THE EFFECT OF NEUROHYPOPHYSIAL EXTRACTS ON THE WATER BALANCE OF LOWER VERTEBRATES

Hormonal effect on the osmotic, electrolyte and nitrogen balance in terrestrial Amphibia

Two main hormones regulate water balance in amphibian. First, mesotocin (MT) acting as a diuretic agent, and second arginine vasotocin (AVT) being an anti-diuretic hormone. In addition, prolactin (PRL), aldosterone, corticosterone, angiotensin II and atriunatriuretic hormones, play a role too in regulating water and ion balance. The hormones affect the epidermis and bladder permeability to water and ions as well as the kidney through the control of the glomerular filtration rate (GFR). The main questions concern the presence and action of these hormones during the amphibian's life history. Are they present in both larval and adult stages? Are these hormones being synthesized in both aquatic and terrestrial adult phases? Under what circumstances are they being stored or released? Would the target organs (epidermis, bladder, kidney) respond in a similar way during all periods? The problem is the fact that under most circumstances an amphibian while in an aquatic environment responds physiologically differently than when on land. Only partial information concerning hormone presence, release and control of water balance is available at the moment, and even that is fragmentary and based on only a very small number of amphibian species.

Water-balance response to salt loads in the toad Bufo bufo: independence of hypophysial pars nervosa function

1. In water, injection of 1 ml of 1 mol/l NaCl/100 g body mass increased the rates of cutaneous water uptake and reduced the rates of bladder urine accumulation both in unoperated and sham operated controls and in toads with denervated or extirpated neuro-intermediate lobe. 2. This initial antidiuretic effect of the salt load gradually changed to a diuretic effect, when the volume expansion caused by cutaneous water uptake overrode the effect of increased osmolarity. 3. In a saturated atmosphere, injections of 0.3 or 0.5 ml of 0.7 mol/l NaCl/toad (mean body mass 33 g) reduced urine accumulation. Only to the largest load was the antidiuretic response significantly smaller in the toads with denervated or extirpated neuro-intermediate lobe. 4. It is argued that the reduced antidiuretic response was due to interference with pars distalis function rather than caused by lack of pars nervosa function.

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.