An in vitro study of the interaction of heart mitochondria with troponin-bound Ca2+

Anaerobic rat heart: mitochondrial role in calcium uptake and contractility

In order to evaluate the manner by which fumarate enhances contractility in the anaerobic heart, we examined Ca++ movements in isolated heart mitochondria and in the isolated perfused heart. Our experiments showed that in isolated antimycin A plus cyanide treated mitochondria: (a) Ca++ uptake was promoted by electron transport generated by fumarate-dependent oxidation of NADH, (b) Ca++ stimulated fumarate-dependent oxidation of NADH, (c) the ratio of Ca++ uptake:NADH oxidized was 1.7 and (d) the Ca++ sequestered is transiently highly mobile and is rapidly released upon collapse of the membrane potential. In anaerobic hearts perfused with glucose plus fumarate, malate and glutamate Ca++ levels were the same as those in oxygenated hearts while in anaerobic organs perfused with or without glucose Ca++ content was appreciably lower. Succinate production in anaerobic heart perfusions was related to: (a) an increased retention of Ca++ by the heart, (b) a diminution in peak aortic pressure generated by cardiac contractions and (c) an increase in heart rate. The information obtained indicates that mitochondria have a capability for Ca++ movement which be used physiologically, particularly in fumarate perfused anaerobic hearts, to assist the mechanism for contraction and relaxation of the heart.

Cellular distribution of sarcoplasmic calcium-binding proteins by immunofluorescence

Specific antibodies against carp paravalbumin, crayfish calcium binding protein and crayfish arginine kinase were used for indirect immunofluorescence localization of the respective proteins. Simultaneous staining of the same muscle sections with human serum containing anti-actin autoantibodies served as a probe to identify the isotropic band. Parvalbumin appears to be evenly distributed in carp white muscle. The crayfish calcium binding protein however shows a distinct localization, in the isotropic band, coincident with the actin staining. Arginine kinase, which has the same molecular weight and is extractible in the same way as the calcium binding protein does not show this distinct localization, but is evenly present in crayfish tail muscle, similarly to parvalbumin. The possible meaning of the different distribution of the two calcium binding proteins is discussed.

Calcium regulation in heart cells. The interaction of mitochondrial and sarcoplasmic reticulum with troponin-bound calcium

A study has been carried out of the interaction of dog heart mitochondria and sarcoplasmic reticulum with Ca2+ bound to troponin or to its Ca2+-binding component (troponin-C). Both organelles are able to release more than 80% of the Ca2+ bound to the two proteins. However, sarcoplasmic reticulum is only able to do so in the presence of oxalate, whereas mitochondria are active also in the absence of permeant Ca-complexing anions. On the other hand, calcium uptake in heart mitochondria is severely inhibited by Mg2+. The results are discussed with reference to the problem of the control of Ca2+ in heart sarcoplasm.

The effect of lead on the calcium-handling capacity of rat heart mitochondria

1. Very low concentrations of Pb2+ decrease the capacity of rat heart mitochondria, oxidizing pyruvate plus malate, to remove Ca2+ from the medium. 2. The primary effect is on the rate of Ca2+ sequestration; this is reflected in the overall extent of Ca2+ removal. 3. Pb2+ has at least two separate actions. Below about 0.5 nmol/mg of protein, it acts solely by competing with Ca2+ (Ki = 0.4 muM); above this concentration it also inhibits the production or use of respiratory energy, so that at 1 nmol of Pb2+/mg of protein, Ca2+ removal is almost completely abolished. 4. Pb2+ inhibits coupled and uncoupled respiratory O2 use by mitochondria oxidizing pyruvate plus malate, but at higher concentrations than those that affect Ca2+ removal; similar concentrations of Pb2+ inhibit pyruvate uptake, but not malate uptake, by the mitochondria. 5. Mg2+ only decreases Ca2+ removal by competition, and is a far-less effective competitor than Pb2+ (Ki = 0.15 mM). It is possible that the primary cause of the second effect of Pb2+ is displacement of membrane Mg2+. 6. The consequences of these results are discussed in terms of the possible involvement of heart mitochondria in excitation-contraction coupling, and the Pb2+ levels that might occur in heart tissue in vivo.

Fluorescence of oxidized flavoproteins from perifused isolated pancreatic islets

In perifused pancreatic islets, the fluorescence of oxidized flavoproteins (FAD) was recorded continuously. Elevation of glucose concentration in the medium form 0 or 5 mM to 20 mM led to decrease in FAD-fluorescence beginning 10 sec after change of medium. L-leucine (10 mM), (+/-)-B-BCH (20 mM) and alpha-ketoisocaproic acid (10 mM) caused typical kinetics of FAD-fluorescence decrease. The results are interpreted to indicate rapid changes of the functional state of B-cell mitochondria induced by the above-mentioned stimulators of insulin release.