Page 273 - Anatomy and Physiology of Farm Animals, 8th Edition

258 / Anatomy and Physiology of Farm Animals

Table 13-1. Continued
VetBooks.ir Endocrine Gland Hormone Action (Target Tissue or Organ)
Calcitonin
Promotes calcium retention (bone)
Thyroid parafollicular cells
Parathyroid PTH Promotes increase in plasma calcium,
reduction in plasma phosphate (bone,
kidney)
Pancreatic islets: β‐cells Insulin Promotes glucose uptake; protein, lipid
synthesis by various tissues, organs,
including skeletal muscle, liver, adipose
tissue
Pancreatic islets: α‐cells Glucagon Promotes glycogenolysis, gluconeogenesis
(liver)
Liver IGF Promote growth in immature animals;
negative feedback on GH release
ACTH, corticotrophin; ADH, antidiuretic hormone, also called vasopressin or arginine vasopressin; CRH,
corticotropin‐releasing hormone; FSH, follicle‐stimulating hormone; GH, growth hormone; GHIH, growth
hormone‐inhibiting hormone; GHRH, growth hormone‐releasing hormone; GnRH, gonadotropin‐releasing
hormone; IGF, insulin‐like growth factor; LH, luteinizing hormone; PRL, prolactin; PTH, parathyroid hormone;
TRH, thyrotrophin‐releasing hormone; TSH, thyrotrophin.

membrane receptor is the first step in a second messenger is a general term for
series of events that brings about changes the intracellular compounds, such as
in the target cell. The subsequent events, cAMP, that function as an intermediate
which vary with the participating peptide in the sequence of steps leading to the
hormones, may include changing the biologic response. Two other common
permeability of membrane channels, second messengers involved in the cellu­
stimulating or inhibiting the activity of lar response to peptide hormones are
membrane‐bound enzymes, stimulating or diacylglycerol (DAG) and inositol
inhibiting the activity of intracellular 1,4,5‐triphosphate (IP ). These second
3
enzymes, the release of vesicle‐bound hor­ messengers are formed by the action of a
mone into the circulation, and initiating membrane‐bound enzyme, phospholi­
gene transcription. In contrast, steroid pase C, on phospholipids in the cell
hormones binding to classical steroid hor­ membrane.
mone receptors inside the cell are only able Ionic calcium (Ca ) concentration in
2+
to initiate gene transcription. the cytosol is normally lower than that in
When enzymatic activity is increased typical extracellular fluid, but some cells
by a hormone‐receptor interaction at the accumulate even higher concentrations of
cell surface, the intracellular concentra­ Ca within their endoplasmic reticulum.
2+
2+
tion of the product of the action of that Increases in cytosolic Ca concentrations
enzyme increases. For example, many may result from the entry of extracellular
peptide hormones acting through G pro­ Ca through membrane channels or by
2+
teins affect the enzyme adenylyl cyclase, Ca release from the endoplasmic reticu­
2+
which increases the intracellular produc­ lum into the cytosol through channels
tion of cyclic AMP (cAMP) by its action within the membrane of the endoplasmic
on adenosine triphosphate (ATP). The reticulum. Increases in intracellular free
2+
cAMP activates other intracellular Ca above the typical low levels may also
enzymes that ultimately bring about the act as a second messenger for secretion of
characteristic biologic response to the certain vesicle bound hormones from the
2+
hormone (e.g., cellular secretion, cellular cell. The increase in Ca begins a series of
contraction, protein synthesis). The term events that ultimately results in changes in