Calcium fluxes in mouse mammary tissue in vitro: intracellular and extracellular calcium pools

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Abstract
The total Ca content of the mammary gland increased from .apprx. 2 to 12 .mu.mol/g tissue during the transition from pregnancy to lactation in the mouse. In tissue from lactating mice at least 2/3 of the total Ca exchanged with external Ca in 6 h. There was little non-exchangeable Ca in tissues from pregnant mice. At 37.degree. C the time courses of influx and efflux of 45Ca in lactating tissues could be analyzed by assuming 3 exponential components with rate constants of .apprx. 0.3, 0.06 and 0.005 min-1 and containing, respectively, 1.7, 1.5 and 4.7 .mu.mol 45Ca/g tissue at the steady state. The rapidly effluxing component showed the time- and temperature-dependence characteristic of bulk-phase-limited diffusion through the extracellular space. The diffusion coefficient was .apprx. 1/4 of the self-diffusion coefficient of Ca in aqueous solution, consistent with a tortuosity factor of .apprx. 2. A portion of the Ca in this component was displaced by La3+. The amount remaining in the presence of 3 mM La3+ was close to that expected for free extracellular Ca. The rapid component was therefore interpreted as originating from an extracellular compartment containing both free and bound Ca. The rate of efflux of the intermediate component was slowed by a factor of 10 when the temperature was decreased from 37 to 0.degree. C giving a Q10 of 2.7, expected for membrane transport. The slow component present at 37.degree. C was not displaced by EGTA [ethylene glycol bis(.beta.-aminoethyl ether) N,N,N'',N''-tetraacetate] or La3+, suggesting that it is not localized extracellularly. It was not apparent in the O.degree. C efflux curves. The biphasic time course of uptake of ionophore (A23187 [calcimycin])-releasable 45Ca in particulate fractions obtained by homogenization and centrifugation of tissues which had been incubated with the isotope was consistent with the hypothesis that the 2 slower components of 45Ca flux originate from intracellular compartments. Mitochondrial uptake probably did not contribute significantly to Ca exchange in these tissues. 45Ca fluxes in mammary tissues from pregnant mice also showed 3 components with rate constants similar to those found in tissues from lactating mice. The amount of Ca in each component was much smaller than in lactating tissue when compared on the basis of tissue weight. Intra- and extracellular Ca pools in mammary tissue can be distinguished on the basis of the temperature dependence of their fluxes. The transition from pregnancy to lactation is accompanied by large increases in both intra- and extracellular Ca pools in mammary alveolar cells.