Ursodeoxycholate mobilizes intracellular Ca2+ and activates phosphorylase a in isolated hepatocytes

In isolated hamster hepatocytes, ursodeoxycholic acid (UDCA) mobilized intracellular free calcium ([Ca2+]i) and activated phosphorylase a with a half-maximally effective concentration of 188 and 9 microM, respectively. Addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) did not affect the maximum [Ca2+]i mobilized by UDCA; however, [Ca2+]i returned to basal levels in 4-5 min compared with > 10 min in the absence of EGTA. Both UDCA and vasopressin activated phosphorylase a to the same extent in the presence and absence of extracellular Ca2+, and the effect of both agents was abolished when the cells were depleted in Ca2+. Vasopressin (100 nM) did not further mobilize [Ca2+]i or activate phosphorylase a when combined with 500 microM UDCA. However, unlike vasopressin, UDCA did not stimulate inositol 1,4,5-trisphosphate (IP3) formation. In contrast to taurine-conjugated UDCA (TUDCA), concentration < or = 500 microM of glycine-conjugated UDCA (GUDCA) did not affect either [Ca2+]i or phosphorylase a. Lithocholic acid and taurolithocholic acid (TLCA) displayed the highest affinity for Ca2+. In addition, TLCA, chenodeoxycholic acid, and NaF stimulated Ca2+ efflux at concentrations as low as 100 microM, 200 microM, and 5 mM, respectively. Conversely, UDCA, TUDCA, and GUDCA presented the lowest affinity for Ca2+ and had no effect on Ca2+ efflux. The 28% increase in Ca2+ release induced by TLCA alone was further augmented to approximately 60% when TLCA was combined with UDCA, TUDCA, or GUDCA. However, Ca2+ efflux induced by NaF was not further increased by UDCA and its conjugates.(ABSTRACT TRUNCATED AT 250 WORDS)