Taurolithocholate-induced Ca2+ release is inhibited by phorbol esters in isolated hepatocytes

Enhanced ER Ca2+ store filling by overexpression of SERCA2b promotes IP3-evoked puffs

Liberation of Ca(2+) from the endoplasmic reticulum (ER) through inositol trisphosphate receptors (IP(3)R) is modulated by the ER Ca(2+) content, and overexpression of SERCA2b to accelerate Ca(2+) sequestration into the ER has been shown to potentiate the frequency and amplitude of IP(3)-evoked Ca(2+) waves in Xenopus oocytes. Here, we examined the effects of SERCA overexpression on the elementary IP(3)-evoked puffs to elucidate whether ER [Ca(2+)] may modulate IP(3)R function via luminal regulatory sites in addition to simply determining the size of the available store and electrochemical driving force for Ca(2+) release. SERCA2b and Ca(2+) permeable nicotinic plasmalemmal channels were expressed in oocytes, and hyperpolarizing pulses were delivered to induce Ca(2+) influx and thereby load ER stores. Puffs evoked by photoreleased IP(3) were significantly potentiated in terms of numbers of responding sites, frequency and amplitude following transient Ca(2+) influx in SERCA-overexpressing cells, whereas little change was evident with SERCA overexpression alone or following Ca(2+) influx in control cells not overexpressing SERCA. Intriguingly, we observed the appearance of a new population of puffs that arose after long latencies and had prolonged durations supporting the notion of luminal regulation of IP(3)R gating kinetics.

Effects of bile acids on iodide uptake and deoxyribonucleic acid synthesis in porcine thyroid cells in primary culture

The effects of bile acids on iodide uptake and DNA synthesis were studied in cultured porcine thyroid cells. All five bile acids, which are commonly found in serum, chenodeoxycholic acid (CDCA), cholic acid (CA), deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) dose-dependently inhibited both basal and TSH-induced iodide uptake at concentrations of 25-250 microM. Since CDCA is one of the two major primary endogenous bile acids, were studied mainly the effects of CDCA. The inhibitory effect of CDCA was detected after 24 h treatment of thyroid cells, and was dependent on the time of exposure up to 72 h. Treatment of thyroid cells with CDCA for 72 h inhibited cAMP production stimulated by 50 mU/L TSH or 0.5 mg/L forskolin and also inhibited iodide uptake induced by 0.5 mM 8-bromo cAMP or 0.5 mg/L forskolin. These results suggest that CDCA inhibits iodide uptake by decreasing cAMP production as well as post-cAMP generation. Bile acids except LCA stimulated [3H]thymidine incorporation into the thyroid cells by itself, indicating that the inhibitory effect of bile acids on iodide uptake is not due to cytotoxic effect. In conclusion, these findings suggest that the direct inhibition of thyroid function by bile acids might cause hypothyroidism in chronic liver disease.

Crosstalk between calcium- and cyclic AMP-mediated signalling systems and the short-term modulation of bile flow in normal and cholestatic rat liver

The flow of bile is subject to short-term modulation by glucagon and calcium-mobilizing hormones. Of potential relevance is the crosstalk between the second messenger-mediated signal transducing systems of these agonists. This latter point has revealed an area of investigation that should enable further insights to be made into a physiological network that interrelates bile flow, hepatocellular calcium movements and hormone action. This information in turn may provide insights into the etiology and treatment of human and animal diseases in which cholestasis is an underlying feature.