Introduction

In the 1920s, it was discovered that a crude pituitary extract stimulated growth in rats, and this extract was referred to as somatotropin or growth hormone, after the Greek derivation meaning tissue growth.[1] Results were extended to farm animals when somatotropin (ST) was shown to enhance growth rates in pigs[2] and stimulate milk production in lactating goats[3] and cows.[4] However, the supply of ST was extremely limited until the advent of recombinant technology. In 1982, the first study with recombinant ST in domestic animals was reported, in this case bovine ST (bST) in lactating dairy cows.[5] Commercial use of bST and porcine ST (pST) began in 1994 and 1996 in the United States and Australia, respectively.

Somatotropin is a protein hormone secreted from the anterior pituitary gland under the action of growth hormone-releasing factor (stimulatory) and somatostatin (inhibitory) released from the hypothalamus. Somatotro-pin can vary slightly in size, generally about 191 amino acids, and the amino acid sequence also varies among species. Whereas bST and pST share a high degree of sequence similarity (ca. 90%), both have a much lower homology with human ST (ca. 65%) and hence are inactive in humans. Since ST is a protein, it is digested if consumed orally, and exogenous ST has to be administered via injection or implant.

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