Abstract
Mitochondria obtained from the myocardia of hypothyroid rats, as compared with those from normal euthyroid rats, show the following abnormalities: a three-times greater activation energy of energy-linked respiration (State 3 and 4, but not uncoupled respiration) at temperatures between 37<sup>o</sup> and 21°C, with absolute rates lower only below about 34°C; and phospholipids that contain less oleic acid (18:1) and more linoleic acid (18:2), with the ratio of arachidonic: linoleic acid decreased. Total unsaturation is unchanged. Injecting a hypothyroid rat once with 0.25 μg of l-tri-iodothyronine per g corrects the thermal responses of respiration and the 18:1 content progressively over 3 days, but introduces new abnormalities: a decrease in respiratory control at 32°C, and in the fatty acid unsaturation index. However, one injection of l-thyroxone, 0.5 μg.g<sup>−1</sup>, almost completely corrects temperature responses in 3 days without changing 18:1 content, opening the possibility that other mitochondrial components besides fatty acids are involved.
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