Seasonal effects on osmotic permeability ofRana pipiens in vivo and in vitro

Inoculative freezing by environmental ice nuclei in the freeze-tolerant wood frog, Rana sylvatica

Efficacy of inoculative freezing by ice nuclei in a simulated winter environment was studied in the wood frog (Rana sylvatica), a freeze-tolerant species that overwinters on the forest floor beneath organic detritus. Adult frogs were confined to plastic canisters and cooled to -2 degrees C over 24 hr with their ventral skin in contact with substrate (humic soil hydrated to 40, 10, or 5%, or soil/peat mixture hydrated to 20 or 10%, w/w), or their dorsal skin in contact with damp leaf mould. Whereas only 20% of control frogs cooled in dry, plastic canisters froze, freezing occurred in nearly all (98%) frogs contacting soil or leaf mould. Inoculation was briefly delayed in frogs exposed to drier substrates. Frogs exposed to an unfreezable substrate (humic soil, 5% moisture) themselves froze, apparently due to the action of constituent nuclei which commonly occur in natural materials. Although the surface over which inoculation can occur is greater in larger frogs, inoculation susceptibility was not correlated with body mass in our frogs (mean +/- SE body mass = 14.0 +/- 0.2 g; range, 9.8-17.8 g). We conclude that the high susceptibility to inoculative freezing in R. sylvatica, which is conferred by its moist, highly permeable integument, promotes freeze tolerance by ensuring that inoculation commences at relatively high temperatures.

Role of pars nervosa of the hypophysis in amphibian water economy: a re-assessment

1. Responses in renal function and in water permeability of skin and bladder to wet and dry environments are accomplished within the range of normal hydration of the amphibian organism. 2. Urine production is discontinued at moderate dehydration. 3. Strong dehydration is needed to raise plasma arginine vasotocin (AVT). 4. Surgical interference with hypophysial function may repress water balance responses because of pars distalis dysfunction, with no clear effect of elimination of pars nervosa function. 5. Antidiuretic hormones, along with adrenergic agonists, may be potent stimulators of the water permeability of membranes of variable permeability, such as skin of terrestrial anurans. 6. AVT does not play a key role in amphibian water economy, but may exert a modulatory role in the control of renal function, secondary to nervous control.

Breeding condition, temperature, and the regulation of salt and water by pituitary hormones in the red-spotted newt, Notophthalmus viridescens

In vivo integumental transepithelial potential (TEP) in the Eastern red-spotted newt (Notophthalmus viridescens) increases linearly with external [Na+] from 0.1 to 10 mM and is anion independent. Both integumental TEP and osmotic permeability are higher in laboratory-conditioned (LC, terrestrial) than in breeding-condition (BC, aquatic) newts at temperatures of 5-25 degrees. Prolactin (PRL) treatment of LC newts decreased both TEP and rate of water uptake. Arginine vasotocin (AVT) treatment resulted in a substantial increase in water uptake in LC newts, while little or no AVT response was seen in PRL-treated or BC newts. Hypophysectomy (HX) or ergocryptine treatment increased TEP in BC newts, whereas HX + PRL maintained TEP at control levels. Although ergocryptine and HX were without effect on water uptake in BC newts kept at 5 degrees for 9 days, HX + ACTH increased water uptake. HX produced a substantial fall in serum [Na+] in BC newts, while either PRL or ACTH replacement elevated serum [Na+]. Combined ACTH and PRL treatment returned serum [Na+] to control levels. These data suggest that high endogenous prolactin plays a significant role in maintaining serum [Na+] and integumental permeability and transport characteristics in breeding-condition N. viridescens. Although PRL and ACTH are both sodium retaining in the aquatic breeding stage, these two hormones promote opposite effects on the integument; PRL decreases both water uptake and integumental TEP, whereas ACTH (presumably acting through the adrenals) increases water uptake and possibly TEP.

Water balance of five amphibian species at different stages and phases, as affected by hypophysial hormones

Five amphibian species were studied for the effect of hypophysial hormones on their water balance. The species were three anurans, Rana ridibunda, Bufo viridis and Pelobates syriacus, and two urodeles Salamandra salamandra and Triturus vittatus. In the first four species different stages of development were studied, in the newt both the terrestrial and aquatic phases of the adult were examined. The hormones used were oxytocin (OXY), arginine vasotocin (AVT) and prolactin (PL). Oxytocin caused most water retention when compared with the other hormones, especially responding were juveniles of Rana and Bufo, but also the terrestrial phase of the adult newt Triturus. Arginine vasotocin affected mostly juvenile Pelobates. Prolactin caused water retention in juvenile Rana and in the terrestrial phase of Triturus. In general the hormones affected the juvenile stages more than either larvae or adults.

Bath osmolality: effect on water permeability of epithelial tissue

When hyperosmotic gradients from 100 to 500 mosM are used to produce a water flux, the water permeability of live and potassium cyanide (KCN)-poisoned frog skin decreases with increasing osmotic gradients. In addition, as the total bath osmolality (corium + epithelial) increases there is a reduction in tissue water. Examination of the tissue cellular and extracellular compartments shows that cell shrinkage caused by the increasing hyperosmolality of the bathing medium correlates with the decrease in osmotic permeability. When the bath osmolality is held constant and cell volume decreases, there is a decrease in the water permeability. High potassium in the external bathing medium causes cell swelling that is associated with an increase in water permeability. These data support the hypothesis that a number of conditions known to affect the water permeability of frog skin do so partly or wholly as a result of a change in the cell volume, which either directly or indirectly alters the osmotic permeability of a rate limiting barrier, possibly the cell membrane.