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

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.