Stimulant anterior

Control of secretion of anterior pituitary hormones also includes inhibition by hormones produced by target organs. For example, CRH stimulates the anterior pituitary to secrete ACTH, which in turn stimulates the adrenal cortex to secrete corticosteroids. Corticosteroids then feed back to inhibit the secretion of ACTH. Feedback mechanisms are important for the control of most hormones. For example, insulin (qv) secretion from the pancreas increases in response to increased blood glucose resulting from ingestion of a meal. Insulin increases tissue uptake and metaboHsm of glucose, which lowers blood glucose and in turn reduces insulin secretion. 

During the isolation of inhibin from foUicular fluid, some chromatographic fractions stimulated FSH release from cultured anterior pituitary cells, suggesting the existence of FSH releasing proteins (FRPs). Two FRPs, given the generic term activins, were subsequentiy isolated (131,132). One is composed of two disulfide-finked P-A subunits (activin A) the other consists of similarly finked P-A and P-B subunits (activin AB). 

Of the other Strychnos alkaloids vomicine has been investigated by Ruickoldt, who finds that in mice and rabbits it causes clonic convulsions, due to stimulation above the level of the anterior corpore quad-ragemina. Convulsions can be elicited after intravenous, but not after subcutaneous, injections. The toxicity is low twelve times the convulsive dose does not cause death. No special action is exerted on blood... 

This section of the chapter discusses FSH, LH, GH, and ACTH. FSH and LH are called gonadotropins because they influence the gonads (the organs of reproduction). GH, also called somatotropin, contributes to the growth of the body during childhood, especially the growth of muscles and bones. ACTH is produced by the anterior pituitary and stimulates the adrenal cortex to secrete the corticosteroids. The anterior pituitary hormone, TSH, is discussed in Chapter 51. Prolactin, which is also secreted by the anterior pituitary, stimulates the production of breast milk in the postpartum patient Additional functions of prolactin are not well understood. Prolactin is the only anterior pituitary hormone that is not used medically. 

Corticotropin (ACTH) is an anterior pituitary hormone that stimulates the adrenal cortex to produce and secrete adrenocortical hormones, primarily the glucocorticoids. 

Fig. 5.8(a) Hypophysectomy abolishes urinary stimulation ablation of anterior pituitary removes neurocrine linkage via AOS to testes - - - = before — = after operation (from Johnston, 1985). 

Adrenal hormone production is controlled by the hypothalamus and pituitary gland. Corticotropin-releasing hormone (CRH) is secreted by the hypothalamus and stimulates secretion of adrenocorticotropic hormone (ACTH), also known as corticotropin from the anterior pituitary. ACTH, in turn, stimulates the adrenal cortex to produce cortisol. When sufficient or excessive cortisol levels are reached, a negative feedback is exerted on the secretion of CRH and ACTH, thereby decreasing overall cortisol production. The control of adrenal androgen synthesis also follows a similar negative-feedback mechanism. 

Prolactin is an essential hormone for normal production of breast milk following childbirth. It also plays a pivotal role in a variety of reproductive functions. Prolactin is regulated primarily by the hypothalamus-pituitary axis and secreted solely by the lactotroph cells of the anterior pituitary gland. Under normal conditions, secretion of prolactin is predominantly under inhibitory control by dopamine and acts on the D2 receptors located on the lactotroph cells. Increase of hypothalamic thyrotropin-releasing hormone (TRH) in primary hypothyroidism can stimulate the release of prolactin. 

Eighty percent of aqueous humor drains through the trabecular meshwork, which is a lattice of connective tissue that surrounds the anterior chamber. The size of the trabecular meshwork can be altered by the contraction or the relaxation of the ciliary muscle. Stimulation of muscarinic receptors on the ciliary muscle causes contraction, which in turn causes the pores of the trabecular meshwork to open, increasing aqueous humor outflow. 

Carbachol stimulates the same muscarinic receptor as pilocarpine and also inhibits acetylcholinesterase, the enzyme that metabolizes acetylcholine. Carbachol is more potent than pilocarpine, but it causes more accommodation spasm and brow ache and may also cause anterior uveitis. Carbachol is rarely used today because of the side-effect profile. 

Corticotropin-releasing hormone A hormone released by the hypothalamus that stimulates release of adrenocorticotropic hormone by the anterior pituitary gland. 

---