In Vitro Work Load and Rat Heart Metabolism: II. Effect on amino acid transport

In vitro pressure overload was found to accelerate protein synthesis in the isolated working rat heart (Hjalmarson and Isaksson 1972 a). Since membrane transport of amino acids is considered to be one rate-limiting step in protein synthesis, the amino acid transport in the isolated rat heart was investigated using the non-utilizable amino acids α-aminoisobutyric acid (AIB) and 1-aminocyclopentane carboxylic acid (cycloleucine). Increased pressure load (afterload) accelerated amino acid uptake after a perfusion period of 15 ruin, and a 50—100% increase in the intracellular to extracellular distribution ratio of the amino acids was seen after 60 min of perfusion. This accelerated uptake was not inhibited by cycloheximide, suggesting that the work load effect was not dependent upon a continuous synthesis of proteins. The accumulation rate continued to be stimulated for some time after normalization of the work load and coronary flow, indicating that the work load effect was not directly linked to coronary flow. Increased preload did not stimulate amino acid uptake. Hearts from hypophysectomized rats showed a decreased concentrative uptake but the amino acid uptake was still accelerated by pressure overload. It is suggested that an increased uptake of amino acids could be of physiological significance in relation to the increased protein synthesis under these conditions.