Lithium administration and phosphate excretion

Lithium interactions with Na+-coupled inorganic phosphate cotransporters: insights into the mechanism of sequential cation binding

Type IIa/b Na(+)-coupled inorganic phosphate cotransporters (NaPi-IIa/b) are considered to be exclusively Na(+) dependent. Here we show that Li(+) can substitute for Na(+) as a driving cation. We expressed NaPi-IIa/b in Xenopus laevis oocytes and performed two-electrode voltage-clamp electrophysiology and uptake assays to investigate the effect of external Li(+) on their kinetics. Replacement of 50% external Na(+) with Li(+) reduced the maximum transport rate and the rate-limiting plateau of the P(i)-induced current began at less hyperpolarizing potentials. Simultaneous electrophysiology and (22)Na uptake on single oocytes revealed that Li(+) ions can substitute for at least one of the three Na(+) ions necessary for cotransport. Presteady-state assays indicated that Li(+) ions alone interact with the empty carrier; however, the total charge displaced was 70% of that with Na(+) alone, or when 50% of the Na(+) was replaced by Li(+). If Na(+) and Li(+) were both present, the midpoint potential of the steady-state charge distribution was shifted towards depolarizing potentials. The charge movement in the presence of Li(+) alone reflected the interaction of one Li(+) ion, in contrast to 2 Na(+) ions when only Na was present. We propose an ordered binding scheme for cotransport in which Li(+) competes with Na(+) to occupy the putative first cation interaction site, followed by the cooperative binding of one Na(+) ion, one divalent P(i) anion, and a third Na(+) ion to complete the carrier loading. With Li(+) bound, the kinetics of subsequent partial reactions were significantly altered. Kinetic simulations of this scheme support our experimental data.

Acute lithium administration impairs the action of parathyroid hormone on rat renal calcium, magnesium and phosphate transport

1. Chronic lithium (Li+) treatment commonly produces a state of hyperparathyroidism in humans and rat although the mechanism is unknown. 2. The present study evaluated the acute effect of Li+ on renal electrolyte transport, particularly Ca2+ and Mg2+ in thyroparathyroidectomized (TPTX) and intact rats. 3. The acute administration of Li+ significantly increased water, sodium, potassium and phosphate excretion in both TPTX and intact animals; however, Ca2+ and Mg2+ excretion was only increased in the intact group. Fractional excretion (FE) of Ca2+ and Mg2+ increased from 2.2 +/- 0.2 to 3.5 +/- 0.3% and 12 +/- 2 to 18 +/- 2%, respectively (P < 0.01). 4. In further experiments in TPTX rats, Li+ administration inhibited the usual reduction in urine Ca2+ and Mg2+ excretion following parathyroid hormone (PTH) administration and inhibited the phosphaturia. However, supramaximal concentrations of PTH overcame this inhibitory effect. For example, an FECa of 3.8 +/- 0.2% was reduced to 1.4 +/- 0.2% and 1.7 +/- 0.2% with maximal and supramaximal PTH concentrations, respectively, while in the presence of Li+ an FECa of 4.0 +/- 0.2 was decreased to 2.8 +/- 0.2 and then 1.9 +/- 0.3% with the same PTH concentrations. 5. The inhibitory effect of Li+ was reduced with a lower plasma Li+ concentration (0.7 +/- 0.2 vs 1.6-1.8 mmol/L). The FEMg results were comparable. 6. These results demonstrate that Li+ directly inhibits PTH-mediated renal reabsorption of Ca2+ and Mg2+ and also blunts PTH-mediated phosphaturia. Therefore, the hyperparathyroidism in humans following Li+ treatment may be a consequence of reduced renal Ca2+ reabsorption.

Lithium effects on calcium, magnesium and phosphate in man: effects on balance, bone mineral content, faecal and urinary excretion

Calcium, magnesium and phosphate metabolism was studied in lithium-treated patients, using a metabolic balance technique. Two groups of patients participated in the study: 1) Patients who were to start on a prophylactic lithium treatment, and 2) Long-term lithium-treated patients whose treatment was to be terminated. Lithium treatment produced a positive balance in both calcium, phosphate and magnesium. By continuous lithium treatment the effect on magnesium wore off, whereas the effect on calcium and phosphate persisted. In urine, lithium induced a decrease in both calcium and phosphate excretion, whereas the excretion of magnesium was increased. Bone mineral content was measured by photon absorption, and lithium treatment resulted in a decrease in bone mineral content occurring within the first 6 months of lithium treatment. In the patients, bioavailability of the Li2CO3 preparation was found to be about 95%, and the patients contained about one 24-h dose of lithium just before the next dose of lithium was administered.