Thiazides stimulate calcium absorption in urinary bladder of winter flounder

Thiazides inhibit voltage-independent NaCl absorption in the urinary bladder of the winter flounder presumably by blocking an electroneutral mucosal Na/Cl co-transporter. As thiazides stimulate calcium absorption in mammalian distal convoluted tubule while inhibiting NaCl absorption, we studied the effects of hydrochlorothiazide (HCTZ) on unidirectional 45Ca fluxes and intracellular electrical potential in short-circuited bladders to examine possible mechanisms of HCTZ effects on calcium transport. Basal secretory calcium flux was, on average, slightly larger than absorptive flux, reflecting small net calcium secretion. Mucosal addition of HCTZ (10(-4) M) stimulated absorptive calcium flux by 46% while the secretory flux was unaltered. Thus, HCTZ tended to induce net calcium absorption. Pre-treatment with serosal ouabain (10(-4) M) attenuated the HCTZ-induced increase in absorptive calcium flux. Moreover, HCTZ hyperpolarized the mucosal membrane potential by 18% as measured by conventional open-tip microelectrodes. These effects of HCTZ are consistent with the hypothesis that HCTZ indirectly stimulates Na/Ca exchange located at the serosal membrane. In conclusion, HCTZ in flounder urinary bladder, as in mammalian distal convoluted tubule, simultaneously inhibits NaCl absorption and stimulates calcium absorption. This study expands on the functional similarities between the flounder urinary bladder and the mammalian distal convoluted tubule.