The role of sarcolemma and mitochondria in regulating Ca2+ movements in human myometrium

A subcellular fraction enriched in sarcolemma was isolated by differential and sucrose gradient centrifugation from human myometrium. The Ca2+ binding characteristics of this fraction were compared to that of mitochondria and microsomes that were isolated simultaneously. 1. Oxalate increased ATP-dependent Ca2+ uptake by the sarcolemmal but not the mitochondrial fraction, while azide inhibited mitochondrial but not sarcolemmanl Ca2+ transport. 2. The amount of Ca2+ bound in 10 min by these two fractions from a 0.3 muM free Ca2+ solution was similar, but the rate of Ca2+ uptake at 1 muM Ca2+ was greater for the mitochondrial than the sarcolemmal. fraction. The properties of Ca2+ binding by the microsomal fraction were intermediate between these two fractions. 3. The results suggest that mitochondria have a greater potential for lowering intracellular Ca2+ activity in human myometrium than do sarcolemma or sarcoplasmic reticulum.