A CALMODULIN-STIMULATED CA-2+ PUMP IN RAT AORTA PLASMA-MEMBRANES

An ATP-driven Ca2+-transport system was characterized in a microsomal fraction from rat aorta. Calmodulin enhanced 2.5-fold 45Ca accumulation by EGTA[ethylene glycol bis(.beta.-aminoethyl ether) N,N,N'',N''-tetraacetic acid]-treated microsomes incubated with 10 .mu.M Ca2+ (in the absence of oxalate) by increasing markedly the apparent affinity of the transport system for Ca2+. The ionophore A23187 [calcimycin] induced a rapid release of the sequestered 45Ca. The vesicles that took up 45Ca were distributed like plasmalemmal marker enzymes when the microsomal fraction was subfractionated by density gradient centrifugation. These vesicles were markedly shifted towards higher equilibrium densities after addition to the microsomes of 0.25 mg digitonin/mg protein before isopycnic centrifugation. The calmodulin-stimulated Ca2+ pump associated with the microsomal fraction apparently is located in plasmalemmal elements.