Active transport of calcium across the urinary bladder and colon of the toad Bufo marinus.

Unidirectional fluxes of calcium were measured (in vitro) across the urinary bladder and colon of the toad Bufo marinus in the absence of electrochemical gradients. A net calcium flux was observed in each tissue, but the polarity differed; it occurred from the mucosal (luminal) to the serosal side of the colon and from the serosal to mucosal (urinary) side of the bladder. The active transport in each tissue appeared to involve different mechanisms; that across the colon exhibited a sodium dependence, possibly involving a sodium-calcium exchange, but this was not seen in the urinary bladder. The net flux in the latter was, however, abolished by metabolic inhibitors, possibly reflecting a role of a calcium-adenosine triphosphatase mechanism. The results are discussed in relation to the calcium metabolism of this amphibian.