Mini MCAT passage: Effects of human growth hormone

Also called somatotropin, human growth hormone (hGH) belongs to a class of hormones called somatotropes. hGH is comprised of 191 amino acids compiled into a single chain. The hypothalamic release of growth-hormone-releasing factor prompts growth hormone secretion. hGH stimulates body growth, increases secretion of IGF-1, stimulates breakdown of lipids, and opposes insulin action on carbohydrate and lipid metabolism. Generally, hGH causes growth in almost every tissue in the body capable of growing.

In an experiment set up to elucidate the effect of hGH, ten rats of the same litter were selected. Five rats received daily injections of hGH, while the other five rats served as a control. Accelerated growth, evidenced by increased body weight, was seen throughout the lifespan of the experimental population (Figure 1). Measurements of body mass index (BMI) revealed that in early stages tissue growth accelerated uniformly. However, after reaching the age of sexual maturity, growth in soft tissue continued, but marked decline in growth of dense bone was evident. This phenomenon can be explained by the closure of the epiphyseal plates of long bones.

Figure 1 Average body weight of rats with (purple) or without (green) daily injections of growth hormone as a function of time in days.