Mechanisms of Hormone Actions

Abstract

Compared with other metabolic active compounds, hormones circulate in extremely low concentrations in the body fluids. Blood levels of peptide hormones normally range from 10⁻¹⁰−10⁻¹² M, whereas glucose circulates at the concentration of 10⁻² M. These low hormone levels, therefore, require special mechanisms for recognition by target cells and for their metabolic activities. Marked organ specificity for actions of hormones are well recognized; for example, the hormone adrenocorticotropin (ACTH) acts on adrenal cortical cells specifically. The product of adrenocortical secretion, Cortisol, on the other hand, has several specific targets with special mechanisms to mediate the biologic activity of the hormone. The hormones regulate growth differentiation and metabolic activities of their target tissues. While they do not take part in energy production, hormones are intimately involved in regulation of that process. Actions of most hormones are mediated by two general mechanisms, either through involvement of membrane adenyl cyclase (Sutherland, 1972) or through participation of specific cytosol and receptor molecules (Means and O’Malley, 1972). The detailed mechanisms of actions of insulin and growth hormone actions are not completely clear. Most of the peptide and glycoprotein hormones initiate their actions by activating membrane adenyl cyclase, whereas the steroid hormone actions are mediated by specific cytosol and nuclear receptor proteins. These mechanisms will be summarized below.