A comparative study of the calcium accumulation by mitochondria and microsomes isolated from the smooth muscle of the guinea-pig taenia coli.

The calcium uptake by mitochondria and microsomes isolated from the guinea-pig taenia coli was studied at physiological Ca2+ concentrations, buffered by Ca-EGTA mixtures. The Ca accumulation by the mitochondria was measured from the difference between the amount of Ca taken up in the presence and in the absence of a specific mitochondrial inhibitor. The Ca uptake by the microsomes was determined in a solution containing oxalate and a mitochondrial inhibitor. It was calculated from the difference in Ca uptake measured with and without ATP. By using this procedure, the necessity of extensive purification of the isolated fractions was avoided. The (Ca2+) for half-maximal transport in the mitochondria is 7 X 10(-6) M. At (Ca2+) lower than 2 X 10(-7) M, Ca is taken up in an energy-dependent way. In the microsomes the apparent Km for Ca is 7 X 10(-7) m. accumulation is still stimulated by ATP at a (Ca2+) as low as 4 X 10(-8) M. The results show that the rate of Ca uptake by the cell organelles corresponding to the microsomal vesicles is sufficiently fast to explain the speed of relaxation of the taenia coli. The results also suggest that these cell organelles are more important than the mitochondria in regulating the cytoplasmic Ca concentration.