Calcium incorporation by smooth muscle microsomes.

The purpose of the present work was to study the factors influencing calcium incorporation into a microsomal fraction prepared from the longitudinal smooth muscle of the guinea-pig ileum. Calcium incorporation required the presence of both ATP and Mg2+ and was unaffected by azide. It was enhanced by oxalate; this effect was pH dependent and it was maximal at pH 6.6. The relation between calcium uptake with oxalate and free Ca2+ concentration in the medium was represented by a curve with an optimum for Ca2+ equal to 3-10-5 M. The threshold concentration was comprised between 5-10-7 and 10-6 7. The optimum calcium uptake rate was 4.5 nmol Ca2+/mg protein per min. In the absence of oxalate, two distinct groups of binding sites were identified. Low affinity sites had a binding constant of 7-104 M-1 and a maximum binding capacity of 0.6-106 M-1 and a binding capacity of 33 nmol Ca2+/mg protein; their capacity was sensitive to pH changes. In the absence of oxalate, Ca2+ binding was depressed by Na+ with respect to K+ or choline. When the medium was supplemented with oxalate, the stimulation of 45Ca incorporation was barely detectable in the presence of choline+ and it was lower in a medium containing Na+ instead of K+. The subcellular distribution profiles of calcium incorporation with and without oxalate indicate the microsomal location of both activities. However, the oxalate-stimulated calcium uptake activity sedimented faster than the calcium binding activity. The subcellular distribution of marker enzyme actvities has been examined. The present results indicate that Ca2+ incorporations with and without oxalate are the result of two processes likely related to two different structures. The role of microsomal calcium uptake in excitation-contraction coupling and its modification by the activity of the sodium pump is discussed.