AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 6 1206-R1213, Copyright © 1993 by American Physiological Society
Ca2+ transport pathways in brush-border membrane vesicles of crustacean antennal glands
G. A. Ahearn and P. Franco
Department of Zoology, University of Hawaii, Manoa, Honolulu 96822.
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.
