Protein Turnover in Muscle Cells as Visualized by Autoradiography

Abstract

Quantitative autoradiography in light and electron microscopy has made it possible to investigate slow-turnover proteins in the striated skeletal muscle cell, the myocardial cell, and the smooth muscle cell in young and adult animals sacrificed at short or long intervals after the injection of tritiated leucine or proline. The processes of synthesis for proteins that are to remain inside the cell are similar in the striated and smooth muscle cells. They play a large role in the elaboration of the contractile apparatus. The relative concentration of the label increases in similar fashions in the polysome-rich sarcoplasm and the myofilaments within 30 minutes after injection, thereby confirming that myofibrillar protein biosynthesis occurs rapidly. The smooth muscle cell, especially in the aorta, show that it is capable of synthesizing extracellular connective tissue proteins despite predominant, sedentary protein synthesis up until four hours after injection of tritiated leucine. Hence, this variety of smooth muscle cell, in which the granular endoplasmic reticulum and Golgi apparatus are very active, does exhibit excretory activity.