Thiazides stimulate calcium absorption in the turtle bladder.

The mechanism of action of the thiazide diuretics on calcium transport is not completely understood. The present study was designed to examine the effect of hydrochlorothiazide (HTZ) on Na transport, proton secretion, and Ca45 flux in the turtle bladder, a high resistance membrane. When added to the mucosal solution, 1 mM HTZ had no effect on Na transport or proton secretion, but significantly increased mucosa-to-serosa Ca45 flux at 60 minutes (control 118.9 +/- 39.7 pmol/mg/60 min versus thiazide 286.0 +/- 64.9 pmol/mg/60 min, N = 16, P less than 0.02). In the presence of 5 X 10(-4) M ouabain, a maneuver which inhibits active Na transport, HTZ again significantly enhanced mucosa-to-serosa Ca45 flux. The increment of calcium transport under these conditions was 83.4 +/- 35.2 pmol/mg/60 min (N = 8, P less than 0.05). Mucosal HTZ had no effect on serosa-to-mucosa (that is, bath-to-lumen) Ca45 flux after a 60 minute incubation. Serosal addition of HTZ had no effect on either of the unidirectional Ca45 fluxes or on Na transport. Mucosal tissue Ca45 content was enhanced by HTZ (mucosal) in the presence or absence of ouabain. These results provide support for the view that thiazides have a direct stimulatory effect on calcium absorption at the luminal membrane, perhaps secondary to increased mucosal calcium permeability.