Rat hearts pulse-labeled by perfusion in vitro with 9,10-oleic acid-3H for 15 or 30 sec were shown to take up the fatty acid extensively. In hearts postperfused with unlabeled medium for 15 sec or more, 90% of the radioactivity was recovered in esterified lipids. The radioautographic reaction was localized initially over elements of the sarcoplasmic reticulum and mitochondria. After longer periods of postperfusion (2–20 min), there was concentration of silver grains over lipid droplets. In mitochondria and sarcoplasmic reticulum isolated from hearts postperfused for 1 min or more, most of the esterified lipid was in the form of triglyceride. The ratio of the specific activity of isolated sarcoplasmic reticulum triglyceride to mitochondrial triglyceride changed from a value of 3.2 to 1.3 during 5 min of postperfusion. Under conditions of hypothermia, considerable uptake of free fatty acid occurred. The radioactivity recovered in the heart was mostly in the form of free fatty acid, and the radioautographic reaction was seen over sarcoplasmic reticulum and mitochondria, but not over lipid droplets or myofibrils. The results are interpreted to show that intracellular transport of free fatty acid, which occurs also when esterification is repressed, proceeds through intracellular channels, i.e. the sarcoplasmic reticulum. Esterification of fatty acid into triglycerides occurs mostly in the sarcoplasmic reticulum, especially in the region of the dyad, in the vicinity of which lipid is stored in the form of droplets.

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