In vitro characterisation of calcium influx across skin and gut epithelia of the Pacific hagfish, Eptatretus stoutii
Although hagfishes osmoconform, concentrations of calcium in their extracellular fluids are maintained at levels lower than those of seawater. Ultimately, calcium homeostasis is a product of relative rates of influx and efflux into the animal, but little is known regarding these processes in hagfish. Using in vitro approaches, calcium influx across gut and skin epithelia of the Pacific hagfish, Eptatretus stoutii, was characterised. Analysis of concentration-dependent kinetics showed that accumulation into skin tissue was linear at mucosal calcium concentrations up to 10 mM. However, movement into the serosal compartment was saturable, with a maximal transport rate of 59 nmol cm−2 h−1 and an affinity (substrate concentration to give half maximal transport rate) of 1139 µM. Calcium accumulation into gut tissue and uptake into the gut serosal compartment was also saturable, with maximal transport rates of 5.1 and 2.8 nmol cm−2 h−1, and affinities of 6417 and 3327 µM, respectively. A strong correlation between epidermal and intestinal calcium tissue uptake was noted in assays where tissues were taken from the same individual. Intestinal calcium influx was inhibited by nickel, but not significantly affected by other putative inorganic (lanthanum, zinc) or organic (diltiazem, verapamil) transport blockers. Inhibitors had no effect on skin calcium handling. These data indicate that both gut and skin epithelia mediate calcium influx in hagfishes, albeit via distinct mechanisms and with differences in responsiveness to potential modifying factors.