Extracellular calcium-sensing receptor is expressed in rat hepatocytes - Coupling to intracellular calcium mobilization and stimulation of bile flow

Liver cells respond to changes in Ca-o(2+). The hepatic functions affected include bile secretion, metabolic activity, liver regeneration, and the res ponse to xenobiotics. In the present study, we demonstrate the presence, in the liver, of the extracellular calcium-sensing receptor (CASR), described previously in the parathyroid and thyroid glands and kidney. CASR mRNA was specifically expressed in hepatocytes and was absent in nonparenchymal liv er cells (stellate, endothelial, and Kupffer cells). Western blot analysis using a specific CASR antibody showed staining in both whole liver and hepa tocyte extracts. Immunohistochemistry and in situ hybridization of rat live r sections showed expression of CASR protein and mRNA by a subset of hepato cytes. The known agonists of the CASR, gadolinium (Gd3+; 0.5-3.0 mM) and sp ermine (1.25-20 mM), in the absence of Ca-o(2+), elicited dose-related incr eases in Ca-i(2+) in isolated rat hepatocytes loaded with Fura-2/acetoxymet hyl ester. There was a greatly attenuated response to a second challenge wi th either agonist. The response was also abrogated when inositol 1,4,5-tris phosphate (IP3)-sensitive calcium pools had been depleted by pretreatment w ith either thapsigargin or phenylephrine, an alpha (1)-adrenergic receptor agonist known to mobilize Ca-i(2+) from IP,-sensitive pools. Addition of th e deschloro-phenylalkylamine compound, NPS R-467, but not the S enantiomer, NPS S-467, increased the sensitivity of the Ca-i(2+) mobilization response to 1.25 mar spermine. Bile flow ceased after Ca-o(2+), withdrawal, and its recovery was enhanced by spermine in isolated perfused liver preparations. The CASR agonists Ca2+ and Gd3+ increased bile flow, and the response to a submaximal Ca2+ concentration was enhanced by NPS R-467 but not the S comp ound. Thus, the data indicate that rat hepatocytes harbor a CASR capable of mobilizing Ca-i(2+) from IP3-sensitive stores and that activation of the C ASR stimulates bile flow.