CELLULAR AND REGIONAL EXPRESSION OF TRANSCRIPTS OF THE PLASMA-MEMBRANE CALCIUM-PUMP PMCA1 IN RABBIT INTESTINE

The plasma membrane Ca2+-pumping adenosinetripho sphatas e (PMCA) is the energy-dependent step in the active vitamin D-dependent absorption of dietary Ca2+ by the enterocyte. Studies of the various PMCA genes and splicing variants in humans and rats have indicated that the isoform known as PMCA1b is the predominant form expressed in small intestine. Using an oligonucleotide probe, we have studied the regional and cellular distribution of PMCA1 transcripts in rabbit intestinal tissues by in situ hybridization. On small intestinal RNA blots, this hybridized to species similar in size to those detected by PMCA1-specific cDNA probes; an additional larger transcript was present in rabbit than in rat or human. In situ hybridization signals were principally in the enterocyte population of the mucosa and were maximal in differentiating enterocytes on the lower part of the villus, a pattern similar to that previously demonstrated for other nutrient transporters. Reflecting the capacity of the different small intestinal segments to transport Ca2+, much higher levels of transcript were detected by both methods proximally (in duodenum) than distally (in jejunum and ileum) and were also higher in cecum and ascending colon mucosa than in descending colon. We conclude that as enterocytes differentiate in regions that absorb Ca2+, they express high levels of mRNA for PMCA1. These results confirm the importance of transcriptional regulation of this gene for active Ca2+ absorption.