The effects of ST are directly dependent on the number of ST receptors as well as the activity of the ST receptor second messenger system (16). For example, greater ST receptor amounts or greater function of the ST receptor second messenger system leads to greater concentrations of blood IGF-I (59, 79) (Figure 1). In a variety of species (including farm animals, humans, and laboratory animals) higher concentrations of blood IGF-I are found in young, well nourished, healthy individuals (56, 99). Animals that are old, diseased, or malnourished have low blood IGF-I concentrations that reflect a compromised state of tissue, organ, and cell function (56, 69, 76, 99). The changes in blood IGF-I can be directly linked to changes in ST receptor expression or ST receptor second messenger systems in liver (Figure1).