13 Hormonal regulation of metabolism
Metabolism is regulated at different organizational levels. With regulatory mechanisms that operate entirely within the individual cell, the main purpose is housekeeping, that is to adjust the throughput of catabolic pathways such as to keep up a ready supply of ATP, NADPH and so on. This is accomplished largely by feedback inhibition and feedforward activation by key metabolites such as ATP and acetyl-CoA. In contrast, hormonal control is about the obligations of a cell to the organism as a whole. Hormones are released in response to the prevailing metabolic situation of the body. Key examples are the secretion of insulin in response to high blood glucose levels, and the secretion of glucagon in response to low glucose levels. Hormones may also be secreted in anticipation of an imminent change of the metabolic situation, as is the case with the 'fight-and-flight' hormones epinephrine and norepinephrine. Other hormones with major effects on energy metabolism are the glucocorticoids (cortisone and hydrocortisone) and the thyroid hormones (tri- and tetraiodothyronine).