SOMATOMEDINS

An intact pituitary gland capable of secreting growth hormone has long been considered the prime requirement for the achievement of skeletal growth potential in man. Recent studies have revealed that the growth-promoting action of growth hormone is an in-vivo phenomenon which cannot be mimicked by the addition of the hormone to skeletal tissue in vitro. The humoral agent responsible for skeletal growth has now been identified as somatomedin, a peptide produced in the liver under the stimulus of pituitary growth hormone. Serum levels of somatomedin are measured in a bioassay system by monitoring the stimulation of uptake of labelled sulphate by cartilage. Low levels of somatomedin activity are detected in the serum of children with growth hormone deficiency and short stature; the levels are high in acromegalics and low in patients with cirrhosis of the liver or chronic renal failure. Undernourished children also have low levels despite reaised serum levels of growth hormone; this suggests the presence of an inhibitor which lowers the growth-promoting activity of the somatomedin molecule. Adequate nutrition in these children results in the restoration of serum somatomedin levels to normal. Attempts to isolate and purify somatomedin have led to the identification of a group of substances sharing similar actions on skeletal tissue. Insulin has also been demonstrated to share some of these growth-promoting activities but varies in its organ specificity. Nerve growth factor, epidermal growth factor and proinsulin are other molecules which form a large group of growth promoting peptides which may all be related to the somatomedins.