Anterior lobe, hormones

Anterior lobe hormone Regulatory hormone or factor Structure... 

Acromegaly and gigantism result from (1) hypersecretion of some hormones of the anterior lobe of the hypophysis (especially the growth hormone), (2) decreased secretion of some other anterior lobe hormones, particularly gonadotropins, and (3) compression of the surrounding tissue at the base of the skull. 

Luteinizing hormone-releasing factor. In 1959 we began an intensive search for a possible LH-releasing action of hypothalamic extracts and employed a new sensitive and specific assay for LH, the ovarian ascorbic acid depletion test of Parlow (1). In immature female rats which have been pretreated with gonadotrophins, minute doses of LH evoke a decline in ovarian ascorbic acid concentration. Other anterior lobe hormones are without effect. 

In a hyperfunctioning of the thyroid gland, secretion of an excess quantity of thyroid hormones leads to a hyperthyroid condition (Basedow s disease, goiter). In this condition, drags are used that suppress production of thyrotropic hormones in the anterior lobe of the hypophysis (diiodotyrosine), in the thyroid gland (propylthiouracil, methylthiouracil,... 

Drugs used for hyperthyroidism can be classified as drugs that suppress thyroid hormone synthesis in the anterior lobe of the hypophysis, and they consist of diiodotyrosine and iodine, as well as drugs that suppress thyroid hormone synthesis in thyroid glands (propylthiouracil, methylthiouracil, methimazole, and carbimazole). 

Diiodotyrosine does not possess pronounced hormonal activity. However, it stops production of thyrotropic hormone by the anterior lobe of the hypophysis, which activates thyroid gland activity. 

Formation of these drugs is under the direct control of a polypeptide adrenocorticotropic hormone (ACTH, corticotropin), which is processed by the anterior lobe of the hypophysis. Human ACTH consists of 39 amino acids and has a molecular weight of about 4500. It differs from animal ACTH in the amino acid compositions at positions 29-33. 

Figure 3.2 The pituitary gland is a pea size gland located at the base of the human brain. The pituitary gland has two rounded projections or lobes. Cells of the anterior lobe produce growth hormone and five other protein hormones involved in regulating various body functions, when stimulated by specific signals from the hypothalamus. The optic chiasma is where the optic nerves from each eye cross before entering the brain.

The correlation of the hypothalamus and its hormones with the hormones of the anterior pituitary gland is summarized in figure 5.2. There is no direct vascular connection between the hypothalamus and the posterior lobe of the pituitary that would correspond to the portal vein system for the anterior lobe of the gland. Not all hypothalamo-pituitary hormones will be discussed in the subsequent sections. Only those that are well-defined chemical entities or have a direct connection with drug action are considered. 

The anterior lobe secretes various trophic hormones, the posterior lobe is responsible for the secretion of oxytocin and antidiuretic hormone (vasopressin) and middle lobe secretes melanocyte-stimulating hormone (MSH) which may affect the synthesis of melanin. 

Anterior lobe of pituitary is the master gland of the endocrine system as a whole because it produces peptide trophic hormones which affect the other ductless/endocrine glands. The anterior lobe secretes the following hormones ... 

Inhibiting ovulation by blocking the release of follicle stimulating hormone and luteinising hormone from the anterior lobe of pituitary gland. 

Figure 30-1 (A) Median sagittal section of the human brain. From Maya Pines.15 (B) Drawing illustrating the synthesis of peptide hormones in the hypothalamus and transport via portal blood vessels into the anterior lobe of the pituitary gland or via nerve tracts into the posterior lobe.16...

Connected to the brain by a stalk (Fig. 30-1), the pituitary gland releases at least ten peptide or protein hormones that regulate the activity of other endocrine (hormone-producing) glands in distant parts of the body. The pituitary is composed of several distinct parts the anterior lobe (adenohypophysis), a thin intermediate portion (pars intermedia), and a posterior lobe (neurohypophysis). Each has its own characteristic endocrine functions. 

The anterior lobe of the pituitary secretes a series of ten or more peptide hormones ranging in size from the 20-residue (i-melanotropin to the 200-residue growth hormone (somatotropin). Several of these contain a common heptapeptide unit, which is marked in green in the following structure ... 

Vasopressin and oxytocin. In contrast to the large peptide hormones made in the anterior lobe of... 

Thyrotropin-releasing hormone is one of several small peptide hormones secreted by the anterior lobe of the pituitary gland. These are the master" hormones that function to stimulate hormone secretion from other endocrine glands. Thyrotropin stimulates the functioning of the thyroid gland. 

The action of HA is indirect, mediated primarily via activation of corticotropin-releasing hormone (CRH) originating in parvocellular neurons in the PVN and secondly via vasopressin (AVP) originating in parvo- and magnocellular neurons in the PVN and in the SON [18-24]. The effect of CRH is predominantly mediating in character (i.e. HA releases CRH which subsequently stimulates ACTH secretion) [18] while the effect of AVP seem to be mediating as well as permissive in character (i.e. AVP has to be present in order for HA to exert its effect on ACTH secretion) [25]. Besides these two important mediators, prostaglandins are involved in HA-induced release of the POMC-derived peptides from the anterior lobe [26], whereas catecholamines, oxytocin (OT) and serotonin (5-HT) do not participate [27-28, Willems et al. (unpublished observations)]. 

Pituitary Hormones Anterior lobe Principal Effects... 

Anterior Lobe. The anterior pituitary, or adenohypophysis, secretes six important peptide hormones. The anterior pituitary releases growth hormone (GH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and prolactin (Pr). The physiologic effects of these hormones are listed in Table 28-1. 

Dopamine can thus be added to the list of hormones and neurotransmitters which can stimulate or inhibit cyclic AMP formation, depending upon their tissue of action. Thus, while dopamine stimulates cyclic AMP formation in parathyroid cells, superior cervical ganglia, retina and striatal tissue (27, 58-61), it inhibits the accumulation of the cyclic nucleotide in cells of the intermediate and anterior lobes of the pituitary gland. Opposite effects on the cyclic AMP system are also found with LHRH which stimulates and inhibits cyclic AMP levels in the anterior pituitary gland (62) and ovary (63), respectively. Similarly, alpha-adrenergic agents show opposite effects on cyclic AMP formation in brain (64) and platelets (65). PGE, stimulates cyclic AMP formation in the anterior pituitary gland (62) while it inhibits the same parameter in fat cells (66). 

ACTH (adrenocorticotropic hormone, corticotropin) is a 39-amino-acid peptide synthesized and secreted by the corticotrope cells of the anterior lobe of the pituitary gland. ACTH acts on several target tissues, including the adrenal cortex, adipose tissue and brain. It is synthesized as part of proopiomelanocortin (POMC) as amino acids 132-170 of this molecule, which is proteolytically cleaved to produce ACTH [1],... 

The pituitaries of male Sprague-Dawley rats weighing about 100 g are quickly removed after decapitation. The posterior lobe is discarded and the anterior lobe is divided into two halves by a midsagittal cut. Five bisected hemipituitaries are incubated in plastic vials containing 4 ml TCM 199 with 0.1 % BSA, 15 pg/ml penicillin and 25 fig/ml streptomycin. The vials are gassed with 95 % O2 and 5 % C02. After 30 min of control incubation, the medium is changed and various doses of standard and test substances are added for an incubation of 90 min. GH content in the medium and in the pituitary tissue after incubation is determined by a specific radioimmunoassay (Schalch and Reichlin 1966). Other of hormones may be tested in the same procedure, for example prolactin. 

In summary, during periods of some forms of stress TIDA and PHDA neurons receive a convergence of inhibitory inputs mediated, at least in part, by opioid, cholinergic, histaminergic, serotoninergic and GABAergic neurons. The inhibition of TIDA neurons (in female rats) and PHDA neurons (in both sexes) culminates in hormonal responses the release of prolactin from lactotrophs in the anterior lobe of the pituitary and of aMSH from melanotrophs in the intermediate lobe of the pituitary. 

In h)rpopituitarism there is a partial or complete deficiency of hormones secreted by the anterior lobe of the pituitary. The posterior lobe hormones (see below) may also be deficient in a few cases, e.g. when a tumour has destroyed the pituitary. Patients suffering from hypopituitarism may present in coma, in which case treatment is as for a severe acute adrenal insufficiency. Maintenance therapy is required, using hydrocortisone, th5Toxine, oestradiol and progesterone (in women) and testosterone (in men). For growth hormone see above. 

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