Effects of ATP on calcium binding to synaptic plasma membrane.
The release of labeled norepinephrine from preloaded synaptosomes requires the presence of potassium and calcium. ATP-dependent binding of calcium to synaptic plasma membranes (SPM) may provide a means of maintaining the cation in a readily available pool for the triggering of transmitter release. A high Ca-binding capacity was demonstrated in SPM. The Km for calcium is 5.5 X 10(-5) M. The dependence of the system on the gamma phosphate of ATP was demonstrated by an increase in Ca-binding with increasing ATP concentration and by competitive inhibition of binding by ADP and AMP. Magnesium is also required for ATP-dependent Ca-binding. The optimum pH for the Ca binding was 7.0. Pretreatment of SPM with phospholipase A2 lowered the binding capacity. Sulfhydryl groups are also critical for ATP-dependent Ca binding to occur. A model for ATP-dependent Ca-binding was proposed.
D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D009638:Norepinephrine / Q000378:metabolism; D012453:Saimiri; D013570:Synaptic Membranes / Q000187:drug effects / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism
D002132:Calcium Radioisotopes; D015636:Magnesium Chloride; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium; D009638:Norepinephrine