Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 18

Hypothalamic Regulation of Hormonal Functions 277

OTHER FACTORS REGULATING

WATER INTAKE

A number of other well-established factors contribute to the
regulation of water intake. Psychologic and social factors are
important. Dryness of the pharyngeal mucous membrane
causes a sensation of thirst. Patients in whom fluid intake
must be restricted sometimes get appreciable relief of thirst by
sucking ice chips or a wet cloth.
Dehydrated dogs, cats, camels, and some other animals rapidly
drink just enough water to make up their water deficit. They stop
drinking before the water is absorbed (while their plasma is still
hypertonic), so some kind of pharyngeal gastrointestinal “meter-
ing” must be involved. Some evidence suggests that humans have
a similar metering ability, though it is not well developed.

CONTROL OF POSTERIOR

PITUITARY SECRETION

VASOPRESSIN & OXYTOCIN

In most mammals, the hormones secreted by the posterior pi-
tuitary gland are
arginine vasopressin (AVP)
and
oxytocin.
In hippopotami and most pigs, arginine in the vasopressin
molecule is replaced by lysine to form
lysine vasopressin.
The
posterior pituitaries of some species of pigs and marsupials
contain a mixture of arginine and lysine vasopressin. The pos-
terior lobe hormones are nonapeptides with a disulfide ring at
one end (Figure 18–5).

BIOSYNTHESIS, INTRANEURONAL

TRANSPORT, & SECRETION

The hormones of the posterior pituitary gland are synthesized
in the cell bodies of the magnocellular neurons in the supraoptic

and paraventricular nuclei and transported down the axons of

these neurons to their endings in the posterior lobe, where they

are secreted in response to electrical activity in the endings.

Some of the neurons make oxytocin and others make vaso-

pressin, and oxytocin-containing and vasopressin-containing

cells are found in both nuclei.

Oxytocin and vasopressin are typical

neural hormones,

that is, hormones secreted into the circulation by nerve cells.

This type of neural regulation is compared with other types in

Figure 18–6. The term

neurosecretion

was originally coined

to describe the secretion of hormones by neurons, but the

term is somewhat misleading because it appears that all neu-

rons secrete chemical messengers (see Chapter 7).

Like other peptide hormones, the posterior lobe hormones

are synthesized as part of larger precursor molecules. Vaso-

pressin and oxytocin each have a characteristic

neurophysin

associated with them in the granules in the neurons that

secrete them—neurophysin I in the case of oxytocin and neu-

rophysin II in the case of vasopressin. The neurophysins were

originally thought to be binding polypeptides, but it now

appears that they are simply parts of the precursor molecules.

The precursor for arginine vasopressin,

prepropressophysin,

contains a 19-amino-acid residue leader sequence followed by

FIGURE 18–5

Arginine vasopressin and oxytocin.

Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH 2

1 2 3 4 5 6 7 8 9

SS

Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH 2

1 2 3 4 5 6 7 8 9

SS

Arginine vasopressin

Oxytocin

FIGURE 18–6
Neural control mechanisms.
In the two situations on the left, neurotransmitters act at nerve endings on muscle; in the two
in the middle, neurotransmitters regulate the secretion of endocrine glands; and in the two on the right, neurons secrete hormones into the hy-
pophysial portal or general circulation.

Acetylcholine

Acetylcholine Acetylcholine Acetylcholine

Vasopressin

General

circulation

Norepinephrine

Epinephrine,

norepinephrine

ACTH, TSH,

GH, FSH, LH,

prolactin

Norepinephrine

or

acetylcholine

Releasing

and

inhibiting

hormones

Portal

vessels

Motor nerves

to skeletal

muscle

Motor nerves

to smooth and

cardiac muscle

Juxta-

glomerular

cells

Adrenal

medulla

Anterior

pituitary

Posterior

pituitary

Renin