Glucose Catabolism in Fetal and Adult Heart

Abstract

The cells of growing and developing organisms are generally characterized by a higher rate of protein synthesis than is found in the cells of mature forms, and it has been shown repeatedly (1) that increases in rate of protein synthesis are associated with higher RNA content.It might then be expected that enaymes necessary for ribose synthesis would be in greater supply in rapidly proliferating or secretory tissues, and that metabolic pathways leading toward ribose should be in greater evidence.Recent work (2-5) has shown two pathways of ribose synthesis, both of which represent portions of the pentose cycle.These pathways are quantitatively more significant in proliferating and secretory tissues than others (6-10).Cardiac muscle has been consistently found to utilize the Embden-Meyerhof mechanism almost exclusively in catabolism of glucose.This seems reasonable since cardiac muscle is low in RNA (11).In fetal heart tissue, however, with a higher rate of protein synthesis than found in adult cardiac tissue, a higher RNA content and pentose cycle activity might be anticipated.The experiments reported here indicate that this is true.EXPERIMENTAL Preparation of Homogenates-Pork hearts, obtained immediately after slaughtering of the animals, were immersed in icecold physiological saline until used.A portion of ventricular tissue, freed from fat and connective tissue, was finely minced with a razor and homogenized in four volumes of a medium similar to that of Wenner et al. (12); i.e. 0.0124 M KH2POI, 0.99 per cent KC1 and 0.001 M Versene (ethylenediaminetetraacetic acid, Dow Chemical Company), adjusted to pH 7.4.In some experiments a medium was used containing 0.2 M Tris,l 0.99 per cent KCl, and 0.001 M Versene, adjusted to pH 7.4.All solutions were made in glass-distilled water.The homogenization was carried out with a precooled, handoperated Dounce homogenizer (13).In order to obtain the least possible destruction of cell components, only three or four strokes