title: Ca2+ transport pathways in brush-border membrane vesicles of crustacean antennal glands.
abstract: 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.
authors.affiliation: Department of Zoology, University of Hawaii, Manoa, Honolulu 96822.
authors.forename: G A
authors.initials: GA
authors.lastname: Ahearn
authors.forename: P
authors.initials: P
authors.lastname: Franco
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D017920:Antiporters; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000493:pharmacokinetics*; D002135:Calcium-Binding Proteins / Q000378:metabolism; D027682:Cation Transport Proteins; D004058:Diffusion; D006859:Hydrogen / Q000378:metabolism; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism; D008121:Nephropidae / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D013997:Time Factors; D014700:Verapamil / Q000494:pharmacology
chemical_list: D017920:Antiporters; D002135:Calcium-Binding Proteins; D027682:Cation Transport Proteins; C027614:calcium-hydrogen antiporters; D000584:Amiloride; D006859:Hydrogen; D012964:Sodium; D014700:Verapamil; D002118:Calcium