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

Abstract

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. Oxalate increased ATP-dependent Ca²⁺ uptake by the sarcolemmal but not the mitochondrial fraction, while azide inhibited mitochondrial but not sarcolemmal Ca²⁺ transport. The amount of Ca²⁺ bound in 10 min by these two fractions from a 0.3 μM free Ca²⁺ solution was similar, but the rate of Ca²⁺ uptake at 1 μM 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. The results suggest that mitochondria have a greater potential for lowering intracellular Ca²⁺ activity in human myometrium than do sarcolemma or sarcoplasmic reticulum.