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WATER AND ELECTROLYTE HOMEOSTASIS

The hydroosmotic response of frog urinary bladder to serosal hypertonicity is dependent on adenylate cyclase for its maintenance and affected by [Cl^–]_o changes

Department of Physiology, University College Cork, Cork, Ireland

Submitted 6 September 2005 ; accepted in final form 30 January 2006

The role of adenylate cyclase (AC) in the maintenance of the hydroosmotic response to serosal hypertonicity (SH) in anuran urinary bladder is disputed. In this study, norepinephrine (NE) significantly reversed the hydroosmotic response of Rana temporaria bladders in hypertonic medium (330 mosmol/kgH₂O). The reversal was inhibited by yohimbine but was unaffected by prazosin and propranolol, indicating that NE action was mediated via ₂-adrenergic receptors. Preincubation of bladders with indomethacin did not interfere with the inhibitory action of NE, contraindicating a role for prostaglandins. The SH hydroosmotic response was abolished in the presence of 5-n-ethyl-N-isopropyl amiloride (EIPA), but the antidiuretic hormone (ADH) hydroosmotic response was not. EIPA inhibits Na⁺/H⁺, known to be activated by cell shrinkage. An investigation of the anionic requirement of the SH hydroosmotic response revealed that replacement of bath Cl^– with the nonpermeable anion gluconate reversibly abolished this response. In contrast, the hydroosmotic response to ADH was unaffected by Cl^– removal; however, when Cl^– was absent, it was no longer augmented in hypertonic bath. The SH response was inhibited by the Cl^– channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate but not by the Na/K/2Cl inhibitor bumetanide. Our results show that not only the onset but also the maintenance of the SH hydroosmotic response is dependent on AC activity and does not differ in this respect to the ADH hydroosmotic response. The effect of modifying extracellular Cl^– concentration, suggests that this anion, possibly functionally linked with Na⁺/H⁺ activity, may be involved in invoking the SH hydroosmotic response in anuran urinary bladder.

anuran; water balance; extracellular chloride