PLASMA-MEMBRANE CALCIUM-PUMP ISOFORMS IN HUMAN AND RAT-LIVER

Plasma-membrane Ca2+-pumping ATPases (PMCAs) extrude Ca2+ from the cyt oplasm of all cells. Some previous studies of ATP-dependent Ca2+ trans port by liver membranes suggested there exist specific properties of t he hepatic PMCA, including regulation by hormones which affect calcium signalling. Multiple PMCA isoforms are now known to result from expre ssion of four different genes (known as PMCA 1-4) and alternative RNA splicing at three possible sites (A, B and C). We investigated which i soforms are expressed in adult human and rat liver RNA using reverse-t ranscription polymerase chain reaction with mixed primers designed to amplify parts of all the known PMCA transcripts. In human liver, produ cts were identified by sequencing from PMCA1, PMCA2 and PMCA4, but not from PMCA3 or from any new gene. In rat liver, by contrast, only PMCA 1 and PMCA2 were detectable, although we confirmed that the primers we re able to amplify from rat lung a new sequence which is part of rat P MCA4. Of the alternatively spliced variants, at site A in the PMCA2 se quences, all the exons were included in both adult and fetal human liv er. In human liver, the exon at site B was excluded in some products f rom PMCA1 and PMCA4, and at site C, only PMCA1b and one form of PMCA4 were found. Blots of human liver RNA showed PMCA1 and PMCA4 were abund antly expressed, unlike PMCA2. On blots of rat liver RNA, PMCA1 was mo re abundant than PMCA2, and purified rat parenchymal cell RNA gave sim ilar findings. In summary, no new hepatic PMCA isoforms have been demo nstrated, but differences between the predominant human and rat isofor ms may have consequences for Ca2+ signalling or the response to liver cell injury.