Interrelationship of chlorothiazide and parathyroid hormone: a micropuncture study

Sodium and calcium are normally reabsorbed in parallel in the renal tubule. Both parathyroid hormone (PTH) and thiazide diuretics may influence this relationship. This study was designed to show whether the dissociation of Na from Ca transport produced by thiazides is dependent upon the presence of PTH. Hydropenic thyroparathyroidectomized (TPTX) dogs were given chlorothiazide alone and together with PTH. Chlorothiazide alone significantly increased fractional excretion of sodium (0.5 +/- 0.3-5.6 +/- 0.3%) and calcium (0.74 +/- 0.18-1.4 +/- 0.24%). However, the Ca/Na excretion ratio fell markedly from 1.57 to 0.24%. Micropuncture revealed this dissociation to occur at the distal tubule. Proximal reabsorption of water, sodium, and calcium were inhibited to an equal extent. However, distal fractional sodium reabsorption fell 10% whereas calcium reabsorption remained unchanged following chlorothiazide administration in TPTX animals. When phosphaturic doses of PTH were administered with chlorothiazide, no significant changes were observed in calcium or sodium reabsorption. It is concluded that PTH plays no role in the dissociation of sodium from calcium reabsorption resulting from acute chlorothiazide administration.