REGULATION OF TAUROCHOLATE AND URSODEOXYCHOLATE UPTAKE IN HAMSTER HEPATOCYTES BY CA2-MOBILIZING AGENTS()

In isolated hamster hepatocytes, the Ca2+ ionophore A-23187 immediatel y decreased the uptake rate of taurocholic acid (TCA) by 60-70%, where as it slowly inhibited that of ursodeoxycholic acid (UDCA) by a maximu m of 35-45%, with an inhibition constant (K-i) of 0.36 and 1.93 mu M, respectively. In contrast to ionomycin, which mimicked the effect of A -23187, vasopressin inhibited the bile acid uptake rate by 40 and 45%, respectively, only after a 5- to 10-min preincubation. The Na+-depend ent bile acid transport was exclusively inhibited by these agents, and this inhibition was independent of extracellular Ca2+. However, intra cellular Ca2+ depletion with ethylene glycol-bis(beta-aminoethyl ether )-N,N,N',N'-tetraacetic acid or chelation with ,2-bis(2-aminophenoxy)e thane-N,N,N',N,-tetraacetic acid resulted in 40-50% inhibition of the uptake rate of both bile acids. The exogenous protein kinase C activat or, phorbol 12-myristate 13-acetate (PMA), but not the nonactive 4 alp ha-phorbol, significantly inhibited TCA uptake rate. Although both A-2 3187 and ionomycin immediately increased and decreased the cellular Na + and K+ concentration, respectively, neither vasopressin nor PMA had a significant effect on the cellular concentration of these cations, e ven after a 10-min incubation. Furthermore, the effect of A-23187 and ionomycin on TCA uptake and Na+ flux, respectively, disappeared after a 40-min preincubation, and additional ionophore remained without effe ct. However, after a 40-min incubation with A-23187, PMA was still abl e to inhibit TCA uptake. Therefore, A-23187 and ionomycin transiently inhibited Na+-dependent uptake of both TCA and UDCA, in part because o f transient alteration of the cellular Na+ and K+ concentration. Vasop ressin and PMA inhibited Na+-dependent bile acid uptake, at least in p art, through protein kinase C activation.