Page 274 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 259

intracellular enzymatic activity and a bio­ production, and this brings about the
biologic response.
logic response.
VetBooks.ir peptide hormones is rapid and relatively roid hormones typically develop more
In many cases the biologic response to
Biologic responses to steroid and thy­
quickly reversed. For example, the action slowly but last longer than responses to
of antidiuretic hormone on cells in the kid­ peptide hormones. In part, this is because
ney to change their permeability to water all effects are related to changes in protein
can occur within a matter of minutes. Such synthesis. The synthesis of new protein or
rapid effects are possible because pathways the degradation of protein already present
leading to the biologic effects may only requires more time than the activation or
require activation of enzymes (proteins) inactivation of enzymes already present.
that are already in the cell. When the hor­
mone is removed or degraded, the effects
are reversed by the inactivation of these Negative and Positive Feedback
enzymes. Regulation
In some cases, the biologic response to
peptide hormones is longer lasting because Assuming that adequate numbers of func­
the intracellular pathways lead to an tioning receptors are available, the biologic
increase in DNA transcription and the for­ effect of any hormone is directly propor­
mation of messenger RNA (mRNA). In tional to the concentration of the hormone
these cases, the effect is prolonged by the in the body fluids available to bind to the
presence of newly synthesized intracellular receptors. This concentration is primarily
proteins. The general term for agents that determined by two factors: the rate of hor­
directly influence DNA transcription is mone release from endocrine cells and the
transcription factor. So peptide hormones rate of elimination from the body fluids. In
may bring about changes in transcription normal conditions, the concentration is
factors within cells and therefore modify typically determined by the rate of release.
gene transcription. Steroid hormones are not stored and must
be synthesized just prior to release. Peptide
hormones and amines are stored within
Cellular Effects of Steroid endocrine cells readily available for release
and Thyroid Hormones in membrane bound vesicles, or secretory
granules. Docking of these vesicles to the
Receptors that bind steroid and thyroid plasma membrane for release of the aggre­
hormones are found either in the cytosol gated hormones requires an increase in
or in the nucleus of the cell. Steroid and calcium within the cell for exocytosis of the
thyroid hormones are lipid soluble vesicle contents.
(hydrophobic). Often these hormones are The release, and thus concentration, of
accompanied by a carrier protein to make most hormones in the peripheral circula­
them more hydrophilic in the extracellu­ tion is controlled by some type of negative
lar environment, but once they reach the feedback regulation. In this type of regu­
target cell they may leave their carrier lation, the rising levels of the hormone
protein behind and bind to intracellular bring about a biologic response that inhib­
receptors after diffusing into the cell its further hormone release. Much like the
through the cell membrane. The receptor, thermostat on an air conditioner, once the
with its bound hormone, acts as a tran­ desired biological response has been
scription factor for specific genes within achieved (cooling), the thermostat detects
the DNA to increase or decrease the for­ that change and turns off the air condi­
mation of mRNA that corresponds to the tioner and the production of cooler air. For
specific gene. Ultimately, the change in example, β‐cells in the pancreatic islets are
mRNA results in a change in protein directly affected by the concentration of