Release of protein hormones

In general, secretion of hormones does not occur at a constant rate. In some cases, secretion is pulsatile, occurring in short bursts, as for many pituitary hormones. The biochemical basis of this pulsatile release is not fully understood, but the release pattern may have profound effects on hormone function, i.e., the pulsatile administration of luteinizing hormone releasing hormone (LHRH) stimulates the release of LH by the pituitary, whereas the constant infusion of the same amount of hormone per unit time has the opposite effect. 

After secretion, the hormones circulate in blood for periods ranging from minutes (insulin) to a few hours (glycoprotein hormones). The protein hormones, being water-soluble, circulate in free form and are not bound to specific proteins. Their concentrations in blood are very low, the resting concentrations being 1CT11 M to 10 10 M. Under stimulated conditions the concentrations of peptide hormones in blood may rise 5- to 100-fold. 

The time course for internalization varies, depending on the hormone. Uptake of insulin, EGF and NGF is rapid (2-30 min) [67-69], whereas that of gonadotropins is much slower [63]. 

Internalization of protein hormones is a principal means of hormone degradation in lysosomes through a receptor-mediated pathway [70], There may also be membrane-associated degradative activities for insulin (insulinase) and other protein hormones, however for EGF, degradation occurs exclusively through an intracellular process [68,71]. 

Primarily, internalization is a means of removing excess, extracellular hormones by an active degradative pathway in lysosomes, thus terminating the hormone signal. Some other mechanisms of clearance of protein hormones are the following. 

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