Fibroblast growth factor-23 and parathyroid hormone suppress small intestinal magnesium absorption

In the present study, we examined the systemic and direct effects of parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23) on duodenal, jejunal, and ileal Mg²⁺ absorption. The rats were injected with FGF-23 or PTH for 5 h before collecting the duodenum, jejunum, and ileum for Mg²⁺ transport analysis in Ussing chambers. The duodenum, jejunum, and ileum were directly exposed to FGF-23, PTH, or FGF-23 plus PTH with or without cell signaling inhibitors for 150 min in Ussing chambers prior to performing the Mg²⁺ transport study. The small intestinal tissues were also subjected to western blot analyses for FGF receptor (FGFR), PTH receptor (PTHR), Klotho, transient receptor potential melastatin 6 (TRPM6), and cyclin as well as the cystathionine β-synthase domain divalent metal cation transport mediator 4 (CNNM4) expression. The small intestine abundantly expressed FGFR and PTHR proteins, whereas, Klotho was not expressed in rat small intestine. Systemic PTH or FGF-23 injection significantly suppressed transcellular Mg²⁺ transport in the duodenum and jejunum. Direct FGF-23-, PTH-, or FGF-23 plus PTH exposure also suppressed transcellular Mg²⁺ absorption in the duodenum and jejunum. There was no additional inhibitory effect of PTH and FGF-23 on intestinal Mg²⁺ absorption. The inhibitory effect of PTH, FGF-23, or FGF-23 plus PTH was abolished by Gö 6850. Systemic PTH- or FGF-23-injection significantly decreased membranous TRPM6 expression, but increased cytosolic CNNM4 expression in the duodenum, jejunum, and ileum. In the present study, we propose a novel magnesiotropic action of PTH and FGF-23 by modulating small intestinal Mg²⁺ absorption.