Renal calcium metabolism and diuretics.

Diuretic agents have variable effects on calcium excretion as studied in vivo and in isolated kidneys and nephron segments. Generally, by increasing sodium and water excretion, diuretics will cause a concomitant increase in calcium excretion. As they diminish blood volume and alter renal hemodynamics, diuretics enhance calcium reabsorption in the proximal tubule, modulating their usual effects on calcium excretion. These general effects can be further modulated by additional metabolic actions. For instance, chronic administration of thiazide diuretics may diminish calcium excretion on the basis of altered levels of or responsiveness to PTH. Agents such as acetazolamide, which diminish bicarbonate reabsorption in the proximal tubule, will cause a modest calciuria, if any, because of reabsorption of the increased delivery of calcium, but not sodium, at the distal nephron. Agents acting in the loop of Henle that increase chloride excretion relative to sodium tend to cause greater calcium excretion. Finally, agents that act beyond the loop of Henle, which have their primary effects on cation excretion, tend to cause lesser degrees of calcium excretion, especially relative to sodium. These principles indicate that it may be appropriate to select a specific diuretic agent for different patients, depending upon the state of their calcium balance. It also may be possible to predict alterations in calcium balance, so that these may be anticipated and compensated for with patients on long-term therapy with various diuretic agents.