Role of the CNS in the control of the water economy of the toadBufo arenarum Hensel
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The water uptake (WU), urine production (U), overall water balance (WB) and body weight variations (bwv) were measured in normal (N) as well as in totally hypophysectomized, retrochiasmatic sectioned, infundibulum lesioned (HR) toads. Groups of animals were studied while adapted to tap water and during immersion in sucrose solutions of different concentrations (46, 92, 138, 220 and 230 mM). Subsequently the rate of water uptake (Jw) and the osmotic permeability coefficient (LPD) were calculated. Also the effect on these variables of the subcutaneous administration of a non-competitive α-blocker (Phenoxybenzamine: POB) was tested.
No correlation was found in normal controls between the osmotic pressure outside and WU, U, WB, bwv orJw.Jw remained relatively stable from 46 to 184 mOsm of external osmotic pressure, apparently due to a compensatory increase of LPD in the same group.
At a higher concentration of the external environment (220 mM) a drastic increase inLPD was observed in normal animals but the effect was markedly attenuated in HR animals.
Alpha-adrenergic blockade with POB induced a significant and almost constant increase in WU, WB andJw in normal animals in solutions up to 138 mM sucrose. These effects ceased when the external concentration reached 170 mM. A concomitant and stable increment inLPD was also observed. These values provide an indirect estimate of the adrenergic factor controlling water movements across the skin of the toad in vivo (Segura et al. 1982b), which appear to be osmotically independent within this range. The copious increase in WU due to adrenergic blockade was unaffected by amiloride (10−4 M), so water transport cannot be linked with sodium transport.
The present results appear to establish that: (a) Permeability of the skin to water is related to the osmolarity of the external environment in the normal toad; (b) hypothalamic-hypophyseal mechanisms seem only in part to account for this responsiveness; (c) normal toads are able to compensate in some degree for the reduction in water uptake caused by an increase in the osmolarity of the external environment by an increase in the osmotic permeability of the skin.
KeywordsWater Uptake Water Balance Osmotic Pressure External Environment Amiloride
overall water balance
body weight variations
influx of water
osmotic permeability coefficient
normal (control) toads
hypophysectomized retrochiasmatic sectioned, infundibulum lesioned toads
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