[The role of sarcolemma and mitochondria in calcium-dependent control of myometrium relaxation].

Using atomic absorption spectroscopy, it was shown that the amount of firmly bound Ca2+ in cattle mitochondria and myometrium sarcolemma is 160 +/- 10 and 30 +/- 10 mumol/kg of wet tissue, respectively. The Ca2+ 1 accumulating capacity of mitochondria (350 nmol per mg of protein) markedly exceeds that of sarcolemmal vesicles (30 nmol per mg of protein). Using a Ca2+-EGTA buffer, it was found that the affinity of ionized Ca for the mitochondrial transport system (Km = 5.69 microM) is higher than that for the Na+-Ca2+ system of sarcolemma exchange (Km = 30 microM), but is markedly lower than that for the Mg2+, ATP-dependent Ca2+ efflux (Km = 0.35 microM). A kinetic analysis demonstrated that the sarcolemmal Ca2+ pump is incapable of causing complete relaxation of the smooth muscle within the physiologically significant time, whereas the Ca2+ transport system of mitochondria evokes this process within 21 s. However, the contribution of the Ca2+ pump to the regulation of the Ca2+ content in myocytes is paralleled with the accumulation of Ca2+ in mitochondria and is realized at low concentrations of this cation in the myoplasm, i.e., at late steps of relaxation. A mechanism of Ca2+ control over myometrium relaxation is proposed. The system of non-electrogenic Na+-Ca2+ exchange maintains Ca2+ concentration in the myoplasm as high as 10(-5) M. Mitochondria which accumulate the bulk of Ca2+ rapidly decrease its concentration in the cytoplasm down to 10(-6)-10(-7) M; at these values, the activity of the sarcolemmal Ca2+ pump with a high affinity for the transfer substrate is manifested. In this way, the Ca2+ pump accomplishes fine regulation of Ca2+ concentration in the myocytes.