Calcium distribution and exchange in the rat uterus |
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| Authors: | C Van Breemen E E Daniel D Van Breemen |
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| Affiliation: | From the Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada. Dr. and Mrs. van Breemen’s present address is the Department of General Physiology, University of Leicester, Leicester, England |
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| Abstract: | ![]()
The calcium content and distribution of the rat uterus were determined employing flamephotometry and Ca45 determinations. The total uterine calcium concentration wasfound to be 2.25 millimoles (mmoles) per kilogram wet weight, 0.45 of which wasinexchangeable. The exchangeable Ca could be divided into 0.8 mmole/kg wet weightextracellular and 1.0 mmole/kg wet weight intracellular. The concentration of ionic Ca inrat serum was obtained by equilibrium dialysis as 1.5 mM or 53 % of the totalserum Ca. The observed Ca distribution required that its active transport be postulated,since the membrane was shown to be permeable to Ca and the internal Ca concentration wasfar below its electrochemical equilibrium value. Metabolic inhibition by iodoacetate ordinitrophenol caused a net Ca uptake, but cooling to 4°C and ouabain did not.Iodoacetate did not affect the Ca45 efflux, but did increase the influx,suggesting that active Ca transport is accomplished by an exclusion mechanism. Inexperiments with varied external sodium concentrations, no evidence was obtained thatsodium competes with calcium for inward transport. Cellular Ca binding was measured underconditions of prolonged metabolic inhibition, which abolished both active transport andthe membrane potential. The association constants obtained were compatible withintracellular Ca binding to proteins, but insufficient to account for the low level ofintracellular ionic Ca believed essential for relaxation. Hence metabolically dependentintracellular Ca binding was postulated. The Ca45 efflux was slowed down byCa-free efflux media. The presence of Sr or EDTA could completely prevent this decrease inefflux rate, and Ba could partly prevent it. Changes in Mg and Na concentration did notaffect the rate of Ca45 efflux. A model to explain Ca exchange across smoothmuscle membranes has been proposed. |
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