326
Chapter 11
Inputs that Control
Hormone Secretion
Many hormones are secreted in short bursts, with little or
no release occurring between bursts. Therefore, plasma con-
centrations of hormones may fl uctuate rapidly over brief time
periods. Hormones may also undergo 24-hour cyclical varia-
tions in their secretory rates, with different circadian patterns
for different hormones. Some are clearly linked to sleep; for
example, the secretion of growth hormone increases during
the early period of sleep and decreases during the rest of the
night and day. The mechanisms underlying these cycles are
traceable to cyclical variations in the activity of neural path-
ways involved in the hormone’s release.
Hormone secretion is mainly under the control of three
types of inputs to endocrine cells (
Figure 11–10
): (1) changes
in the plasma concentrations of mineral ions or organic nutri-
ents; (2) neurotransmitters released from neurons impinging
on the endocrine cell; and (3) another hormone (or, in some
cases, a paracrine/autocrine agent) acting on the endocrine
cell. There is actually a fourth type of input—chemical and
physical factors in the lumen of the gastrointestinal tract—but
it applies mainly to the hormones secreted by the gastrointes-
tinal tract and will be described in Chapter 15.
Before we look more closely at each category, we must
stress that more than one input may infl uence hormone secre-
tion. For example, insulin secretion is controlled by the extra-
cellular concentrations of glucose and other nutrients, by both
sympathetic and parasympathetic neurons to the insulin-
secreting endocrine cells, and by several hormones acting on
these cells. Thus, endocrine cells, like neurons, may be sub-
ject to multiple, simultaneous, often opposing inputs, and the
resulting output—the rate of hormone secretion—refl ects the
integration of all these inputs.
The term
secretion
applied to a hormone denotes its release
by exocytosis from the cell. In some cases, hormones such as
steroid hormones are not secreted
per se
, but instead diffuse
through the cell’s plasma membrane into the extracellular
space. Secretion or release by diffusion is sometimes accom-
panied by increased synthesis of the hormone. For simplicity
in this chapter and the rest of the book, we will generally not
distinguish between these possibilities when we refer to stim-
ulation or inhibition of hormone “secretion.”
Control by Plasma Concentrations of Mineral
Ions or Organic Nutrients
There are multiple hormones whose secretion is directly con-
trolled, at least in part, by the plasma concentrations of specifi c
mineral ions or organic nutrients. In each case, a major func-
tion of the hormone is to regulate, through negative feedback,
the plasma concentration of the ion or nutrient controlling
its secretion. For example, insulin secretion is stimulated by
an increase in plasma glucose concentration. Insulin, in turn,
acts on skeletal muscle and adipose tissue to promote facili-
tated diffusion of glucose across the plasma membranes into
the cytosol. The effect of insulin, therefore, is to decrease the
plasma glucose concentration (
Figure 11–11
). Another example
is the regulation of Ca
2+
homeostasis by parathyroid hormone
(PTH), as described in detail in Section F. This hormone is pro-
duced by cells of the parathyroid glands, which, as their name
implies, are located in close proximity to the thyroid gland.
When plasma Ca
2+
concentration decreases, PTH secretion is
directly stimulated. PTH exerts several actions, in coordination
with other hormones, that restore plasma Ca
2+
to normal.
Control by Neurons
As stated earlier, the adrenal medulla is a modifi ed sympa-
thetic ganglion and thus is stimulated by sympathetic pregan-
glionic fi bers. In addition to controlling the adrenal medulla,
the autonomic nervous system infl uences other endocrine glands
(
Figure 11–12
). Both parasympathetic and sympathetic
inputs to these other glands may occur, some inhibitory and
some stimulatory. Examples are the secretion of insulin and
the gastrointestinal hormones, which are stimulated by neu-
rons of the parasympathetic nervous system and inhibited by
sympathetic neurons.
Endocrine cell
Alters rate of hormone secretion
Ions or nutrients
Neurotransmitters
Hormones
Figure 11–11
Example of how the direct control of hormone secretion by the
plasma concentration of a substance, in this case by an organic
nutrient, results in negative-feedback control of the substance’s
plasma concentration. In other cases, the regulated plasma substance
may be a mineral, such as Ca
2+
.
Figure 11–10
Inputs that act directly on endocrine gland cells to stimulate or
inhibit hormone secretion.
Insulin's target cells
Actions of insulin
(transport of glucose from
plasma to intracellular space)
Insulin-secreting cells
Insulin secretion
Plasma glucose concentration
Plasma insulin concentration
previous page 354 Vander's Human Physiology The Mechanisms of Body Function read online next page 356 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off