Impaired glucose tolerance, glucagon, and insulin responses in mice lacking the loop diuretic-sensitive <i>Nkcc2a</i> transporter

Abstract

The Na⁺K⁺2Cl^- cotransporter-2 (<i>Nkcc2</i>, <i>Slc12a1</i>) is abundantly expressed in the kidney and its inhibition with the loop-diuretics bumetanide and furosemide has been linked to transient or permanent hyperglycemia in mice and humans. Notably, <i>Slc12a1</i> is expressed at low levels in hypothalamic neurons and in insulin-secreting β-cells of the endocrine pancreas. The present study was designed to determine if global elimination of one of the <i>Slc12a1</i> products, i.e., <i>Nkcc2</i> variant <i>a</i> (<i>Nkcc2a</i>), the main splice version of <i>Nkcc2</i> found in insulin-secreting β-cells, has an impact on the insulin and glucagon secretory responses and fuel homeostasis in vivo. We have used dynamic tests of glucose homeostasis in wild-type mice and mice lacking both alleles of <i>Nkcc2a</i> (<i>Nkcc2a^KO</i>) and assessed their islet secretory responses in vitro. Under basal conditions, <i>Nkcc2a^KO</i> mice have impaired glucose homeostasis characterized by increased blood glucose, intolerance to the sugar, delayed/blunted in vivo insulin and glucagon responses to glucose, and increased glycemic responses to the gluconeogenic substrate alanine. Further, we provide evidence of conserved quantitative secretory responses of <i>Nkcc2a^KO</i> islets within a context of increased islet size related to hyperplastic/hypertrophic glucagon- and insulin-positive cells (α-cells and β-cells, respectively), normal total islet Cl^- content, and reduced β-cell expression of the Cl^- extruder <i>Kcc2</i>.