Neurophysins are derived from the processing of the same larger precursor as the hormone which they bind, though an identical 90 amino acid sequence in both permits either to bind ADH or oxytocin. The neurophysin-hormone dimers are packaged in the neuronal soma in microscopically visible granules. The granule contents are released into the general circulation either from terminals which end directly on capillary endothelial cells or on cells which adjoin the vessel wall. ADH acts by permeabilizing the tubules to water and does not retain or secrete particular cations or anions. At high doses, ADH causes contraction of arteriles and capillaries, especially those of the coronary vessels, to produce localized increases in blood pressure.
Oxytocin stimulates contraction of uterine smooth muscle.
It is secreted during labor to effect delivery of the fetus. Oxytocin causes milk ejection, which is necessary for adequate lactation, but not milk production. Receptors, mechanism, and actions ADH has two types of receptors. The increased resorption of water by the renal tubule decreases urine volume and increases urine osmolality. Drugs which work via the sympathetic nervous system to increase peripheral resistance without decreasing coronary blood flow are better choices for the treatment of low blood pressure.