Some aspects of calcium uptake by human myometrial mitochondria and microsomes relevant to relaxation

Abstract

Calcium uptake by mitochondria and microsomes isolated from the human myometrium was studied at physiological Ca++ concentrations. The initial rates as well as the maximum velocity of Ca uptake by mitochondria were 10-20 times higher than those by microsomes. The Ca++ concentration for half-maximal transport in the mitochondria and microsomes was about 1 microM and 0.5 microM, respectively. The Ca uptake capacity of mitochondria measured after 20 min of uptake (1 microM Ca++ in the medium) was 10-30 times higher than that of microsomes. The capacity but not the initial rates of Ca uptake by microsomes was increased in the presence of 5 mM oxalate. These were only minor differences in the Ca uptake kinetics of subcellular fraction isolated from the pregnant and non-pregnant myometria. The results of this study reinforce the argument for the domineering role of mitochondria in the relaxation of the human myometrium.