EFFECTS OF TAUROLITHOCHOLATE, A CA-2+-MOBILIZING AGENT, ON CELL CA-2+ IN RAT HEPATOCYTES, HUMAN PLATELETS AND NEUROBLASTOMA NG108-15 CELL-LINE

The monohydroxy bile acid taurolithocholate permeabilizes the endoplasmic reticulum to Ca2+ in rat liver cells. To assess whether this action on the endoplasmic reticulum was restricted to this tissue, the effects of bile acid were investigated in two cell types quite unrelated to rat hepatocyte, namely human platelets and the neuronal NG108-15 cell line. The results showed that taurolithocholate (3-100-mu-M) had no effect on free cytosolic [Ca2+] in human platelets and NG108-15 cells, whereas it increased it from 180 to 520 nM in rat hepatocytes. In contrast, in cells permeabilized by saponin, taurolithocholate initiated a profound release of the stored Ca2+ from the internal Ca2+ pools in the three cell types. The bile acid released 90 % of the Ca2+ pools, with rate constants of about 5 min-1 and half-maximal effects at 15-30-mu-M. The results also showed that, in contrast with liver cells, which displayed an influx of [C-14]taurolithocholate of 2 nmol/min per mg, human platelets and the neuronal cell line appeared to be resistant to [C-14]taurolithocholate uptake. The influx measured in these latter cells was about 100-fold lower than in rat liver cells. Taken together, these data suggest that human platelets and NG108-15 cells do not possess the transport system for concentrating monohydroxy bile acids into cells. However, they show that human platelets and neuronal NG108-15 possess, in common with liver cells, the intracellular system responsible for taurolithocholate-mediated Ca2+ release from internal stores.