Insulin, Growth Hormone and Cortisol Through Lactation
Oxytocin Oxytocin is a 9 amino acid long peptide. The amino acid structure of oxytocin is: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly It has a molecular mass of 1007 daltons. Oxytocin has a disulfide bond between the two cysteines. Reduction of the disulfide bond inactivates oxytocin. One IU (international Unit) is approximately 2 micrograms of pure peptide.
Oxytocin Synthesis Oxytocin is synthized in the hypothalamus in specific nuclei, the paraventricular nucleus and the supraoptic nucleus in the hypothalamus. [A cluster of nerve cells in the brain is often called a nucleus. This is different from the nucleus of a single cell.] Neurons in these hypothalamic nuclei synthesize the oxytocin precursor and package it into vesicles. Oxytocin is initially synthesized as a large molecular weight precursor which also consists of the oxytocin-carrier peptide neurophysin. The precursor is proteolytically cleaved in the neuron in the oxytocin- containing vesicle to yield oxytocin bound to neurophysin. The oxytocin- neurophysin complex is the intracellular storage form of oxytocin. The oxytocin-containing vesicles are transported from the cell body (which is in the hypothalamus), down the axons to the neuron endings in the posterior pituitary. This is called the hypothalamo-neurohypophysial tract. The oxytocin-neurophysin complex is stored in neurosecretory granules called herring bodies in the axon ending.
Oxytocin Release and Half-Life It is estimated that the bovine pituitary has about 800 micrograms of oxytocin. This is about 40X what is in the blood under resting conditions. Only about 1/3 of pituitary oxytocin is released at a milking. Oxytocin receptors on myoepithelial cells can respond to very low levels of oxytocin. Oxytocin has a short half-life in the blood = 0.55 to 3.6 min. This means that the removal of milk by machine or by nursing must be closely timed with stimulation of the teats.
Factors Modifying Milk Letdown Autonomic nervous system – Stress gives epinephrine release – Inhibits oxytocin release – Inhibits myoepithelial cell contraction – Inhibits blood flow to udder Conditioned reflex – Letdown in response to sights, sounds associated with milking