Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: I. MgATP-dependent and MgATP-independent calcium uptake.

The components of 45calcium (Ca) uptake were studied in saponin skinned rat caudal artery. The steady-state Ca content increased when the free Ca concentration was varied from 10(-8) to 10(-4) M but was reduced by azide when the free Ca concentration exceeded 3.1 microM. The azide sensitivity and low affinity for Ca were consistent with functional mitochondria. The azide-insensitive component consisted of a small bound and a larger releasable Ca fraction. After skinning in Triton X-100, approximately 4 mumol Ca/kg wet tissue remained, which represented a tightly bound but slowly exchangeable Ca pool. The Ca content was independent of the free Ca concentration and MgATP, and it was not released with A-23187 or Ca. The Ca content of the larger fraction was a higher order function of the free Ca concentration and was released with A-23187, indicating it resided within a membrane-bounded structure. Ca uptake by the releasable fraction was increased by oxalate, MgATP, phosphocreatine, temperature, phosphate, and ruthenium red and represents Ca sequestered by the sarcoplasmic reticulum (SR) with little contribution from other Ca binding or storage sites. It is described by the coefficients Umax = 96.94 mumol/kg wet tissue, K1/2 = 0.75 microM, and Hill coefficient = 1.70. The SR in this preparation regulates cytosolic Ca concentrations under physiological conditions and can accumulate Ca by MgATP-dependent and MgATP-independent process. The larger, MgATP-dependent Ca uptake is described by the coefficients Umax = 72.87 mumol/kg wet tissue, K1/2 = 0.8 microM, and Hill coefficient = 2.09 and is consistent with Ca sequestered by the Ca-transport ATPase of smooth muscle SR. The smaller, MgATP-independent uptake is described by the coefficients Umax = 24.14 mumol/kg wet tissue, K1/2 = 0.56 microM, and Hill coefficient = 1.01 and represents Ca sequestered by an unidentified mechanism or by a subpopulation of SR.