D. Cremaschi et al., Apical Na+-CI- symport in rabbit gallbladder epithelium: A thiazide-sensitive cotransporter (TSC), J MEMBR BIO, 176(1), 2000, pp. 53-65
Cl- apically enters the epithelium of rabbit gallbladder by a Na+-Cl- sympo
rt, sensitive to hydrochlorothiazide (HCTZ). Since HCTZ also activates an e
pical SITS-sensitive Cl- conductance (G(Cl)), the sym port inhibition might
be merely due to a short circuit of the symport by G(Cl) rather than to a
direct action of HCTZ on the symporter. To examine whether the symport is d
irectly inhibited by HCTZ and whether the symporter belongs to the family o
f thiazide-sensitive cotransporters (TSC), radiochemical measurements of th
e apical Cl- uptake, electrophysiological determinations of intracellular C
l- and Na+ activities (a(i,Cl) and a(i,Na)) with selective theta microelect
rodes and molecular biology methods were used. The Cl-36(-) uptake proved t
o be a measurement of the apical unidirectional Cl- influx (J(mc)) and of t
he symport only (without backflux components), with measuring times of 45 s
ec under all experiment conditions; its inhibition by HCTZ was unaffected b
y G(Cl) activation or abolition. After HCTZ treatment the decrease in a(i,C
l) (measured as the initial rate or in 3 min) was larger than the decrease
in a(i,Na). The difference was reduced to one third in a group of epithelia
in which the elicited G(Cl) was reduced to one third; moreover it was abol
ished in any case when G(Cl) was abolished with 10(-4) M SITS. The SITS-ins
ensitive rate of a(i,Cl) decrease was equal to that of the a(i,Na) decrease
in any case. Thus the a(i,Cl) decrease displays a component dependent on G
el activation and a second component dependent on symport inhibition. Using
the RT-PCR technique a cDNA fragment was obtained that was 99% identical t
o the corresponding region of the rabbit renal TSC isoform. The results ind
icate that in rabbit gallbladder epithelium HCTZ displays a dual action, na
mely G(Cl) activation and Na+-Cl- symport inhibition. This Na+-Cl- symporte
r is the first TSC found to be functionally expressed in a nonrenal or nonr
enal-like epithelium.