A. Role of VP (Antidiuretic Hormone) in the Regulation of Body Fluids
The primary action of arginine vasopressin (AVP) is in the inhibition of water diuresis (loss of water in the urine), and this action operates through the actions of membrane receptors in the kidney. AVP also elevates blood pressure and constricts coronary arteries, and it stimulates intestinal and uterine contractions, as well as slightly stimulating milk ejection. However, the last two activities modestly cross over to the major functions of OT. In most cases, the secretion of OT will be signaled separately from the signaling to release AVP.
Figure 4-13 shows an overview of some of the main features of water balance (additional discussion is presented in Chapter 15). The hypothalamic osmoreceptor detects elevations in serum tonicity. The mechanism is thought to involve the shrinking of the osmoreceptor neuron to the point where a signal (chemical or electrical) is generated and transmits this signal down its axon to its nerve ending. Alternatively, the changed ionic environment produces a conformational alteration in the osmoreceptor, which generates the signal. The signal is then received by the cell body of the vasopressinergic neuron and transmitted to its own nerve ending for the release of VP-NP. Signals to baroreceptors that control the release of AVP-NP are exemplified by emotional states that inhibit the release of AVP-NP and lead to transitory frequency of urination. Nicotine intake and pain may also operate through baroreceptors of the CNS repressive center, which may stimulate AVP-NP release through adrenergic stimulation. The AVP-NP complex is briefly stable in blood, having an experimental half-life of 10-20 min. AVP dissociates from NP and binds to a membrane receptor located in distal tubular cells of the kidney. The second messenger of this interaction is cyclic AMP, which is elevated in the kidney cell and leads to the phosphorylation of proteins. Phosphorylation of proteins causes the admission of water from the lumen (urine side) at
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