Magnesium Transport by the Urinary Bladder of the Crab, Cancer Magister

Anuric crabs, Cancer magister, in 100% sea water lose most of their ability to regulate serum magnesium levels below that of the external medium, indicating that the antennal gland is the site of most of the crab’s hypo-regulatory ability. In vitro measurement of unidirectional fluxes of 28Mg across tissue from the urinary bladder (the terminal element of the antennal gland) showed a significant, serosa-to-lumen (SL) net flux of 0.280 ± 0.059 µequiv cm-2h-1 which was greatly reduced by 5 mm ouabain. Based on the calculated surface area of the bladder in the crab, the net SL flux of magnesium in vitro is sufficient to account for the in vivo rate of magnesium excretion by the antennal gland. Bladder tissue from magnesium-depleted crabs which had stopped concentrating magnesium in the urine did not show a significant, net SL flux of MMg in vitro. It is speculated that magnesium enters the bladder cell by a sodium-coupled process at the serosal border and is actively transported into the urine at the luminal border. Eyestalk ablation caused no significant changes in urinary rate or magnesium levels in serum or urine ; thus neurosecretory centres in the eyestalk are apparently not involved in control of magnesium secretion by the antennal gland. Large, nearly equal, net effluxes of 22Na (1.33 ± 0.19 µequiv cm-2 h-1, ouabain-insensitive) and 36C1 (1.26 ± 0.34 µequiv cm-2 h-1) from the urine were measured in bladder preparations in vitro. It is speculated that this net efflux of salt may be the driving force for fluid reabsorption from the urine by the antennal gland.