Inhibition of biliary bicarbonate secretion in ethinyl estradiol-induced cholestasis is not associated with impaired activity of the Cl-/HCO-3 exchanger in the rat.
OBJECTIVE
Bicarbonate is a major component of bile salt independent bile flow, which is impaired in ethinyl estradiol (EE)-cholestasis. To examine this subject in EE-cholestasis, we studied: 1) basal and glucagon-stimulated biliary bicarbonate secretion both in vivo and in the isolated perfused rat liver (IPRL); 2) H+/HCO-3 transport processes in isolated rat hepatocyte couplets.


METHODS
Rats received EE (5 mg.kg b.w.-1) for 5 days. Intracellular pH (pHi) was measured (BCECF-AM) using a single-cell microfluorimetric setup.


RESULTS
Bile flow was markedly (p < 0.01) decreased in EE-treated rats. Bicarbonate concentration in bile was decreased (p < 0.01) and bicarbonate secretion was 2.5-fold lower in EE-treated animals than in controls, both in bile-fistula rats [19.5 +/- 5.1 (n = 23) vs 54.2 +/- 5.7 (n = 20) nmol.min-1g liver-1; p < 0.01] and in the IPRL [11 +/- 2 (n = 8) vs 24 +/- 3 (n = 8) nmol.min-1.g liver-1; p < 0.01]. In control IPRL, a bile/perfusate gradient for bicarbonate is maintained, while it is lost in EE-treated IPRL because of the lower bicarbonate concentration in bile. Glucagon stimulated bile flow and bicarbonate secretion to a similar extent in EE-treated and control IPRL (+25% vs +23%). Resting pHi of EE-treated hepatocyte couplets was higher in comparison with controls in KRB [7.25 +/- 0.07 (n = 35) vs 7.20 +/- 0.05 (n = 33); p < 0.02] but similar in Hepes [7.08 +/- 0.07 (n = 24) vs 7.05 +/- 0.06 (n = 26)]. Basal activity of the Cl-/HCO-3 exchanger was similar in EE-treated and control hepatocyte couplets [H+ flux = 2.87 +/- 1.12 (n = 18) vs 3.01 +/- 1.23 mM/min (n = 15)] and was stimulated to a similar extent by glucagon. Na+/HCO3-symport activity was increased in EE-treated hepatocyte couplets (p < 0.05) while the Na+/H+ exchanger was unchanged.


CONCLUSIONS
Bicarbonate biliary secretion is markedly impaired during EE-cholestasis in association with a marked decrease of bile salt independent bile flow. However, the Cl-/HCO-3 exchanger and its hormonal regulation are normal, indicating that the lower bicarbonate excretion in EE-cholestasis is not due to a compromised activity of this anion exchanger. Since the bile/perfusate gradient for bicarbonate is dissipated in EE-treated IPRL, the impaired bicarbonate excretion could be caused by a reflux of biliary bicarbonate via leaky tight junctions.
Alvaro
D
D
II Department of Gastroenterology, University of Rome, La Sapienza, Italy.
Gigliozzi
A
A
Piat
C
C
Carli
L
L
Fraioli
F
F
Romeo
R
R
Francia
C
C
Attili
A F
AF
Capocaccia
L
L
D000818:Animals; D017920:Antiporters / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism*; D001646:Bile / Q000378:metabolism*; D001835:Body Weight / Q000187:drug effects; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides / Q000378:metabolism; D002779:Cholestasis / Q000139:chemically induced*; D045166:Estradiol Congeners / Q000494:pharmacology*; D004997:Ethinyl Estradiol / Q000494:pharmacology*; D005934:Glucagon / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D009929:Organ Size / Q000187:drug effects; D010477:Perfusion; D051381:Rats; D017208:Rats, Wistar; D012636:Secretory Rate / Q000187:drug effects
D017920:Antiporters; D001639:Bicarbonates; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides; D045166:Estradiol Congeners; D004997:Ethinyl Estradiol; D005934:Glucagon