Defect in parathyroid-hormone-induced luminal calcium absorption in connecting tubules of Klotho mice

Background. Homozygous Klotho mutant mice (KL-/- mice) exhibit multiple phenotypes resembling human ageing. Increases in the ratio of urinary calcium to urinary creatinine (uCa/uCr) and in serum Ca concentration and decreases in urinary Cr excretion and serum parathyroid hormone (PTH) concentration were reported; however, precise information about renal Ca handling was not reported in these animals. Methods. We evaluated the PTH-induced increase in intracellular Ca2+ concentration ([Ca2+]i) in cells of isolated perfused connecting tubules (CNTs) of KL-/- mice. We also determined fractional excretion of Ca from the urine and serum samples of the same animals (n = 7), and compared them with KL+/+ mice and hemi-nephrectomized KL-+/+ mice (n = 10 in each) as controls. Results. FECa was significantly higher in KL-/- mice than in controls (0.67 +/- 0.13 vs 0.20 +/- 0.04%). The PTH (10 nM)-induced increase in [Ca2+]i was diminished in KL-/- mice (58 +/- 5 vs 231 +/- 15 nM). Addition of 10 nM of 8-(4-chlorophenylthio)-cyclic adenosine 3',5'-monophosphate had a similar effect. The PTH-induced increase had completely disappeared by the removal of Ca from lumen and bath in both groups of animals. Removal of sodium (Na) from the solution increased [Ca2+]i to a similar extent in both groups. Conclusion. We conclude that renal Ca excretion estimated by determining FECa was defective in the KL-/- mice. Impairment of Ca absorption from the lumen by stimulation of PTH in CNTs is one of the mechanisms of this defect. Activity of the basolateral Na/Ca exchanger was preserved in this strain. Therefore, the pathway downstream after generation of second messengers following stimulation of PTH (such as the sorting of transporters of Ca absorption) might be impaired by disruption of the Klotho gene.