Plasma membrane calcium pump expression in human placenta and small intestine.

To identify the forms of the plasma membrane calcium pump present in tissues that transport calcium, cDNA from human placenta and proximal small intestine was amplified by the polymerase chain reaction using a pair of mixed primers based on all the known human and rat plasma membrane calcium pump sequences. Clones were identified from the two human forms HPMCA1 and HPMCA4, but no new sequences were found in either tissue. RNA blots probed with HPMCA1 showed two bands in both tissues; probing with HPMCA4 gave a single, larger species. In placenta, HPMCA4 was the more abundant form and similar expression was found in full-term and second-trimester placentas. In contrast, in the small intestine, HPMCA1 was more abundant, suggesting that calcium absorption is not associated with any one specific isoform in calcium transporting cells.