mones are immediately released from the cell. Thus sleroid hormones are not stored

mones are immediately released from the cell. Thus sleroid hormones are not stored. In the blood the steroid hormones combine with a prolein, synthesized by the liver. For example, cortisol binds with the cortisol binding globulin, CBG. A part of the circulating hormone, however remains free (= unbound). For example, about 90%, the circulating cortisol remains bound with CBG, but the rest of the cortisol (ie, 10%) remains free. lt is the free cortisol that is active (ie, can bind with the receptor of the target cell). Thus in some conditions (eg, pregnancy) the circulating steroid hormone concentration may be higher than normal but the concentration of the free hormone (say, per ml of blood) remains same and thus no sign of excess mrtisol activity is presented by the subject. [NB. It is imporiant to note that in some forms of malignancies of non endocrinal tissues, peptide hormones may be secreted (eg. peptide hormone secreting lung cancers). This is unknown with steroid hormones]. CONTROL OF HORMONE SECRETION Recall, hormones are one of the fundamental agents of achieving the homeostasis. Therefore, exactly that much hormone is released for circulation which is exactly needed by the body, neither more nor less. That is, only the eustate of secretion is permitted, but not the hyper (excess) nor the hypo (deficient) state of secretion is allowed in the healthy slable states. Question is how this becomes possible ? Answer is, by the feed back mechanism (fig. 1.3.1, chap 3 sec I). As stated there, there are two varieties of feed back, (i) the -ve, and (ii) the +ve feed back. Table 6.1.2 gives a summarized version of control of hormone secretion in general. Explanation of table 6.1.2 Note, there are 4 major controlling agents or factors. Short explanatory note follows : (1) Anterior pituitary controls (a) thyroid, (b) suprarenal cortex and (c) gonadal (= testicular or ovarian) hormones through their 'tropic' (also spelt, trophic, by the British authors) hormones. Thus, there are thyrotropic, gonadotropic, adrenocorticotropic (ACTH) hormones causing stimulation of thyroid, gonads and adrenal cortex respectively. (2) Hypothalamus. 7 This part of brain is on the under surface of the brain (fig. IOD.1.1) and from hypolhalamus hangs the anterior pituitary. The vascular connection (chap 5 sec VI) that exists between the hypothalamus and the anterior pituitary is a portal syslem, ie, it starts (in the hypothalamus) as capillaries, and ends (in the anterior pituitary) as capillaries. Many hormones called hypothalamic releasing factors are produced by the hypothalamic neurons enter the capillaries (of the portal circulation) of the hypothalamus reach the capillaries at the level of anterior pituitary comes out of the blood vessels, to enter the cellular masses of anlerior pituitary. In this way, by a forward flowing blood flow, the hypothalamic hormones reach the anterior pituitary and either stimulate or inhibit their cells (who in turn secrete the tropic and other hormones of the anterior pituitary). A retrograde flow also occurs , that is, hormones of anterior pituitary enter Ihe capillary blood by diffusion reach the hypothalamic level hypothalamic neurons where they (= the anterior pituitary hormones) exert an inhibitory effect; this is also called short loop feed backfrom the anterior pituitary. In short the hypothalamus controls (= stimulates/ inhibits) the anterior pituitary by its releasing hormones (releasing factors of the yesleryears). For details see chap 5, sec VI. (3) Some of the endocrinal secretions (notably the cortisol and the ACTH) occur vigorosly at a predetermined hour of the day and the rate of secretion drops sharply at another given hour of a 24 hr, day night cycle. For example cortisol and ACTH are secreted very vigorosly in the early morning, whereas their secretion is greatly reduced in the night. This occurs in most persons and occurs without any obvious cause (though some cellular level explanations have recently been forwarded, see chap 4, sec VI). This is called circadian rhythm. (4) Stress. As mentioned already (chap 3, sec I) anything which tries to destabilize homeostasis is called a stress. Thus, physical injury, menial injury, disease, sharp changes in the environmental temperature are examples of stress. It is now known, several hormones viz, ACTH, cortisol adrenalin, ADH and thyroxine are secreted in increased rate during stress. Some further details Feed back. The general fealures of feed back mechanism have been described in chap 3 sec I. In endocrinology, most of the feed back mechanism belong to the variety of-ve feed backs allhough +ve feed backs (eg, LH surge) are also known. In -ve feed back mechanism (fig. 1.3.1) as in, say, the control of cortisol secretion, level of plasma hormone (cortisol) is the signal the signal reaches and operates at, usually two levels, anlerior pituitary and hypothalamus higher level of blood cortisol causes inhibition of release of both anterior pituitary tropic hormone(= ACTH) as well as hypolhalamic releasing hormone (= CRH) ultimately low level of anterior pituitary tropic hormone (= ACTH) withdrawal of stimulation on the adrenal cortex. Reverse occurs when the blood hormone level is low. Plasma or blood level of hormone is not the only signal. Feed back signals may be constituted by other conslituents of blood Further, such signals can also operate on glands other than anterior pituitary or hypothalamus. Thus blood sugar levels act as signals and act on a and β cells of islets of pancreas controlling glucagon and insulin secretions. Na+ and K+ levels of serum ultimately act as signals which control the aldosterone secretion by feed back. Ca++ level