Background: Thiazides are associated with glucose intolerance and new onset diabetes mellitus. Previous studies demonstrated that thiazides attenuate insulin secretion, but the molecular mechanisms remain elusive. We hypothesized that thiazides attenuate insulin secretion via one of the known molecular thiazide targets in β-cells.

Methods: We performed static insulin secretion experiments with islets of wild-type, NCC (SLC12A3) and NDCBE (SLC4A8) knock-out (KO) mice and with murine Min6 cells with individual knock-down of carbonic anhydrase (CA) isoforms to identify the molecular target of thiazides in β-cells. CA5b KO mice were then used to assess the role of the putative thiazide target CA5b in β-cell function and in mediating thiazide sensitivity in vitro and in vivo.

Results: Thiazides inhibited glucose- and sulfonylurea-stimulated insulin secretion in islets and Min6 cells at pharmacologically relevant concentrations. Inhibition of insulin secretion by thiazides was CO2/HCO3--dependent, not additive to unselective 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 or NDCBE. 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 oxaloacetate synthesis. CA5b KO mice were resistant to thiazide-induced glucose intolerance, and thiazides did not alter insulin secretion in CA5b KO islets.

Conclusions: In summary, our study reveals that thiazides attenuate insulin secretion via inhibition of the mitochondrial CA5b isoform in β-cells.