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Biomedical subjects

A Weber

Publications and source records attributed to A Weber.

At least 703 records · Page 39Linked to original sources
PubMed1971

Regulatory mechanisms of the calcium transport system of fragmented rabbit sarcoplasmic rticulum. I. The effect of accumulated calcium on transport and adenosine triphosphate hydrolysis.

The rate of ATP hydrolysis decreases very rapidly during the first 2 sec of calcium uptake. It changes with time in a manner similar to that described for calcium net uptake by other workers, suggesting that the two activities are coupled. The decline in both rates may be ascribed to an inhibitory effect of accumulated calcium on calcium influx and ATPase activity for the following reasons. During the steady state, Ca-Ca and Sr-Ca exchange and the rate of ATP hydrolysis are much slower than the initial rate of net calcium uptake and the associated ATP hydrolysis. If the accumulation of free calcium is prevented by calcium-oxalate precipitation the initial rate of net calcium uptake does not decay during prolonged periods of transport. Furthermore, passive preloading of vesicles with calcium inhibits the rate of hydrolysis in proportion to the extent of preloading. The inhibition of steady-state flux is alleviated by free ATP; i.e., not chelated with magnesium, but not by free ITP.

Adenosine Triphosphate↗
PubMed1971

Regulatory mechanisms ofthe calcium transport system of ramented rabbit sarcoplasmic rticulum. II. Inhibition of outflux in calcium-free media.

The outflux of calcium from vesicles of sarcoplasmic reticulum is controlled by the concentration of ionized calcium in the medium. In a calcium-free medium calcium outflux is at a minimum provided ATP and Mg are present. If they are removed calcium outflux becomes rapid and has a rate constant similar to that of outflux during steady-state (40)Ca-(45)Ca exchange with saturating calcium concentrations in the medium. In order to maintain the low permeability state of the membrane only the presence but not the continued hydrolysis of ATP is necessary.

Adenosine Triphosphate↗
PubMed1969

Parallel response of myofibrillar contraction and relaxation to four different nucleoside triphophates.

The Mg chelates of ITP, GTP, and UTP in addition to that of ATP were shown to be capable of causing complete relaxation of myofibrils as indicated by the complete inhibition of syneresis and the reduction of the NTPase activity to that of isolated myosin. For ITP and GTP the required concentrations were about 100 times higher than those for ATP, whereas UTP was maximally effective also in low concentrations (0.2 mM). For all NTP's the concentrations for relaxation were related to those necessary for contraction so that NTP concentrations which gave 80-90% maximal NTPase activity in the presence of Ca caused complete relaxation in the absence of Ca. Thus, these experiments do not support the view that relaxation is caused by the binding of NTP to a special inhibitory site but are quite compatible with the idea that relaxation depends on the extent to which the hydrolytic site is saturated with NTP. The Ca concentration required for contraction depends on the nature of the NTP base and its concentration; it is lower for ITP than for ATP and decreases with decreasing concentrations of ITP.

Adenosine Triphosphatases↗