Ca2+ and Zn2+ are transported by the electrogenic 2Na+/1H+ antiporter in echinoderm gastrointestinal epithelium
45Ca2+ uptake by purified brush-border membrane vesicles of starfish (Pycnopodia helianthoides) pyloric ceca was stimulated by an outwardly directed H+ gradient and this stimulation was enhanced by the simultaneous presence of an induced membrane potential (inside negative; K+/valinomycin). External amiloride (competitive inhibitor; Ki=660 &micro;mol l-1) and a monoclonal antibody raised against proteins associated with the lobster (Homarus americanus) electrogenic 2Na+/1H+ antiporter both inhibited approximately half of the proton-gradient-stimulated 45Ca2+ uptake. These results suggested that Ca2+ might be transported by the electrogenic antiporter and that the crustacean antibody was inhibitory to the exchange function in echinoderms, as was recently shown in crustacean epithelial brush-border membrane vesicles. Carrier-mediated 45Ca2+ influx by amiloride-sensitive and amiloride-insensitive systems displayed the following kinetic constants: (amiloride-sensitive) Kt=66&plusmn;2 &micro;mol l-1; Jmax=0.173&plusmn;0.002 pmol &micro;g-1 protein 8 s-1; (amiloride-insensitive) Kt=18&plusmn;0.3 &micro;mol l-1; Jmax=0.100&plusmn;0.001 pmol &micro;g-1 protein 8 s-1. Zn2+ was a mixed inhibitor of 45Ca2+ influx by carrier-mediated transport, displaying a Ki of 920 &micro;mol l-1. Mn2+, Cu2+, Fe2+ and Mg2+ also inhibited 45Ca2+ uptake, but the mechanism(s) of inhibition by these other cations was not disclosed. An equilibrium shift experiment showed that both Na+ and Zn2+ were able to exchange with equilibrated 45Ca2+ in these vesicles, suggesting that both monovalent and divalent cations were able to enter pyloric cecal cells through a common carrier-mediated transport system. In addition, the echinoderm electrogenic system appeared to exhibit a molecular component recognized by the crustacean antibody that may imply a similar epitope in the two animals.
Zhuang
Z
Duerr
J
Ahearn
G