[Calcium components in vascular smooth muscle (author's transl)]

Using Scatchard plots, high and low affinity CA binding sites were detected on the surface membrane of vascular smooth muscle. Superficial low affinity bound Ca was rapidly lost in Ca-depleted solution and K-induced contraction appears to depend upon this Ca source. A part of this low affinity bound Ca exchanged slowly and was rapidly released by norepinephrine. Superficial high affinity bound Ca was not readily lost in Ca-depleted solution and was removed by La and EGTA, suggesting that the sustained contraction induced by norepinephrine may be supported by this Ca component. The La-inaccessible (cellular) Ca was also separated into high and low affinity bound Ca. Cellular low affinity bound Ca increased in high-K solution but not by norepinephrine. In depolarized muscle, however, norepinephrine increased the cellular low affinity bound Ca. Inhibitors of mitochondrial respiration inhibited Ca binding to the cellular low affinity binding site but not the K-induced contraction, suggesting that this Ca component represents mitochondrial Ca uptake. Cellular high affinity binding site took up Ca slowly and the bound Ca was released rapidly by norepinephrine. This source of Ca may be utilized for norepinephrine-induced phasic contraction.