Calcium transport by skeletal muscle sarcoplasmic reticulum in the hypothyroid rat.

The rate of calcium transport by isolated sarcoplasmic reticulum from rat skeletal muscle increases markedly during the first 4 wk of life and thereafter remains relatively constant. When animals are made hypothyroid during the first 3 wk of life, there is a marked inhibition of the increase in calcium transport by the sarcoplasmic reticulum. Production of hypothyroidism after 4 wk of age, at which time the calcium transport by sarcoplasmic reticulum has reached maximum levels, results in a depression in the rate of calcium transport. There is no clear alteration in ATPase activity of the sarcoplasmic reticulum to account for the low calcium transport in hypothyroidism. It is proposed that the decrease in calcium transport by sarcoplasmic reticulum may account for observed alterations in the intrinsic contractile properties of muscle in the hypothyroid animal.