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Metabolic Diseases and Immunology Research Unit, National Animal Disease Center, Agricultural Research Service, United States Department of Agriculture, Ames, Iowa 50010
The transcellular Ca2+
fluxes required for milk production must be rigorously regulated to
maintain the low cytosolic Ca2+
concentrations critical to cell function.
Ca2+-ATPases play a critical role
in the maintenance of this cellular Ca2+ homeostasis. Using RT-PCR and
sequencing, we identified six Ca2+
pumps in lactating mammary tissue. Three plasma membrane
Ca2+-ATPases (PMCAs)
were found (PMCA1b, PMCA2b, and PMCA4b). Two sarco (endo)plasmic
reticulum Ca2+-ATPases (SERCAs)
were identified (SERCA2 and SERCA3), and the rat homologue to the yeast
Golgi Ca2+-ATPase RS-10 was also
found. The pattern of mRNA expression of each of these pumps was
examined in rat mammary tissue from the 7th day of pregnancy to the
21st day of lactation. Northern blots revealed increased mRNA
expression for all Ca2+ pumps by
the 14th day of lactation, and transcripts continued to increase
through the 18th day of lactation. PMCA1b, PMCA4b, SERCA2, and SERCA3
showed the lowest levels of expression. RS-10 transcripts were more
abundant than SERCA2, SERCA3, PMCA1b, and PMCA4b. RS-10 was the only
pump to increase in expression before parturition. PMCA2b was the most
abundant transcript found in lactating mammary tissue. At peak
lactation, expression of PMCA2b approached that of actin. The high
expression, high affinity for Ca2+, and high activity at low
calmodulin concentrations exhibited by PMCA2b suggest that it is
uniquely suited for maintenance of Ca2+ homeostasis in the lactating
mammary gland. The pattern of expression and abundance of RS-10 suggest
that it is a candidate for the Golgi
Ca2+-ATPase shown to be important
in maintaining the Golgi Ca2+
concentration required for casein synthesis and micelle formation.
breast; calcium pumps; lactation; plasma membrane calcium-adenosinetriphosphatase; sarco(endo)plasmic reticulum calcium-adenosinetriphosphatase
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