11th Class Biology Chemical Coordination and Integration Local Hormones Pheromones, And Insect Endocrine Glands Local Hormones

Hormones described so far are called circulating hormones, because these circulate in whole body with blood. When stimulated by physical or chemical stimuli, all body cells, except red blood corpuscles (RBCs), secrete certain such compounds which transmit coded informations of metabolic adjustments between neighbouring cells and hence remain ECF instead of diffusing into the blood. These compounds are called local tissue hormones or autocoids. These are short-lived, because various enzymes present in ECF continue degrading these at a fast rate.

Local hormones are of two main categories-paracrine and autocrine. Paracrine hormones affect metabolism of cells located in the neighbourhood of those which secrete them. Autocrine hormones affect metabolism of the every cells which secrete them. Most local hormones are paracrine. These belong to the following categories : 

(1) Eicosanoids : These are a category of lipids derived from a fatty acid, arachidonic acid, synthesized in the plasma membrane of cells, and released in ECF. These are of four categories, viz. Prostaglandins, prostacyclins, thromoboxanes and leukotrienes.

(i) Prostaglandins (PGs) : In 1935, Ulf von Euler discovered that human semen contains a very active compound presumably secreted by prostate gland and, hence, named as such. He found that after the semen is discharged in woman’s vagina, this compound contracts uterine muscles to facilitate the sperms to ascend into fallopian tubes and reach ova to fertilize these.

(ii) Prostacyclins : These are found in walls of blood vessels and induce vasodilation. These also facilitate flow of blood in vessels and prevent thrombosis by inhibiting aggregation of platelets.

(iii) Thromboxanes : These are secreted by blood platelets. These help in blood clotting by instigating aggregation of platelets at the place of injury. These also instigate vasoconstriction at places of injury to prevent excessive loss of blood.

(iv) Leukotrienes : These are secreted by eiosinophils of blood and mast cells of connective tissues. These serve as mediators in inflammatory and allergic reactions, induce bronchoconstriction (constriction of bronchioles), constrict arterioles and induce migration of neutrophils and eosinophils towards the places of inflammation. These can cause asthma, arthritis, colitis, etc.

(2) Neuroregulators : These are a category of proteins which function as paracrine hormones in nervous tissues. These can be classified in three categories as follows :

(i) Neurotransmitters : These are synthesized in nerve cells and are secreted by exocytosis by axon terminals of these cells. These serve to transmit nerve impulses from one neuron to other neighbouring neurons, or muscles, or glands across synapses. About 60 to these have so far been discovered, but the most common of these are acetylcholine, norepinephrine, dopamine, serotonin and histamine.

(ii) Neuromodulators : In nervous tissues, the neurons secrete such paracrine hormones which modulate (increase or decrease) the excitability of other neighbouring neurons. These hormones are called neuromodulators. The main positive neuromodulators which increase the excitability of other neurons are the amino acids glutamate and aspartate, and polypeptide named ‘P’ substance. Contrarily, the main negative modulators which decrease the excitability of neighbouring neurons are the amino acid glycine and gama aminobutyric acid (GABA), polypeptides named enkephalins, endorphins, dynorphins and tachychinins, and the nitric oxide (NO).

(iii) Nerve growth factors : The supporting glial cells of nervous tissues and cells of muscles, salivary glands and many other tissues secrete such polypeptide paracrine hormones which play important role in growth, development and survival of nerve cells. That is why, these hormones are collectively called neurotrophins.

Pheromones

These are defined as chemicals excreted or released by one animal to the exterior, but evoke a physiological or behavioral response in another animal of the same species. Some pheromones, release on body surface, evoke a response in the recipient when tasted by the latter by licking, but most pheromones are volatile and odorous fatty acids (hydrocarbons) whose air borne molecules are received by recipient animals through olfaction. Certain insect pheromones are well-known examples. For instance, certain insects secrete bombykol or gyplure to attract their mating partners. Some other insects release geranoil to transmit information of food souirce of danger to their fellows.

In mammals, presumably including humans, certain volatile fatty acids secreted in vaginal fluid by females acts as pheromones. These may evoke sex drive in males, or affect menstrual cycle in other females. It has been observed that there is a tendency of synchronized menstrual cycles in female roommates. This “dormitory effect” must be due to pheromones.

Types of pheromones

Insect endocrine glands

The endocrine system of cockroach comprises intercerebral gland cells, corpora cardiaca, corpora allata, and prothoracic glands.

(1) Intercerebral gland cells : These cells lie in the brain between the two cerebral ganglia. They secrete a hormone called the brain hormone. This hormone activates the prothoracic glands to secrete their hormone.

(2) Corpora cardiaca : These are a pair of rod-like bodies situated on the sides of the oesophagus just behind the brain. They secrete a growth hormone.

(3) Corpora Allata : These are a pair of small, rounded bodies lying close behind the corpora cardiaca. They secrete a juvenile hormone in the nymphal stages. This hormone causes retention of the nymphal characters and checks the appearance of adult characters. In other words, it keep the insect young. In the last nymphal form, corpora allata become inactive, thereby resulting in the absence of juvenile hormone. The absence of this hormone permits the appearance of adult features. In the adult, the corpora allata again become active and secrete a gonadotropic hormone, which regulates egg production and development and functioning of the accessory sex glands.

(4) Prothoracic glands : These are fairly large, irregular glands situated in the prothorax. They secrete a hormone called ecdyson, which controls moulting of the nymphs. The prothoracic glands degenerate after metamorphosis.