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P Mermier

Publications and source records attributed to P Mermier.

7 recordsLinked to original sources
PubMed1976

Comparison between strontium and calcium uptake by the fragmented sarcoplasmic reticulum.

The ATP-supported uptake of strontium by the fragmented sarcoplasmic reticulum is monophasic and proceeds more rapidly than the fast uptake of calcium. Strontium uptake is not activated by Pi. The accumulation of strontium is nearly proportional to the external strontium concentration even in the millimolar range. Internal and external strontium quickly equilibrate. One mole of strontium is stored for every mole of ATP split by the Sr2+-activated ATPase. In the absence of oxalate most of the strontium is taken up with a transport ratio of one. On the opposite, the transport ratio of calcium decreases immediately, especially when ADP is not instantaneously phosphorylated to ATP. In this case, energy conversion is uncoupled more effectively by the simultaneous action of ADP and free internal calcium, resulting in the interruption of the fast uptake. After depletion of ATP most of the stored strontium is released and the remaining fraction appears to be not exchangeable. Strontium activates the slow uptake of calcium, but reduces the amplitude of the fast uptake. The calcium induced release of strontium, and vice versa, is partial and transient. The strontium activated ATPase does not transport calcium at low ionic calcium concentrations.

Adenosine Triphosphatases↗
PubMed1976

The biphasic active transport of calcium by the fragmented sarcoplasmic reticulum as revealed by the flow dialysis method.

The calcium accumulated by the fast uptake has an apparent association constant of 0.8 X 10(6) M-1 and a maximum of 80 nmol/mg protein. The fast uptake and the initial rate of the slow uptake show a similar dependence on the calcium concentration when the latter ranges from 5 to 50 muM. The fast uptake is a linear and the slow uptake rate an exponential function of the reticulum concentration. Both uptakes of calcium display a fast and nearly total isotopic equilibration between intra- and extravesicular calcium. After depletion of ATP the calcium accumulated during the slow uptake is released, while that sequestered during the fast uptake is retained by the vesicles, though it remains rapidly exchangeable. After depletion of ITPor acetylphosphate, or addition of EGTA, the release is more substantial, but is almost complete only after addition of ionophore X537A or deoxycholate. The presence of oxalate strongly reduces the rates of these releases. It is concluded that in the steady state observed after the depletion of ATP, a Ca2+ gradient exists through the sarcoplasmic membrane, and the sarcoplasmic pump works at equilibrium. The fast uptake is an active transport and not an active binding. The slow uptake represents an extension of the calcium capacity of the vesicles due to the phosphate liberated by the sarcoplasmic ATPase.

Adenosine Triphosphate↗
PubMedDate not supplied

The biphasic Ca2+-uptake by the fragmented sarcoplasmic reticulum.

The non-equilibrium dialysis has been used for kinetic studies of ATP dependent calcium uptake by the sarcoplasmic reticulum. The uptake displays two phases, which are defined as fast and slow uptake. The former is an exponential function of time, with a half-life time of approximately 15--20 sec, the latter presents the characteristics of an autocatalytic reaction. The fast uptake is characterized by its amplitude, the slow uptake by its rate. Compared with the fast uptake, the slow uptake requires higher amounts of Mg2+ or ATP, and is more sensitive to pH variations and aging. The reasons which formerly prevented the resolution of the second phase from the first are discussed. It is concluded that the fast uptake is not a simple binding reaction, and that the slow uptake is more sensitive to changes by the vesicular membrane.

Adenosine Triphosphatases↗
PubMedDate not supplied

The effect of calcium and phosphate on the biphasic calcium uptake by the sarcoplasmic reticulum.

The amplitude of the fast uptake and the initial rate of the slow uptake increase with increasing free calcium concentrations, up to 30 muM. In that range, both processes are correlated to each other. At higher concentrations, the slow uptake is more inhibited than the fast uptake. The fast uptake shows a maximum amplitude which remains unchanged in the presence of phosphate. The slow uptake leads to a nearly complete depletion of the external calcium, and its rate is proportional to the phosphate concentration, even at physiological range. The sarcoplasmic ATPase liberates inorganic phosphate and the slow uptake is an autocatalytic process.

Adenosine Triphosphatases↗