Reversible resistance to the phosphaturic effect of db-cAMP in lithium-treated rats

Acute lithium administration (5 mmol/kg b.w.) to parathyroidectomized (PTX) rats induces extracellular acidosis, lower plasma phosphate concentration and increased phosphate reabsorption. The present studies evaluate the effect of lithium administration on tissue phosphate distribution, metabolites content in the kidneys and renal phosphate, 2-oxoglutarate and citrate transport in the presence and absence of db-cyclic AMP. Lithium decreased plasma and renal phosphate concentrations and increased phosphate concentration in the skeletal muscle, db-cyclic AMP was not phosphaturic in lithium-treated PTX rats. In PTX rats infused with 20 mM phosphate lithium depressed fractional phosphate excretion induced by db-cyclic AMP from 20 +/- 0.3% to 3.2 +/- 1.0%. However, metabolic or respiratory acidosis restored the responsiveness to db-cyclic AMP. Citraturia and ketoaciduria induced by lithium were depressed in db-cyclic AMP-treated rats. Kidney citrate and 2-oxoglutarate concentrations increased drastically, ATP level fell significantly whereas cAMP content did not change after lithium. We conclude that lithium administration increases phosphate uptake by the muscle which largely accounts for hypophosphatemia. The kidney responds with increased phosphate reabsorption independent of plasma and kidney phosphate concentrations, and with refractoriness to the phosphaturic effects of db-cyclic AMP. Acute lithium administration to rats induces extracellular acidosis and, probably, renal intracellular alkalosis as reflected by citraturia and ketoaciduria as well as the renal metabolite profile. The phosphaturic responsiveness to db-cyclic AMP is dependent, at least in part, on intracellular pH.