Inhibition of calcium absorption in the cortical thick ascending limb of Henle's loop by furosemide.

The effect of luminal furosemide on calcium transport across the cortical thick ascending limb of Henle's loop was studied. Cortical thick ascending limbs were dissected from rabbit kidneys and perfused in vitro. Calcium concentrations in perfused and collected tubule fluids were measured by continuous-flow microcolorimetry. Tubules were perfused at slow flow rates and parathyroid hormone was added to the bathing solution to maximize the difference between perfusate and collectate calcium concentrations. Furosemide simultaneously and reversibly inhibited the lumen-positive transepithelial voltage and calcium absorption. In the presence of furosemide perfusate-minus-collectate calcium concentration difference was smallest under conditions that minimized the residual transepithelial voltage. It was concluded furosemide inhibits calcium absorption in the cortical thick ascending limb of Henle's loop by reducing the lumen-positive transepithelial voltage that drives passive calcium ion transport.