Chapter 8.7

Endocrinal Functions of Other Organs and Local Hormones

In addition to the main endocrinal glands described in the previous chapters, the other organs which have endocrinal functions are heart, kidney, pineal gland, thymus and others.

Hormones of the heart

The heart also acts as an endocrine organ. The hormones secreted by heart include are given below.

Atrial natriuretic peptide (ANP). It was the first natriuretic hormone isolated from the heart.

Brain natriuretic peptide (BNP). It was the second natriuretic hormone, first isolated from the porcine brain and hence named as BNP. In humans, it is present in the heart.

C-type natriuretic peptide (CNP). It was the third natriuretic hormone to be isolated in sequence and so named C-type natriuretic peptide.

Actions

1. Increase in sodium excretion by kidneys. ANP and BNP increase excretion of sodium ion in urine by:

• Increasing glomerular filtration and

• Inhibiting Na⁺ reabsorption at the level of renal tubules.

2. ANP lowers the blood pressure by:

• Increasing the capillary permeability leading to extravasation of fluid and a decline in blood pressure.

• Relaxing vascular smooth muscle (VSM) in arterioles and venules.

Hormones of the kidney

The kidneys secrete three hormones:

• Renin,

• 1,25-dihydroxycholecalciferol (see page 393) and

• Erythropoietin.

Renin

Renin is a glycoprotein with a molecular weight of 37,326 in humans secreted by the granular cells of juxtaglomerular apparatus of the kidneys into the blood stream.

Actions. The only action of active renin is to convert angiotensinogen (renin substrate) into angiotensin-I. For further details about renin–angiotensin system (see page 187).

Erythropoietin

Erythropoietin is glycoprotein. In adults it is mainly (85%) secreted by the juxtaglomerular apparatus of the kidneys with some contribution (15%) from the perivenous hepatocytes in the liver.

Actions. The main role of erythropoietin is to stimulate the bone marrow and cause erythropoiesis (for details see page 77).

Pineal gland

Functional anatomy

Pineal gland, also known as epiphysis, is a small structure (5 mm × 7 mm) shaped like a pine cone. It is situated in the groove between the two superior colliculi in diencephalic area of brain above the hypothalamus (Fig.8.7-1).

Fig. 8.7-1 Location of pineal body in the groove between the two superior colliculi.

Structure. The pineal stroma has two types of cells: neuroglial and parenchymal. The parenchymal cells are large epithelial cells having secretory function.

• In infants, the pineal gland is large and the cells tend to be arranged in alveoli.

• In adults, the pineal gland gets calcified, i.e. small concretions of calcium phosphate and carbonate (pineal sand) appear in the tissue.

Melatonin

The hormone melatonin is synthesized by the parenchymal cells of the pineal gland.

Functions of melatonin are:

1. Role in circadian rhythm of the body. Melatonin secretion shows diurnal variation. It is secreted more during dark period of the day than during the day light hours. This correlates with various internal activities in different periods of the day, i.e. circadian rhythm.

2. Effects on the gonads. Melatonin exerts both inhibitory and facilitatory effects on the gonads due to the diurnal change in the melatonin secretion. It inhibits onset of puberty.

3. Effect on MSH and ACTH secretion. An inhibitory effect of melatonin on MSH and ACTH secretion has been reported.

4. Sleep. Melatonin has a role in induction of sleep.

Thymus

Functional anatomy

Thymus is a small lymphoid structure located in the lower part of neck in front of the trachea, below the thyroid gland. At birth, it is small (weighing 10–12 g), gradually enlarges till puberty and then it starts decreasing in size in old age.

Histologically, thymus consists of the inner medulla and outer cortex.

• Medulla. It comprises reticular epithelial cells, a few lymphocytes and concentric corpuscles of Hassall.

• Cortex. It includes actively multiplying, closely packed lymphocytes and contains no Hassall's corpuscles.

Functions

Thymus has two functions:

• Immunological functions and

• Endocrinal functions.

1. Immunological functions of thymus

(i) Development of immunologically competent Tlymphocytes is an essential function of the thymus (see page 99).

(ii) Maintenance of adequate pool of T-lymphocyte. The hormone thymosin produced by the thymus also stimulates lymphopoiesis in the peripheral lymphoid tissue and thus plays a role in maintenance of an adequate pool of T-lymphocytes in adult life.

2. Endocrine function of thymus. Thymus tissue secretes two hormones, thymosin and thymin.

(i) Thymosin. It is a peptide, which, as described above, promotes proliferation of T-lymphocytes in the thymus and peripheral lymphoid tissue.

(ii) Thymin, also called thymopoietin, inhibits acetylcholine release at motor nerve endings and thus suppresses neuromuscular activity.

Local hormones

The local hormones are the substances which are produced in many tissues, and when activated in certain circumstances, execute their actions in the same area or in immediate neighbourhood. Commonly produced local hormones are discussed below.

Prostaglandins and related substances

Prostaglandins (PGs) and related substances include thromboxanes, prostacyclin, leukotriene and lipoxin. These substances are called eicosanoids, reflecting their origin from arachidonic acid, linoleic and linolenic acid.

Prostaglandins were so named by Von Euler in 1937 because they were first isolated from prostatic secretion in semen. However, now they are known to be synthesized in almost all tissues of the body. Presently, a variety of PGs are identified.

Actions of prostaglandins. Prostaglandins have multitudinous and varied actions on almost all tissues of the body. Many of them are discussed in the chapters on the systems in which they play an important role. Some important actions of PGs are:

1. Actions on cardiovascular system. Peripheral arteriolar dilatation, especially in splanchnic and muscular bed.

2. Actions on kidneys. PGA₂ increases renal cortical blood flow and increases urinary excretion of sodium, potassium and water.

3. Actions on female reproductive system

• Initiate labour by stimulating contraction of gravid uterus.

• Responsible for painful uterine contractions during menstruation (dysmenorrhoea).

• Promote secretion of hypothalamic gonadotropin releasing hormone (GnRH).

4. Role of PGs in inflammation. Prostaglandins mediate following effects of inflammation:

• Increase vascular permeability and cellular infiltration.

• Pain-producing effect of bradykinin by sensitizing cutaneous nerves.

5. Actions on blood platelets

• PGE₁ inhibits platelet aggregation.

6. Action on bronchial musculature

• PGE_2α causes contraction of bronchial smooth muscles and may precipitate bronchial asthma.

7. Actions on GIT

• Prostaglandins: Inhibit the secretion of gastric HCl.

• Cause inhibition of sodium and water absorption producing profuse watery, cholera-like diarrhoea.

• Increase intestinal motility.

8. Actions on nervous system

• On CNS, the PGs function as transmitters or modulators of neuron activity.

Actions of thromboxane A₂. Thromboxane A₂ is synthesized by platelets. It promotes:

• Vasoconstriction and

• Platelet aggregation.

Actions of prostacyclin. Prostacyclin is produced in vascular endothelium. It produces vasodilatation.

Actions of leukotrienes. Leukotrienes are mediators of allergic responses and inflammation. Their release is provoked when specific allergens combine with IgE antibodies on the surfaces of mast cells.

Actions of lipoxins. Physiological role of lipoxins is uncertain. Their actions include:

• Dilatation of microvasculature (by lipoxin A), and

• Inhibition of cytotoxic effects of natural killer cells (by lipoxin A and lipoxin B).

Other local hormones synthesized in tissues

In addition to PGs and related substances, the other local hormones synthesized in the tissues are:

• Acetylcholine,

• Serotonin,

• Histamine,

• Substance P,

• Heparin and

• Gastrointestinal hormones.