Oxalate transfer across the membranes of sarcoplasmic reticulum during the uptake of Ca++ |
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Authors: | V M Madeira |
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Affiliation: | Centro de Biologica Cellular, Departmento de Zoologia, 3049 Coimbra Codex, Portugal |
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Abstract: | The kinetic profile of Ca++ uptake in the presence of oxalate is biphasic. An initial phase independent on oxalate is followed by an oxalate-dependent phase delayed in time. The ionophore X-537A only abolishes the net Ca++ uptake if added before the onset of the oxalate phase. However, during this phase, X-537A suddenly releases an amount of Ca++ similar in quantity to that released in the initial phase. The delay of the oxalate-dependent phase is a function of pH. At pH of about 5.5, the oxalate phase and simultaneous calcium oxalate precipitation would theoretically start at the beginning, with no delay. Ejection of protons during Ca++ uptake is strongly depressed by oxalate, but not by other organic anions which do not trap Ca++. It is suggested that oxalate is transferred to the inside of the vesicles as a monoprotonated species at expense of protons ejected by the Ca++-pump during the uptake of Ca++. |
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