Identification of an mRNA species which encodes a voltage-operated Ca2+ channel in rat liver mRNA

Abstract

cDNA which encodes part of the alpha 1-subunit of the rabbit skeletal muscle L-type voltage-operated Ca2+ channel (VOCC cDNA) was employed to search for the presence in whole liver and hepatocytes of poly (A+) RNA homologous to mRNA which encodes VOCCs. Such homologous mRNA would be a candidate for mRNA which encodes the putative hepatocyte receptor-activated Ca2+ inflow system (RACIS). Northern blot analysis showed that poly (A+) RNA prepared from intact liver tissue, but not hepatocytes, contained a poly (A+) RNA species comparable in size to that which encodes the alpha 1-subunit of the L-type VOCC. It is concluded (a) that hepatocytes do not possess VOCCs or that the levels of VOCC poly(A+) RNA in hepatocytes are too low to be detected by Northern analysis and (b) that another cell type present in liver tissue does possess a VOCC. In a low stringency screen of a rat liver cDNA library employing VOCC cDNA as a probe, seven positive cDNA clones were obtained. While regions of the 2.3 kb cDNA insert from one of these clones showed sequence similarities with regions of VOCC cDNA, the 2.3 kb sequence did not appear to encode a Ca2+ channel. The present results suggest that the approach of low stringency cDNA library screening is unlikely to allow isolation of RACIS cDNA.