18 Mechanisms Controlling Heart Growth in Mammals

Abstract

The heart is a four-chambered organ (two ventricles and two atria) responsible for circulating blood throughout the body. Growth of the mammalian heart, for the most part, is regulated by functional demand. The heart is composed of a number of cell types, including cardiac muscle cells (cardiac myocytes), extracellular matrix, interstitial cells such as fibroblasts, and blood vessels. During postnatal life, growth of the heart occurs primarily as a result of an increase in the size of ventricular cardiac myocytes, known as hypertrophy. Even though ventricular cardiac myocytes make up only one-third of the total cell number, they account for ~70–80% of the heart’s mass (Rakusan 1984). For this reason, the mechanisms controlling the growth of ventricular myocytes form the main focus of this review. Growth of cardiac myocytes requires the induction of a number of events, including changes at the level of gene expression, an increase in the overall rate of protein synthesis, and organization of contractile proteins into sarcomeric units.

CARDIAC MYOCYTES
Cardiac myocytes are specialized muscle cells composed of bundles of myofibrils that contain myofilaments. The myofibrils have distinct, repeating, micro-anatomical units, called sarcomeres (Fig. 1). The sarcomere is the basic contractile unit of the heart. Approximately 50 sarcomeres end to end make up a myofibril, and a bundle of 50–100 myofibrils makes up a muscle cell. The sarcomere is defined as the region of the myofilament structures between two Z discs. The sarcomere consists of a number of major and minor proteins organized into...