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Biomedical Engineering Fundamentals
effector activation and thus unable to transduce a meaningful signal to the cell. However, the difficulty with
these studies has been to distinguish receptor-activation and signal-generation defects from hormone-
binding and effector activation defects. A parallel technique is to explore the use of agonists, which are
molecules that behave like the “normal” hormone, bind the receptor and induce all the postreceptor events
that yield a physiological effect. Natural hormones are themselves agonists and, in many cases, more than
one distinct hormone can bind to the same receptor. For a given receptor, different agonists can have
dramatically different potencies. The corollary to this is to explore the use of antagonists, that is molecules
that bind the receptor and block-binding of the agonist, but fail to trigger intracellular signaling events.
Hormone antagonists have obvious pharmaceutical potential in clinical endocrinology.
The peptide and protein hormones circulate at very low concentrations relative to other proteins in
the blood plasma. These low concentrations are reflected in the very high affinity and specificity of the
receptor sites, which permit recognition of the relevant hormones amid the prolusion of protein molecules
in the circulation. Adaptation to a high concentration of a signal ligand in a time-dependent reversible
manner enables cells to respond to changes in the concentration of a ligand instead of to its absolute
concentration. The number of receptors in a cell is not constant; synthesis of receptors may be induced
or repressed by other hormones or even by their own hormones. Adaptation can occur in several ways.
Ligand binding can inactivate a cell surface receptor either by inducing its internalization and degradation
or by causing the receptor to adopt an inactive conformation. Alternatively, it may result from the changes
in one of the nonreceptor proteins involved in signal transduction following receptor activation. Down
regulation is the name given to the process whereby a cell decreases the number of receptors in response
to intense or frequent stimulation and can occur by degradation or more temporarily by phosphorylation
and sequestration. Up regulation is the process of increasing receptor expression either by other hormones
or in response to altered stimulation.
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