Ca2+ transport pathways in brush-border membrane vesicles of crustacean antennal glands

Calcium uptake by brush-border membrane vesicles of Atlantic lobster (Homarus americanus) kidneys (antennal glands) in independent experiments was stimulated by outwardly directed Na or H gradients. In the absence of external amiloride, 45Ca uptake was strongly stimulated by an outwardly directed Na gradient, and this stimulation was enhanced by the addition of an inside-negative membrane potential. External amiloride (2 mM) reduced 45Ca uptake sixfold and lowered sensitivity to membrane potential. 45Ca influx kinetics (2.5-s uptake) in the presence of an outwardly directed H gradient and inside-negative membrane potential were composed of three components: 1) an amiloride-sensitive carrier system, 2) an amiloride-insensitive carrier system, and 3) a verapamil- and membrane potential-sensitive process that may represent diffusional transfer through a calcium channel. It was concluded that 45Ca entry by the amiloride-sensitive process occurred by a previously described electrogenic 2 Na-1 H antiport mechanism [Ahearn, G., and L. Clay. Am. J. Physiol. 257 (Regulatory Integrative Comp. Physiol. 26): R484-R493, 1989; Am. J. Physiol. 259 (Renal Fluid Electrolyte Physiol. 28): F758-F767, 1990; Ahearn, G., P. Franco, and L. Clay. J. Membr. Biol. 116: 215-226, 1990]. 45Ca influx by the amiloride-insensitive mechanism occurred by an apparent electroneutral 1 Ca-2 Na exchange. Transport stoichiometry of the latter mechanism was tentatively established by experiments determining intravesicular Na binding properties and by its apparent lack of response to a membrane potential. At physiological Na, Ca, and H concentrations in the antennal gland lumen and epithelial cytosol, these three calcium transport pathways individually may make significant contributions to net calcium reabsorption to the blood.