Thiazides are associated with glucose intolerance and new onset diabetes mellitus, but the molecular mechanisms remain elusive. The aim of this study was to decipher the molecular basis of thiazide-induced glucose intolerance. In mice, hydrochlorothiazide induced a pathological glucose tolerance, characterized by reduced first phase insulin secretion but normal insulin sensitivity. In vitro, thiazides inhibited glucose- and sulfonylurea-stimulated insulin secretion in islets and the murine β-cell line Min6 at pharmacologically relevant concentrations. Inhibition of insulin secretion by thiazides was CO2 /HCO3- -dependent, not additive to unselective carbonic anhydrase (CA) inhibition with acetazolamide and independent of extracellular potassium. In contrast, insulin secretion was unaltered in islets of mice lacking the known molecular thiazide targets NCC (SLC12A3) or NDCBE (SLC4A8). CA expression profiling with subsequent knock-down of individual CA isoforms suggested mitochondrial CA5b as molecular target. In support of these findings, thiazides significantly attenuated Krebs cycle anaplerosis through reduction of mitochondrial oxalacetate synthesis. CA5b KO mice were resistant to thiazide-induced glucose intolerance, and insulin secretion of islets isolated from CA5b KO mice was unaffected by thiazides. In summary, our study reveals attenuated insulin secretion due to inhibition of the mitochondrial CA5b isoform in β-cells as molecular mechanism of thiazide-induced glucose intolerance.
