Role of sarcolemma and mitochondria in calcium-dependent control of myometrial relaxation

It was shown by the method of atomic absorption spectrophotometry that the content of tightly bound calcium in bovine myometrial mitochondria and sarcolemma is equal to 160 +/- 10 and 30 +/- 10 μmoles/kg of wet tissue, respectively. The Ca2+-accumulating capacity of the mitochondria (350 nmoles/mg protein) greatly exceeds that of the sarcolemmal vesicles (30 nmoles/mg protein). It was demonstrated using Ca2+-EGTA buffer that the affinity of ionized calcium for the mitochondrial transport system (K/sub m/ = 5.69 μM) is greater than that for the sarcolemmal Na+-Ca2+ exchange system of the sarcolemma (K/sub m/ = 30 μM), but less than that for the Mg2+, ATP-dependent Ca2+-transporting system (K/sub m/ = 0.35 μM). A kinetic analysis indicated that the calcium pump of the sarcolemma is incapable of providing relaxation of smooth-muscle tissue within a physiologically significant time, whereas the mitochondrial Ca2+-transporting system is capable of this (within 21 sec). However, the contribution of the calcium pump to the regulation of the Ca2+ concentration in myocytes is manifested against a background of Ca2+ accumulation by the mitochondria and is realized at low Ca2+ concentrations in the myoplasm, i.e., at a late stage of relaxation. A mechanism of the calcium-dependent control of myometrial relaxation is proposed