Spontaneously beating myocardial fragments prepared by mechanical disaggregation have hyperpermeable sarcolemmae. Such preparations were used to study mitochondrial function in situ. The myocardial fragments suspended in a phosphate-buffered salt solution containing 1-3 mM MgCl2 showed a low rate of oxygen uptake. Addition of succinate, pyruvate plus malate or glutamate was followed by an increase in the rate of O2 uptake. Addition of ADP to fragments engaged in State 4 respiration was followed by initiation of more rapid State 3 respiration, with respiratory control ratios routinely greater than 3 for succinate and glutamate. If the fragments were suspended in the same medium containing 3 mM ATP (a medium in which contractile activity occurs), State 3 was initiated upon addition of substrate. The suspension medium used in these experiments contained about 8 muM calcium as contamination. Addition of calcium chloride to give a final concentration of 0.14 to 0.57 mM stimulated State 4 respiration of the myocardial fragments. In contrast similar additions made during State 3 inhibited respiration. The maximum degree of inhibition brought respiration close to the State 4 rate. If calcium was added prior to ADP, respiratory stimulation by the nucleotide was diminished. Respiratory function of myocardial fragments and of mitochondria isolated from them was similar in terms of response to substrate, ADP, and calcium addition in State 4. Response to calcium in State 3 was different in that inhibition was long-lived only at low [Pi] in the case of mitochondria, but at low or high [Pi] in the case of the fragments.
Animals Calcium