Submitted to: American Journal of Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 1998
Publication Date: N/A
Interpretive Summary: Most metabolic and infectious diseases in dairy cows are problems whose genesis starts with the stresses of the initiation of lactation. Calcium flows rapidly into the mammary gland, prior to the start of lactation, into as yet unidentified storage sites. The rapidity of this calcium flux contributes to milk fever and the complications that result from milk fever rand subclinical hypocalcemia (low calcium levels). Fine control of mammar milk cell calcium also is key to the health, hormone responsiveness, and the quality of milk produced by the secretory cell. The well-being of the cow and her profitability could be greatly enhanced by understanding those factors that regulate the mammary glands' transition to milk production and the concomitant excretion of calcium. This paper represents the first in a series of studies examining factors controlling calcium flux and storage in the mammary at and around calving. We identified 6 separate calcium pumps in the milk-producing mammary gland. We next measured the amount of each pump prior to, at the start, and during milk production. All pumps increased through this period. Two pumps stand out as candidates for controlling most of the calcium movement in the mammary gland and thus the hypocalcemia that exacerbates disease at this time. Their scientific names are plasma membrane calcium ATPase 2b and the secretory pathway calcium ATPase. Further studies on their function and roles, which will ultimately benefit the dairy industry and producers, are underway.
Technical Abstract: 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 calcium homeostasis. Using RT-PCR and sequencing, we identified 6 calcium pumps in lactating mammary tissue. Three plasma membrane Ca2+-ATPases were found (PMCA1b, 2b and 4b). Two sacoplasmic/endoplasmic reticulum Ca2+-ATPases were identified (SERCA2 and 3) and the rat homologue to the yeast Golgi Ca2+-ATPase RS-10 was found. The patterns of mRNA expression of each of these pumps was examined in rat mammary tissue from rats 7-d pregnant to 21-d lactating. Northern blots revealed increased mRNA expression for all calcium pumps by d 14 of lactation and transcripts continued to increase through d 18 of lactation. PMCA1b and 4b and SERCA2 and 3 showed the lowest level of expression. RS-10 transcripts were more abundant than SERCA2 and 3 and PMCA1b and 4b. RS-10 was the only pump to increase expression prior to parturition. PMCA2b was the most abundant transcript found in lactating mammary tissue. At peak lactation, PMCA2b's expression approached that of actin. PMCA2b's high expression, high affinity for calcium, and high activity at low calmodulin concentrations suggest that PMCA2b is uniquely suited for maintenance of calcium homeostasis in the lactating mammary gland. RS-10's pattern of expression and abundance suggest that RS-10 is a candidate for the Golgi CA2+-ATPase shown to be important in maintaining Golgi CA2+ concentration required for casein synthesis and micelle formation.