The concentration of a hormone in the plasma is also influenced by the rate at which it is metabolized (inactivated) and/or excreted. Inactivation can take place at or near the receptor, or in the liver, the major site for hormone metabolism. In addition, the kidneys can metabolize a variety of hormones, or excrete them in their free (active) form. In fact, the rate of excretion of a hormone in the urine has been used as an indicator of its rate of secretion during exercise (12, 38, 69, 70). Since blood flow to the kidneys and liver decreases during exercise, the rate at which hormones are inac-tivated or excreted decreases. This results in an eleva-tion of the plasma level of the hormone over and above that due to higher rates of secretion.
The concentration of some hor-mones is influenced by the quantity of transport pro_ tein in the plasma. Steroid hormones and thyroxine are transported bound to plasma proteins. For a hormone to exert its effect on a cell, it must be “free” to interact with the receptor and not “bound” to the transport pro-tein. The amount of free hormone is dependent on the quantity of transport protein and the capacity and affinity of the protein to bind the hormone molecules. Capacity refers to the maximal quantity of hormone that can be bound to the transport protein, and affinity refers to the tendency of the transport protein to bind to the hormone. An increase in the quantity. capacity. or affinity of transport protein would reduce the amount of free hormone and its effect on tissue (48, 81). For example, high levels of estrogen during pregnancy increase the quantity of thyroxine’s transport protein, causing a reduction in free thyroxine. The thyroid gland produces more thyroxine to counteract this effect.
Changes in plasma volume will change the hormone concentration independent of changes in the rate of secretion or inactivation of the hormone. During exercise, plasma volume decreases due to the movement of water out of the cardiovascu-lar system. This causes a small increase in the concen-tration of hormones in the plasma. By measuring-changes in plasma volume, it is possible to “correct” the concentration of the hormone to obtain a more accu-rate assessment of endocrine gland activity (81).
■ Endocrine glands release hormones directly into the blood to alter the activity of tissues possess-ing receptors to which the hormone can bind.
■ The free plasma hormone concentration deter-mines the magnitude of the effect at the tissue level.
■ The free hormone concentration can be changed by altering the rate of secretion or inactivation of the hormone, the quantity of transport protein, and the plasma volume.