|Reinhardt, Timothy - Tim|
Submitted to: American Society for Bone and Mineral Research
Publication Type: Abstract only
Publication Acceptance Date: 10/2/1999
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: The large transcellular Ca2+ fluxes required for milk production must be regulated to maintain the low cytosolic Ca2+ concentrations critical to cell function. We examined plasma membrane Ca2+-ATPases (PMCA) and the putative Golgi secretory pathway Ca2+-ATPase (SPCA) protein expression by Western blotting. As lactation started we found dramatic increases in PMCA protein and this increased expression paralleled milk production. Isoform specific antibodies showed that this increased mammary PMCA was primarily PMCA2b, but ca. 4000 daltons larger than expected. RT-PCR showed that the primary mammary PMCA2b transcript was alternatively spliced, at splice site A. This spice form is designated 2bw and accounts for the 4000 dalton increase in PMCA2b's size. PMCA2bw was secreted into milk, associated with the milk fat globule membrane. This result demonstrates that significant amounts of PMCA2bw are located on the apical membrane of the secretory cell. Relatively smaller amounts of PMCA1b and 4b protein were found in mammary tissue. PMCA4b was the major PMCA expressed in developing mammary tissue and declined as lactation started. PMCA1b increased moderately during lactation. While major increases in mammary PMCA2b expression occurred only after lactation started, SPCA protein increased 1 wk prior to parturition and increased further as lactation proceeded. PMCA2b's abundance, cellular location, high affinity for Ca2+, and high constitutive activity suggests that PMCA2b is important for Ca2+ homeostasis in lactating tissue. SPCA's pattern of expression and abundance suggest that SPCA is a candidate for the Golgi Ca2+-ATPase shown to be important in maintaining Golgi Ca2+ concentration required for casein synthesis.