with estimates of threshold [Ca2+] in intact frog muscle fibers.

Adenosine Triphosphatases↗
PubMed1998

Force regulation by Ca2+ in skinned single cardiac myocytes of frog.

Atrial and ventricular myocytes 200 to 300 microm long containing one to five myofibrils are isolated from frog hearts. After a cell is caught and held between two suction micropipettes the surface membrane is destroyed by briefly jetting relaxing solution containing 0.05% Triton X-100 on it from a third micropipette. Jetting buffered Ca2+ from other pipettes produces sustained contractions that relax completely on cessation. The pCa/force relationship is determined at 20 degrees C by perfusing a closely spaced sequence of pCa concentrations (pCa = -log[Ca2+]) past the skinned myocyte. At each step in the pCa series quick release of the myocyte length defines the tension baseline and quick restretch allows the kinetics of the return to steady tension to be observed. The pCa/force data fit to the Hill equation for atrial and ventricular myocytes yield, respectively, a pK (curve midpoint) of 5.86 +/- 0.03 (mean +/- SE.; n = 7) and 5.87 +/- 0.02 (n = 18) and an nH (slope) of 4.3 +/- 0.34 and 5.1 +/- 0.35. These slopes are about double those reported previously, suggesting that the cooperativity of Ca2+ activation in frog cardiac myofibrils is as strong as in fast skeletal muscle. The shape of the pCa/force relationship differs from that usually reported for skeletal muscle in that it closely follows the ideal fitted Hill plot with a single slope while that of skeletal muscle appears steeper in the lower than in the upper half. The rate of tension redevelopment following release restretch protocol increases with Ca2+ >10-fold and continues to rise after Ca2+ activated tension saturates. This finding provides support for a strong kinetic mechanism of force regulation by Ca2+ in frog cardiac muscle, at variance with previous reports on mammalian heart muscle. The maximum rate of tension redevelopment following restretch is approximately twofold faster for atrial than for ventricular myocytes, in accord with the idea that the intrinsic speed of the contractile proteins is faster in atrial than in ventricular myocardium.

Animals↗
PubMed1998

ATPase and shortening rates in frog fast skeletal myofibrils by time-resolved measurements of protein-bound and free Pi.

Shortening and ATPase rates were measured in Ca2+-activated myofibrils from frog fast muscles in unloaded conditions at 4 degrees C. ATPase rates were determined using the phosphate-binding protein method (free phosphate) and quench flow (total phosphate). Shortening rates at near zero load (V0) were estimated by quenching reaction mixtures 50 ms to 10 s old at pH 3.5 and measuring sarcomere lengths under the optical microscope. As with the rabbit psoas myofibrils (C. Lionne, F. Travers, and T. Barman, 1996, Biophys. J. 70:887-895), the ATPase progress curves had three phases: a transient Pi burst, a fast linear phase (kF), and a deceleration to a slow phase (kS). Evidence is given that kF is the ATPase rate of shortening myofibrils. V0 is in good agreement with mechanical measurements in myofibrils and fibers. Under the same conditions and at saturation in ATP, V0 and kF are 2.4 microm half-sarcomere(-1) s(-1) and 4.6 s(-1), and their Km values are 33 and 200 microM, respectively. These parameters are higher than found with rabbit psoas myofibrils. The myofibrillar kF is higher than the fiber ATPase rates obtained previously in frog fast muscles but considerably lower than obtained in skinned fibers by the phosphate-binding protein method (Z. H. He, R. K. Chillingworth, M. Brune, J. E. T. Corrie, D. R. Trentham, M. R. Webb, and M. R. Ferenczi, 1997, J. Physiol. 50:125-148). We show that, with frog as with rabbit myofibrillar ATPase, phosphate release is the rate-limiting step.

Adenosine Triphosphatases↗
PubMed1998

Calretinin-immunoreactive elements in the retina and optic tectum of the frog, Rana esculenta.

The frog retina contains numerous 28 kDa calbindin positive elements in its every layer. At the same time, parvalbumin has been observed only in a few elements in the visual system of amphibian species, whilst calretinin immunoreactivity could be detected in salamander retina and the optic tectum of tench. However, the presence and distribution of calretinin have been described to date neither in the retina nor in the other parts of the visual system of anurans. Therefore, the aim of this study is to describe the calretinin immunoreactive elements in the retina and the optic tectum of the frog and to establish whether or not the expression of this calcium-binding protein is transmitter-related and/or cell type specific in these parts of the central nervous system. In the retina, numerous bipolar cells showed calretinin immunoreactivity. The axon terminals of the bipolar cells branched in both the OFF and ON sublayers of the inner plexiform layer. The few labeled amacrine cells were larger than 10 microns in diameter. Over 50% of the cells in the ganglion cell layer contained calretinin. The labeled cells in the ganglion cell layer were of usually 16-22 microns in diameter, although a few smaller cells were also seen. Accordingly, many optic fibers were also labeled. In colocalization experiments, gamma-aminobutyric acid and calretinin were found in partially overlapping amacrine cell populations, cells with the former marker being much more numerous. At the same time, all the gamma-aminobutyric acid positive bipolar cells also contained calretinin. Most of the calretinin positive neurons in the ganglion cell layer however were only single-labeled. Axons of ganglion cells terminated in B, C and F sublayers of layer 9 in the optic tectum. Local tectal neuron populations in layer 4, 6, 8 and 9 were also labeled and a few calretinin positive cells were detected also in layer 2. Approximately 10% of the tectal cells were found to be immunoreactive for calretinin. Layer 4 and 6 cells were mostly large pear-shaped neurons while cells in the 8th layer were small pear-shaped and ganglion cells labeled too. Coexistence of gamma-aminobutyric acid and calretinin was characteristic in cells of the upper tectal layers while they were not detected in neurons of deep layers of the tectum. After monocular enucleation, contralateral to the removed eye, calretinin-immunoreactivity disappeared almost completely from F sublayer and became less pronounced in sublayers B and C after 90 days. Calretinin-immunoreactivity remained mostly unchanged in local tectal cells. The results show that, although its function remains undetermined, calretinin is the major EF-hand calcium-binding protein in the frog retina and optic tectum.

Animals↗
PubMed1998

Adenosine potentiates the delayed-rectifier potassium conductance but has no effect on the hyperpolarization-activated Ih current in frog melanotrophs.

The effects of adenosine on the voltage-sensitive delayed-rectifier K+ (IK) currents and hyperpolarization-activated cationic inward current (Ih) were studied in cultured frog melanotrophs using the whole-cell configuration of the patch-clamp technique. The A1 receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA; 50 microM) reversibly increased IK. Perfusion of dibutyryl-cAMP (1 mM) in the external solution did not modify the R-PIA-induced enhancement of IK. Pretreatment of melanotrophs with pertussis toxin (1 microg/ml; 12 h) totally abolished the R-PIA-evoked response. Application of hyperpolarizing voltage pulses from -60 to -120 mV to melanotrophs induced a two-component inward current corresponding to an Ih-like conductance. This conductance was characterized by a high K+ selectivity and a low Na+ permeability and was resistant to tetrodotoxin (1 microM). R-PIA had no effect on Ih. The present study demonstrates that in frog melanotrophs adenosine inhibits the electrical activity by activating IK through an A1 receptor subtype coupled to a pertussis toxin-sensitive pathway independent of the cAMP/PKA system. This study also demonstrates the existence of a Ih conductance in frog melanotrophs which is not modulated by A1 receptors.

Adenosine↗
PubMed1995

Isoprenoid lipid metabolism in the retina: dynamics of squalene and cholesterol incorporation and turnover in frog rod outer segment membranes.

Frogs were injected intravitreally with [3H]acetate, and the formation of [3H]-labeled squalene and cholesterol in the retina and their incorporation into rod outer segment (ROS) membranes were evaluated biochemically over a 60-day time course. ROS [3H]squalene specific activity was maximal by 1-3 days, then declined with a half-time of approximately 20-30 days. In contrast, the specific activity of ROS [3H]cholesterol initially increased to a level substantially less than that of [3H]squalene, and then remained constant. Thus, ROS squalene appears to turn over without obligatory conversion to, or coturnover with, ROS cholesterol. When [3H]acetate was injected into one eye, radiolabel in non-saponifiable lipids of the contralateral retina represented < 1% of those recovered from the ipsilateral retina; hence, systemic contributions to de novo synthesis were obviated. Long-term (> or = 8 hr) in vitro incubations of isolated retinas with [3H]acetate resulted in incorporation of [3H]-labeled sterols and squalene into ROS, at levels comparable to those observed in ROS from companion incubated eyecup preparations and from retinas 8 hr after intravitreal injection of [3H]acetate. These results demonstrate that the in vitro system faithfully reflects the in vivo biosynthetic capacity with respect to isoprenoid lipid metabolism, and suggest that de novo synthesis within the neural retina is responsible for generating most, if not all, of the [3H]squalene and [3H]cholesterol formed under the given conditions. Treatment of retinas in vitro with brefeldin A or energy poisons blocked transport of newly synthesized opsin, but not squalene, to the ROS. Furthermore, frogs maintained at 8 degrees C exhibited marked suppression of incorporation of newly synthesized protein into the ROS, while [3H]squalene incorporation was only minimally reduced, compared with frogs maintained at 22 degrees C. These results are consistent with prior findings that suggest that lipids are transported to the ROS by a mechanism distinct and independent from that employed for intracellular trafficking of opsin and other ROS-destined membrane proteins.

Animals↗
PubMed1987

In vivo and in vitro observations on permeability and diffusion pathways of tracers in rat and frog lenses.

Two tracers [horseradish peroxidase (HRP) and lanthanum] with different properties were injected separately into eyes of the rat and frog to determine the diffusion pathways of tracers in the lens. HRP can diffuse rapidly into lens tissue and can also be washed out, whereas lanthanum has opposite properties. In a short (2-5 min) in vivo intravitreal injection of HRP into the rat eye using an automated micro syringe, tracer reached the entire intercellular space between lens epithelium and fiber cells, and between epithelial cells primarily at the peripheral region; however, it reached the central zone in a longer period (10-20 min) of injection, as examined with light microscope and transmission electron microscope. HRP in epithelial intercellular spaces basal to the tight junctions was washed away upon 5-15 min washing with TC-199, but was retained in the spaces apical to the junctions and between epithelium and fiber cells. Lanthanum studies in both rat and frog lenses showed that within 3-4-hr incubation, tracer was seen in epithelial intercellular spaces only basal to the tight junctions at the central epithelium. At the peripheral zone, however, tracer was visualized in the entire intercellular spaces between epithelial cells, and between epithelium and fiber cells. These studies strongly suggest that both tracers seen in the intercellular space between epithelium and fiber cells are derived from the most equatorial region of the lens, and they may not leak out to the anterior lens surface through tight junctions between epithelial cells in the rat and frog. This leads us to conclude that the equatorial region of the lens is the major site of efflux and influx of substances for the anterior lens region.

Animals↗
PubMed1987

Lengths of immunolabeled ciliary microtubules in frog photoreceptor outer segments.

Monoclonal anti-tubulin antibodies were used to label microtubules in the connecting cilia and outer segments of retinal photoreceptors isolated from Rana pipiens. In paraformaldehyde-fixed rods from frogs maintained on diurnal light cycles, the anti-tubulin labeling of ciliary microtubules (mean length = 27 micron) typically extends to slightly over half the length of the outer segments (mean length = 46 micron). Rod outer segments from frogs kept in constant darkness for 3-4 weeks are longer (mean length = 53 micron) than rod outer segments from frogs maintained in cyclic lighting. However, the distribution of fractional lengths of anti-tubulin labeling of ciliary microtubules is the same for both lighting regimens. Incubating retinas in 1.0 mM colchicine prior to outer-segment fixation has no effect on the length of immunolabeling of ciliary microtubules, suggesting that post-mortem elongation artifacts are not significant. Incubating retinas in 10 microM taxol prior to fixation significantly increases the length of stained ciliary microtubules, suggesting that taxol either promotes post-mortem assembly of microtubules, or that taxol reduces post-mortem disassembly. The mean position of the end of anti-tubulin-labeled ciliary microtubules does not correspond to the position of disk shedding for any of the experimental conditions employed.

Alkaloids↗
PubMed2003

Ethane 1,2-dimethane sulphonate is a useful tool for studying cell-to-cell interactions in the testis of the frog, Rana esculenta.

Ethane 1,2-dimethane sulphonate (EDS), a toxin which specifically destroys Leydig cells (LC), has been used to study cellular interactions in the testis of the frog Rana esculenta. Animals received three consecutive EDS injections and were sacrificed on day 4, 8, and 28 from the first injection. No significant morphological differences were observed between present observation and that obtained, in a previous experiment, after four consecutive EDS injections. In fact, on day 4, in the germinal tubules adjacent to apparently normal LC, Sertoli cells surrounding primary spermatogonia (I SPG) show heterochromatic nuclei and loss of cellular adhesion. Interestingly, I SPG surrounded by the heterochromatic Sertoli cells present grossly swollen mitochondria with ballooned cristae. On day 8, sometimes in the interstitium many LC appear strongly damaged and the germinal tubules appear disorganized; the only cell type still distinguishable is the I SPG. On day 28 from the first EDS injection a new population of LC reappear in the interstitium and spermatogenesis normalizes. These data confirm the close relationship between the interstitial and the geminal compartments. Immunocytochemical data obtained using a polyclonal antibody anticonnexin-43 (Cx-43, the most abundant Cx found in mammalian testis) demonstrate the presence of Cx-43 in the frog testis. In particular, Cx-43 is present between LC in the interstitium, between Sertoli and germ cells in the cysts and between Sertoli cells and I SPG. Cx-43 immunopositivity sharply decreases on day 4 from the first EDS injection simultaneously with the loss of cellular adhesion between Sertoli and germ cells. On day 8 and 28 from the first EDS injection Cx-43, immunopositivity is restored and, this data is also supported by Western blot analysis. Our data provide, for the first time, evidence that Cx-43 protein is present in the frog testis and confirm that EDS is a useful tool for studying cellular communication at the paracrine pathway or through direct contact depending on the gap junctional pathway in R. esculenta testis

Animals↗
PubMed2003

Ca2+-recruitment in tachykinin-induced contractions of gut smooth muscle from African clawed frog, Xenopus laevis and rainbow trout, Oncorhynchus mykiss.

Changes in intracellular Ca(2+) concentration control many essential cellular functions like the contraction of smooth muscle cells. The aim of this study was to investigate if the tachykinin substance P (SP) engages external Ca(2+)-sources, internal Ca(2+)-sources, or both in the contraction of the gastrointestinal smooth muscle of rainbow trout (Oncorhynchus mykiss) and the African clawed frog (Xenopus laevis). Strip preparations made of either longitudinal smooth muscle of proximal intestine or circular smooth muscle of cardiac stomach were mounted in organ baths and the tension was recorded via force transducers. Ca(2+)-free Ringer's solution containing the Ca(2+) chelating agent EGTA (2mM) abolished all spontaneous contractions. Exposure to SP in Ca(2+)-free solution decreased the response. Preparations were also treated with the Ca(2+)-ATPase inhibitor thapsigargin (10 microM) during 30 min. Thapsigargin reduced the effect of SP on intestinal longitudinal smooth muscle in rainbow trout and on stomach circular smooth muscle in the African clawed frog and to a less extent in the intestinal longitudinal smooth muscle. The results show that external Ca(2+) is of great importance, but is not the only source of Ca(2+) recruitment in SP-activation of gastrointestinal smooth muscle in rainbow trout and the African clawed frog.

Animals↗
PubMed1985

Ultrastructure and architecture of the sarcoplasmic reticulum in frog sino-atrial fibres: a comparative study with various preparatory procedures.

Various preparatory procedures were tested to preserve the ultrastructure of the sarcoplasmic reticulum (SR) by the best possible method within frog sino-atrial muscle fibres. These procedures were: conventional aldehyde fixation with or without tannic acid, cryofracture, metallic impregnation and quick-freezing followed by freeze-substitution. Our results illustrated that, when optimally preserved, the SR architecture and ultrastructure of frog sino-atrial fibres were not fundamentally different from those described in many other vertebrate muscle fibres, particularly cardiac fibres. The three-dimensional arrangement of the SR and the structure of its main compartments were situated in a precise fashion: the peripheral SR, located close to the plasma membrane, was made of a tight network of tubules and showed typical couplings; the juxtafibrillar SR was made of a loose network of tubules, small cisternae and some tubules near Z-lines; the intermediary SR, associated with the mitochondria, was made of tubules and fenestrated cisternae. Contacts between SR and mitochondrial membranes were also studied; cryofractures revealed no special intramembrane particles at this level. Collapsed portions of the SR were found after quick-freezing. Because of its relative importance and its three-dimensional arrangement, the SR of frog sino-atrial fibres may have comparable functional significance to the SR of other cardiac muscle fibres.

Animals↗
PubMed1988

Cardiac muscle following quick-freezing: preservation of in vivo ultrastructure and geometry with special emphasis on intercellular clefts in the intact frog heart.

Intact frog, mouse and finch hearts were quick-frozen on a liquid He-cooled copper block. Adjacent frozen samples from the same heart were processed by freeze-substitution (followed by embedding and thin sectioning), freeze-fracture/etch (followed by platinum/carbon replication) and frozen sectioning (followed by freeze-drying), respectively, and examined with the electron microscope for fidelity of reproducing the in vivo state of heart muscle geometry, especially that of the narrow intercellular clefts between frog cardiac muscle cells. It was concluded that quick-freezing followed by the above procedures accomplishes that and that, therefore, narrow intercellular clefts are an invariant feature of the normal anatomy of frog cardiac muscle, which must be considered in physiological experiments. The methodology showed that quick-freezing through the epicardial surface is capable of producing superb cryopreservation for ultrathin cryosections, as well.

Animals↗
PubMed2001

Mechanisms of metabolic defense against hypoxia in hibernating frogs.

The cold submerged frog (Rana temporaria) serves as a useful model for many hibernating ectotherms that take refuge in hypoxic ponds and lakes until more favourable conditions of climate and food availability return. In all such animals, entry into a hypometabolic state effectively extends their survival time by lessening the impact of ATP demands on endogenous substrates. At the cellular level, metabolic depression may be brought about by decreasing energy-consuming processes and/or by increasing the efficiency of energy-producing pathways. Since the mitochondrion is the major contributor to the total energy production during aerobic metabolism and frog survival during winter depends on entry into a hypometabolic state, this review focuses on the respiratory properties of mitochondria that serve to increase the efficiency of energy production in hibernation. Energy conservation during overwintering also occurs through decreases in the ATP demand of the energy-consuming processes. For example, hibernating frogs decrease their ATP demands for Na(+)/K(+)-ATPase activity as part of a coordinated process of energy conservation wherein O(2)-limitation initiates a generalised suppression of ion channel densities and/or channel leak activities. The net result is that cell membrane permeabilities are reduced, thereby lowering the energetic costs of maintaining transmembrane ion gradients.

Adaptation, Physiological↗
PubMed1998

The protective effects of metabolic rate depression in hypoxic cold submerged frogs.

Aerobic metabolic rates (MO2) and respiratory quotients (RQ = MCO2/MO2) were measured in hypoxic frogs, hibernating underwater for up to 90 days at 3 degrees C. After 3 months of submergence at a PO2 of 50 mmHg, MO2 levels were 25% of those seen prior to hibernation. In progressive hypoxia, a gradual shift in RQ from 0.85 to 1.0 signaled an increasing reliance upon carbohydrate metabolism. Moreover, the glycogen concentrations of skeletal muscle, liver and heart of hypoxic frogs were more rapidly depleted than in their normoxic counterparts. A plasma lactacidosis revealed that the hypoxic animals recruited anaerobiosis to fuel a large 'Pasteur effect'. Throughout all stages of hypoxia, cellular ATP levels were maintained homeostatic. The ability to depress metabolic rate such that ATP demands can be met by oxidative phosphorylation in an oxygen limited environment is the key to the frogs' overwintering survival.

Acid-Base Equilibrium↗