Ws. Marshall et al., CA2-WATER-ADAPTED EURYHALINE TELEOST, FUNDULUS-HETEROCLITUS( TRANSPORT BY OPERCULAR EPITHELIUM OF THE FRESH), Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 165(4), 1995, pp. 268-277
We examined transepithelial transport of Ca2+ across the isolated oper
cular epithelium of the euryhaline killifish adapted to fresh water. T
he opercular epithelium, mounted in vitro with saline on the serosal s
ide and fresh water (0.1 mmol . l(-1) Ca2+) bathing the mucosal side,
actively transported Ca2+ in the uptake direction; net flux averaged 2
0-30 nmol . cm(-2). h(-1). The rate of Ca2+ uptake varied linearly wit
h the density of mitochondria-rich cells in the preparations. Ca2+ upt
ake was saturable, apparent K-1/2 of 0.348 mmol . l(-1), indicative of
a multistep transcellular pathway. Ca2+ uptake was inhibited partiall
y by apically added 0.1 mmol . l(-1) La3+ and 1.0 mmol . l(-1) Mg2+. A
ddition of dibutyryl-cyclic adenosine monophosphate (0.5 mmol . l(-1))
+ 0.1 mmol . l(-1) 3-isobutyl-1-methylxanthine inhibited Ca2+ uptake
by 54%, but epinephrine, clonidine and isoproterenol were without effe
ct. Agents that increase intracellular Ca2+, thapsigargin (1.0 mu mol
. l(-1), serosal side), ionomycin (1.0 mu mol . l(-1), serosal side) a
nd the calmodulin blocker trifluoperazine (50 mu mol . l(-1), mucosal
side) all partially inhibited Ca2+ uptake. In contrast, apically added
ionomycin increased mucosal to serosal unidirectional Ca2+ flux, indi
cating Ca2+ entry across the apical membrane is rate limiting in the t
ransport. Verapamil (10-100 mu mol . l(-1), mucosal side), a Ca2+ chan
nel blocker, had no effect. Results are consistent with a model of Ca2
+ uptake by mitochondria rich cells that involves passive Ca2+ entry a
cross the apical membrane via verapamil-insensitive Ca2+ channels, int
racellular complexing of Ca2+ by calmodulin and basolateral exit via a
n active transport process. Increases in intracellular Ca2+ invoke a d
ownregulation of transcellular Ca2+ transport, implicating Ca2+ as a h
omeostatic mediator of its own transport.