lecture 7

• lactation occurs/blends with lactogenesis 2 (synthesis of lactose, protein, and fat; minerals and vitamins are being transported from blood)
• synthesis of milk does not occur at a constant rate and is dependent on certain hormones
• galactopoiesis- maintenance of lactation once initiated
• persistency- rate of decline in milk production
• peak milk- point in time when the greatest amount of milk is produced (around 60 days after calving)
• hypophysectomy- removal of pituitary gland for study
• homeorhesis- orchestrated or coordinated control in metabolism of body tissues necessary to support a physiological state; changes in body that permit lactation to occur
• greater rate (slope) of decline = lower persistency
• multiparous cows typically have less persistency than primiparous cows
• mammary gland function is largely hormonally controlled
• removal of the pituitary gland causes a nosedive in hormone levels, must exogenously add hormones back in via shots/implants
• main hormone organs relevant to mammary physiology: hypophysis, ovary, adrenal gland, thyroid, pancreas
• the hypophysis (pituitary) is below the hypothalamus in the brain; hypothalamic neurons cause hormone production in anterior and posterior lobes of pituitary gland
• the portal vessels allow hormones to be produced in the hypothalamus and be transported to the anterior pituitary
• neurons/nuclei in the hypothalamus produce hormones that are released in the posterior pituitary lobe
• pituitary hormones from the adenohypophysis include: prolactin, thyroid stimulating hormone, LH, FSH, somatotropin, and ACTH
• pituitary hormones from the neurohypophysis include oxytocin
• the ovaries produce estradiol and progesterone; both are central for mammary growth, but are not beneficial to milk production
• estradiol produced by a granulosa cell on the ovaries comes from the follicle that forms in response to FSH in anterior pituitary
• progesterone produced from the corpus luteum (yellow body) that forms in response to LH
• the adrenal glands are cranial to the kidneys
• the adrenal glands are split into 2 parts: the adrenal cortex (top) and the adrenal medulla (bottom); produce different hormones
• the zona glomerulosa (outside layer of adrenal cortex) secretes mineralcorticoids (aldosterone)
• the zona fasciculata (largest/most prominent and active) secretes glucocorticoids (cortisol, corticosterone, cortisone)
• the zona reticularis (lower adrenal cortex) secretes androgens (dehydroepiandrosterone)
• the adrenal medulla secretes hormones (epinephrine, noreoinephrine)
• the adrenal gland secretes glucocorticoids (dictated by ACTH); cortisol is responsible for formation of tight junctions (blood-milk barrier), as well as differentiation of ER and golgi apparatus, as well as transcription of casein to RNA and overall milk secretion
• corticosterone (adrenal gland) is important in birds, mice, and fish only
• the thyroid gland produces 2 hormones: thyroxine (T4) - inactive and triiodothyronine (T3) - active
• the pancreas is both an endocrine and exocrine gland; exocrine: produces lipases and protases to digest food
• the endocrine function of the pancreas: produces insulin (uptake of glucose by cells, breakdown glucose) and glucagon (release of glucose from cells into blood)
• the brain is completely glucose dependent (80-85% of brain function)
• insulin inhibits lactation performance by reducing blood glucose; the consequences is less nutrients are available to the mammary gland, reducing overall milk volume
• insulin increases fat and protein percentage
• insulin is not a galactopoietic hormone
• a galactopoietic hormone is capable of increasing milk production
• triiodothyronine (T3) can be injected or fed to cows as thyroprotein (iodinated casein); this is gluconeogenic, increases BMR (basometabolic rate), mammary uptake of glucose and FA, and milk yield by 10-20%
• triiodothyronine (T3) as thyroprotein is short lived (response lasts only 2-4 months), milk production plummets after removal
• triiodothyronine (T3) is a galactopoietic protein
• somatotropin (growth hormone)- Posilac (rBST) is the commercial form; thought to affect nutrient partitioning and increases persistency