In the first series of experiments, we loaded the cells with the low-affinity Ca2+ dye, mag-fluo-4 (KD for Ca2+: 22 μM). It has been shown previously that this dye is preferentially trapped within the lumen of the ER, and, most important, its fluorescence-intensity changes GSK2126458 are proportional to the [Ca2+] within this organelle. 21 Figure 1 shows that, at rest, the fluorescence-signal intensity of WT cells is larger than that of Pkd2KO cholangiocytes, whereas addition of the SERCA inhibitor, thapsigargin (2 μM), in the absence of extracellular Ca2+, resulted in a drop of mag-fluo-4 signal in both control and Pkd2KO cells. However, the drop of mag-fluo-4 fluorescence

caused by thapsigargin was much faster and larger in controls, compared to Pkd2KO cholangiocytes. In a second series of experiments, we measured [Ca2+]c changes after administration of adenosine triphosphate (ATP; 10 μM) or ionomycin (5 μM) in cells loaded with fura-2 and incubated in a Ca2+-free buffer. With any other parameter being similar, differences in the [Ca2+]c peaks reflect differences in the amount of Ca2+ released from intracellular stores. 9, 10, 27 The amount of Ca2+ released from the ER by ATP, an inositol 1,4,5-triphosphate (IP3)-generating agonist, both when measured as peak [Ca2+]c

increase relative to baseline and as the area under the curve (AUC), was significantly reduced in Pkd2KO cholangiocytes (peak increase: 50.12 ± 14 nM; AUC: 16 ± 0.7 AU; n Obeticholic Acid price = 53) with respect to WT (peak increase: 214 ± 16 nM; AUC 38 ± 11 AU; n = 53; P < 0.001) (Fig. 2). Qualitatively similar results were obtained when Ca2+ was not specifically mobilized from the stores using the Ca2+ ionophore,

ionomycin (peak increase in Pkd2KO cells: 38.8 ± 6.5 nM; AUC: 12 ± 5.6 AU) with respect to WT (peak Orotidine 5′-phosphate decarboxylase increase: 219 ± 28 nM; AUC: 42 ± 13 AU; n = 48; P < 0.001). When ER Ca2+ levels are acutely decreased, SOCE is activated. The efficiency of Ca2+ entry resulting from SOC can be conveniently estimated by measuring [Ca2+]c changes upon the readdition of extracellular Ca2+ to cells whose stores have been depleted (in Ca2+-free medium) by thapsigargin or ionomycin. SOCE-dependent [Ca2+]c increase was significantly slower (and the peak smaller) in Pkd2KO cholangiocytes (thapsigargin: rate of [Ca2+]c rise = 3.53 ± 0.52 nM/sec in WT versus 0.38 ± 0.09 nM/sec in Pkd2KO cells; peak increase in WT and Pkd2KO: 135 ± 39 and 34 ± 17 nM [P < 0.001], respectively; ionomycin: rate of [Ca2+]c rise = 7.3 ± 0.31 nM/sec in WT versus 0.52 ± 0.069 nM/sec in Pkd2KO cells; peak increase in WT and Pkd2KO: 245 ± 49 and 30 ± 19 nM [P < 0.001], respectively) (Fig. 3). Western blotting analysis of STIM-1 and Orai expression showed no difference in expression of the main components of SOCE between WT and Pkd2KO cells (Supporting Fig.