I was reading in my pharmacology textbook on lithium in treating bipolar disorder, and I was curious to know if there was any specific action lithium takes to produce symptoms such as polyuria and polydipsia. Any help would be great. Also, what is the most common way to monitor lithium levels in the body?
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Lithium changes the effect of at least three substances that play a role in diuresis:
- aldosterone: lithium partially inhibits its ability to increase the expression of ENaC receptors on apical membrane thus increasing sodium losses [1].
- arginine vasopressin (AVP): because lithium can induce hyperparathyroidism, parathyroid hormone can act as partial agonist to AVP, inhibiting its hydroosmotic action [2].
- antidiuretic hormone (ADH): lithium blocks its stimulatory effect on adenilate cyclase. The result is a decrease of cAMP levels in the cells from collecting tubules. cAMP promotes formation of water channels, thus improving absorption from collecting tubules. Because lithium decreases cAMP levels, diuresis is stimulated [1].
All these result in polyuria.
Lithium levels can be determined from the blood. Usually the test should be performed 12 hours after last dose of Lithium [3].
References:
- Eleanor Lederer, MD; Lithium Nephropathy on http://emedicine.medscape.com/article/242772-overview#a0104
- Carney SL, Ray C, Gillies AH. Mechanism of lithium-induced polyuria in the rat. Kidney Int. 1996 Aug;50(2):377-83.
- http://labtestsonline.org/understanding/analytes/lithium/tab/sample/
