GOLDSTEIN, J; HOFF, K; HILLYARD, S: Effects of angiotensin II and hydration state on water absorption behavior of toads (Bufo punctatus)

Anuran amphibians press their ventral skin to moist surfaces to facilitate osmotic water uptake, a process termed cutaneous drinking. The frequency and duration of two cutaneous drinking behaviors (the water absorption response, WR and seat patch down , SPD) provide an assay for elucidating the hormonal regulation of drinking in amphibians. We examined the modulation of cutaneous drinking by angiotensin II (AII) in toads, Bufo punctatus, that had been held for one hour in tap water(TW)or in deionized water(DI). We found that DI exposure suppressed the initiation of WR relative to the TW toads and neither AII injection nor ionic concentration of the hydration source increased WR frequency in the DI toads. The DI toads initiated SPD at the same high frequency as the TW toads, however, SPD duration shown by the DI toads increased after AII injection when the hydration source was deionized water or 50 mM NaCl but not with 250 mM NaCl. AII injection increased weight gain of DI toads with deionized water as a hydration source, but not with either NaCl solution. The addition of amiloride (a sodium channel blocker) to the hydration source did not affect SPD duration or weight gain in DI toads. These observations suggest that the physiological systems activated AII are primarily sensitive to osmolality as opposed to ionic concentration. Further, the suppression of AII-induced hydration behavior in DI toads occurs at or beyond the site where AII binds to its receptors.