Seasonal changes of antidiuretic hormone action on sodium transport across frog skin

Skeletal muscle substrate utilization is altered by acute and acclimatory temperature in the American bullfrog (Lithobates catesbeiana)

We investigated the effect of acute and acclimatory temperature on the relative contribution of g9lucose and lactate to metabolism in resting sartorius muscle of the American bullfrog (Lithobates catesbeiana). We examined the fate of these metabolites in vitro by supplying radiolabeled [(14)C]glucose, [(14)C]lactate and [(14)C]palmitate to isolated muscle bundles from frogs (1) acutely exposed to incubation conditions of 5, 15 or 25 degrees C, (2) acclimated for 2-6 weeks to 5 or 25 degrees C or (3) acclimated for 2-6 weeks to 5 or 25 degrees C and the muscles incubated at 15 degrees C. Under all three temperature conditions tested, net rate of lactate metabolism exceeded that of glucose. Acute exposure to 5 degrees C reduced net rate of glucose metabolism by 15x and net lactate metabolism by 10x as compared with 25 degrees C-exposed tissues. Acclimation to 5 degrees C favored glucose storage as glycogen and increased the proportion of lactate oxidized (versus stored or converted to glucose) when compared with 25 degrees C-acclimated tissues. Net rates of storage of lactate as glycogen (glyconeogenesis) were significantly higher in muscles from 5 degrees C-acclimated frogs during incubation at a common temperature of 15 degrees C. These data suggest that lactate is the predominant fuel for resting skeletal muscle over this temperature range, and particularly so under cold conditions. Ready use of lactate as a substrate, and enhancement of glyconeogenic pathways in response to cold acclimation, could play a role in the tolerance of this species to seasonal temperature changes by promoting sequestration and storage of available substrate under cold conditions.

Effects of arginine vasotocin and vasopressin receptor antagonists on Na+ and Cl- transport in the isolated skin of two frog species, Hyla japonica and Rana nigromaculata

Physiological function of arginine vasotocin (AVT) and effect of receptor antagonists of vasopressin were electrophysiologically investigated on transepithelial transport of ions in the abdominal skin of Hyla japonica and Rana nigromaculata by means of the Ussing chamber system. Administrations of AVT and forskolin (adenylate cyclase activator) in the serosal side of normal Ringer's solution significantly increased transepithelial potential difference (PD) and short-circuit current (Isc) accounting for Na(+) influx, mucosal to serosal direction, across the skin of H. japonica. In contrast, AVT administrations significantly decreased PD but not Isc on the skin of R. nigromaculata in a concentration-dependent manner ranging from 10(-11) to 10(-8)M. Administration of 10(-5)M forskolin also significantly decreased PD in normal and low Na(+) Ringer's solution and in the presence of amiloride (Na(+) channel blocker) on the mucosal side of normal Ringer's solution. On the other hand, forskolin significantly increased PD and Isc in the Cl(-) free Ringer's solution. These results suggested that AVT and forskolin stimulated mainly Cl(-) influx across the skin of R. nigromaculata. In two frog species, the AVT actions on ion transports were inhibited by pretreatment of OPC-31260 (a vasopressin V(2) receptor antagonist) but not OPC-21268 (a vasopressin V(1) receptor antagonist). These results suggested that AVT activates adenylate cyclase via V(2)-like receptor and stimulates actively net Na(+) and net Cl(-) transports in the abdominal skin of H. japonica and R. nigromaculata, respectively.

Digitalis-like compounds in the toad Bufo viridis: tissue and plasma levels and significance in osmotic stress

Digitalis-like compounds (DLC), constituents of animal tissues, are possible regulators of the Na+, K(+)-ATPase implicated in water and salt homeostasis. The distribution of DLC in the toad (Bufo viridis) was determined following methanol extraction and partial purification. DLC highest levels were found in the skin but it was also detected in the plasma and many internal organs. Short term (hours) exposure of the toad to hypertonic shock (1.5% NaCl) induced an increase in plasma osmolarity due to an increase in Na+ and Cl- levels. This treatment induced a transient, three fold, increase of DLC levels in the brain and transient reduction of its levels in the ventral skin. Acclimation of the toads to burrowing conditions for six weeks resulted in an increase in plasma osmolarity due to a large increase in plasma urea with a small increase in ion concentrations. Under these conditions DLC levels in the dorsal skin increased by 100% without alteration of its levels in the plasma, brain and ventral skin. DLC levels in the toad brain of control animals, showed a significant dependence on season, being highest in the summer and lowest in the winter. DLC levels in the skin peaked in May while the levels in the plasma were season independent. The changes in DLC levels induced by the short- as well as long-term perturbations in the animal environmental salinity together with the seasonal differences suggest that DLC in the toad is involved in water and salt homeostasis of these animals, but may also participate in other unknown functions.

Intracellular electrolyte concentrations in the frog skin epithelium: effect of vasopressin and dependence on the Na concentration in the bathing media

The intracellular electrolyte concentrations of the frog skin epithelium have been determined in thin freeze-dried cryosections using the technique of electron microprobe analysis. Stimulation of the transepithelial Na transport by arginine vasopressin (AVP) resulted in a marked increase in the Na concentration and a reciprocal drop in the K concentration in all epithelial cell layers. The effects of AVP were cancelled by addition of amiloride. It is concluded from these results that the primary mechanism by which AVP stimulates transepithelial Na transport is an increase in the Na permeability of the apical membrane. However, also some evidence has been obtained for an additional stimulatory effect of AVP on the Na pump. In mitochondria-rich cells and in gland cells no significant concentration changes were detected, supporting the view that these cells do not share in transepithelial Na transport. Furthermore, the dependence of the intracellular electrolyte concentrations upon the Na concentration in the outer and inner bathing solution was evaluated. Both in control and AVP-stimulated skins the intracellular Na concentration showed saturation already at low external Na concentrations, indicating that the self-inhibition of transepithelial Na transport is due to a reduction of the permeability of the apical membrane. After lowering the Na concentration in the internal bath frequently a Na increase in the outermost and a drop in the deeper epithelial layers was observed. It is concluded that partial uncoupling of the transport syncytium occurs, which may explain the inhibition of the transepithelial Na transport and blunting of the AVP response under this condition.

Tyrosine aminotransferase activity of frog (Rana esculenta) liver. III. A circannual study

A circannual study of tyrosine aminotransferase and other metabolic enzymes in frog liver is reported. The subcellular distribution of all enzymatic activities under investigation was also studied. Results show significant oscillations of all enzymatic activities throughout the year; in particular tyrosine aminotransferase has a marked summer maximum. The subcellular distribution of tyrosine aminotransferase shows significant variations: the soluble activity of the enzyme presents a bimodal circannual distribution, which has its counterpart in an increased activity of heavier fractions.

Seasonal changes in sodium transport and amiloride sensitivity in the isolated intestine of the earthworm, Lumbricus terrestris (L.)

Temporal changes in sodium flux rates and the electrical properties of two regions of the intestine appear to occur in a yearly cycle. In RIIIA, the anterior portion of the mid-intestine, the short-circuit current in January and April preparations is 60.2 and 10.2 microA cm-2, respectively and the net sodium fluxes are 1.50 and 1.24 mu Eq cm-2 hr-1, respectively. In RIIIB, the posterior portion of the mid-intestine, the short-circuit current in January and April preparations is 50.7 and 27.9 microA cm-2, respectively, while the net sodium fluxes are 1.78 and 0.59 mu Eq cm-2 hr-1, respectively. Sodium transport in RIIIB is inhibited by amiloride (10(-4)M) in January preparations but is refractory to amiloride (less than or equal to 10(-3)M) in April preparations.

Effects of frog-skin angiotensin II in amphibians

The role of frog-skin angiotensin II (AII) in amphibia was studied by comparing the sodium and water permeability effects of three angiotensins (AII): frog skin (Ala-Pro-Gly-[Ile3, Val5]-Ang II), human [( Asp1, Ile5]-AII), and Japanese goosefish [( Asn1-Val5]-AII). Frog-skin AII increased the short-circuit current (SCC) significantly after it was added to the dermal side of the isolated skin of the South American frogs, Leptodactylus chaquensis and ocellatus, and the toad, Bufo arenarum, in concentrations of 10(-6) M. In frogs, the effect was significant at 15 minutes and reached 45% over control after 2 1/2 hours. The effect cannot be achieved with concentrations lower than 10(-7) M. Since amiloride (10(-4) M) blocked the SCC response, and absence of chloride in the bathing fluid did not, the effect is probably dependent on sodium transport. Human AII (10(-6) M) produced a similar response in summer frogs that had been treated with 0.1% NaCl for 14 days. Goosefish AII was ineffective at similar concentrations, and none of the angiotensins modified SCC in the toad bladder. Hydrosmotic effects could be achieved with the three angiotensins, the response being dependent on seasonal and species factors but always considerably lower than that of the neurohypophyseal peptides. Vascular reactivity of the isolated frog hindlimbs was compared by dose-response curves. Potency ratios on a molar basis against frog-skin AII was 1.136 for human AII and 1.193 for goosefish AII. The results show that the effects of the angiotensins differ in both the response of SCC to frog-skin angiotensin and its higher vascular effects.

The effect of photoperiod on changes in plasma, glucose, cholesterol, and free fatty acids during cold acclimation in frogs

1. Many previous experiments on cold acclimation in frogs were carried out on animals maintained in total darkness. I tested the effect of photoperiod on changes in plasma FFA, glucose, and cholesterol during cold acclimation. 2. Hematocrit levels were lower whereas plasma FFA levels were higher in frogs kept in total darkness during cold acclimation. 3. Plasma glucose and plasma cholesterol levels were not affected by photoperiod during cold acclimation.

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.

Variation in the effects of antidiuretic hormone on the isolated skin of the toad, Scaphiopus couchi

The rate of active sodium transport as measured by short-circuit current across the isolated skin of the toad, Scaphiopus couchi, was elevated following vasopressin (0.2 units/ml) or arginine vasotocin (0.1 units/ml) treatment of skins from active animals at all times of the year tested. Skins from dormant animals showed no such elevation at any time of the year. The rate of active sodium transport was elevated following treatment with dibutyryl cyclic AMP (2.5mM) plus theophylline (10 mM) in all skins tested. The hydraulic conductivity of isolated skins from both active and dormant animals showed no significant change following treatment with vasopressin (0.2 units/ml) or arginine vasotocin (0.1 units/ml except on the first day following emergence from dormancy in the field. A correlation was, therefore, observed between the occurrence of a hydroosmotic response to antidiuretic hormones and the seasonal exposure of S. couchi to standing water. A small but significant elevation of hydraulic conductivity was observed across the skins of dormant toads following treatment with dibutyryl cyclic AMP (2.5 mM) plus theophylline (10 mM) whereas a substantial elevation was observed with the skins of active animals.

Salt-induced changes in sodium transport across the skin of the euryhaline toad, Bufo viridis

1. Euryhaline toads (Bufo viridis) were adapted to either distilled water, 115 or 200 mM-NaCl solutions. Na transport across the skin of these animals was studied in in vitro preparations. 2. Salinity adaptation caused a reduction in transepithelial potential and short-circuit current across the skin, and increased its electrical resistance. 3. Na influx across the skin was decreased. The reduction accounted fairly well for the measured reduction in short-circuit current. 4. Short-term Na uptake at the outer barrier of the skin was greatly reduced in preparations from salt adapted animals. Amiloride, which usually inhibits Na uptake, did not affect it in skins from salt adapted toads. 5. Sodium conductance decreased by a factor of three in skins from 115 mM-NaCl adapted toads, and to zero in skins from 200 mM-NaCl adapted animals. Shunt conductance was only little affected by the salinity of adaptation. 6. 'Amiloride-sensitive component' of transepithelial Na transport was 55, 21-5, and 4-5 muA/cm-2 in skins from toads which were adapted to distilled water, 115, and 200 mM-NaCl solutions, respectively. 7. It is concluded that the effect of salinity adaptation on the skin of the toad is to reduce the number of Na selective sites at the outer barrier. This change is reflected in a decrease in the 'amiloride-sensitive component' of Na transport across the skin, and could not be increased by aldosterone. Other changes may occur as secondary to that.

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).

Effects of amiloride on active sodium transport by the isolated frog skin: evidence concerning site of action

1. Amiloride reduces short-circuit current and potential difference across the isolated frog skin.2. Isotopically measured sodium influx and efflux are diminished.3. Total electrical conductance and partial sodium conductance are diminished, the reduction in total conductance being entirely accounted for by the reduction in partial sodium conductance.4. The effect of antidiuretic hormone (ADH), cyclic 3'5'-adenosine monophosphate (cyclic AMP) and theophylline can be antagonized by pretreatment with amiloride but the antagonism can be abolished by increasing the concentration of these compounds.5. Amiloride has no effect on oxygen consumption in concentrations which inhibit sodium transport. However, it prevents the stimulatory effect of ADH on oxygen consumption.6. The results are consistent with an action of amiloride at the passive outside membrane of the transporting cells of isolated frog skin.