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Somato-mammotropin Group Of Hormones And Investigations On Hormones Of The Anterior Pituitary
The Human Growth Hormone- A Powerful Anabolic Hormone
The Growth Hormone And Prolactin
Growth Hormone (GH): GH is a single polypeptide with 191 amino acids. The quantity of GH exceeds that of the other pituitary hormones. Human pituitary contains 6-10mg of extractable GH. Postmortem pituitary is the only source of this hormone used for diagnostic and therapeutic purposes.
Fasting levels of GH are below 4ng/ml. Secretion occurs in bursts lasting 1-2 hours. Maximal secretion occurs during the first part of sleep, during stages III and IV of NREM sleep. Factors such as exercise, hypoglycemia and rise in aminoacid level enhance secretion. Secretion of GH by the pituitary is increased by its releasing factor (GRF) and inhibited by somatostatin secreted by hypothalamus. GRF has not been isolated so far.
Growth hormone has effect on intermediary metabolism and on cartilage growth. It increases protein synthesis. Lipolysis is increased, thereby the circulating level of free fatty acids is also increased. This, in turn, interferres with the uptake and metabolism of glucose by cells, in this manner, GH behaves like an anti-insulin.
Action of GH on epiphyseal cartilage is exerted through a group of insulin-like peptides called somatomedins. Somatomedins are secreted by the liver in response to circulating GH. The metabolism and multiplication of chondrocytes are increased and this accounts for linear growth. The exact mechanism of action of GH at the tissue level is not well understood.
Prolactin (PRL): Prolactin resembles GH closely. Prolactin can be separated by radioimmunoassay. This hormone is also secreted in bursts lasting for a few minutes to hours, maximal secretion occurring during sleep. Hypothalamus exerts an inhibitory effect on PRL secretion through prolactin inhibitory factor (PIF). It is most likely that prolactin-releasing factors may also exist. Thyrotropin releasing hormone also stimulates release of prolactin. Neuroendocrine reflexes caused by suckling and stimulation of breast in females also help to release PRL. Prolactin exerts its main action on the breast which is primed by the action of estrogens, progesterone, GH and cortisol. Prolactin stimulates the synthesis of milk proteins. The physiological role of PRL in male is not clearly known.
Basal PRL level is about 10-20 ng/ml in men and non-pregnant women. Towards the end of pregnancy, levels as high as 200 ng/ml are observed. During puerperium, values fall to non-pregnant levels even though lactation is continued. Prolactin is release during suckling and this prepares the breast for the next feeding.
Investigations in hypothalamo-pituitary disorders are designed to bring out morphological and functional abnormalities. Tumours may present as space occupying lesions with pressure effects on neighboring neural structures. Endocrine disturbances resulting from increased or decreased secretion of hormones are also seen commonly, but these may not be constant. Some tumours secrete excess of hormones while others compress normal tissue and suppress its function. The investigations may be grouped under three heads-neurological, radiological and hormonal studies.
Neurological Investigations: Electroencephalography (EEG): This may help in detecting hypothalamic lesions which are too small to produce focal neurological deficits.
Perimetry: Pressure of tumours in the region of the pituitary causes compression of the optic chiasma leading to defects in the visual fields. These can be documented by perimetry.
Radiological studies: Lateral view of the skull may show enlargement of sella, double flooring, destruction of clinoid processes or supra sella calcification. More detailed information of this area may be obtained by cone views of the pituitary fossa.
Pneumoencephalography (PEG): This study may reveal lesions in the third ventricle. However, since this procedure is invasive, it is not accepted as the first line of investigation at present. Morever, in patients with adrenal cortical deficiency, this may precipitate adrenal crisis.
Carotid angiography: This is also an invasive procedure, which is undertaken to demonstrate the vascular pattern in this region.
CT Scanning coupled with contrast studies: These studies give definite information in almost all cases.
Echoencephalography: This may help in detecting shift of midline structures and cysts.
Radio immunoassay: Anterior pituitary hormones and secretion of its target glands can be measured precisely by radio-immunoassay.
Feedback control: Integrity of hypothalamo-hypophyseal system can be assessed by tests involving suppression or stimulation tests.
Finally, when the measurement of hormones in blood is not practical, for example ADH, the functions of the target organs are studied. A good illustration is in diabetes insipidus, studies of concentrating function of the kidney give valuable information.
© 2014 Funom Theophilus Makama