Metabolic effects of l-thyroxine and l-triiodothyronine on the intact rat and selected excised tissues

Abstract

Time-response curves for the oxygen consumption of the intact rat and excised tissues were determined after single equimolar injections of l-thyroxine or l-triiodothyronine. In the whole animal, injection of either material resulted in significant increases in BMR within 12 hours. Thyroxine treatment of the intact rat caused an increased oxygen utilization until the sixth day, while the triiodothyronine-treated group began declining after the 4th day. Tissue oxygen consumption was determined in kidney, liver, heart, gastric mucosa, skeletal muscle, diaphragm and duodenal smooth muscle during the same time periods as used in the study of the whole animal. The oxygen consumption of all tissues except smooth muscle showed a significant increase above the control value. The earliest response to either hormone occurred 12 hours after injection in liver, kidney and cardiac muscle and all reactive tissues reached their maximal values in 3 days with the exception of skeletal muscle. All tissues from thyroxine-injected animals maintained a near maximum oxygen consumption through the 9th day except for cardiac muscle; all tissues from triiodothyronine-injected animals (except skeletal muscle) showed significant decreases by the 7th day.