Torasemide (1-isopropyl-(4-(3-methylphenylamino)pyrid-3-yl)urea) is a new diuretic. The present study examines the effects of this substance in the isolated perfused thick ascending limb (TAL) of mouse and rabbit kidney. In cortical TAL segments of the rabbit, torasemide added to the lumen perfusate led to a fall in equivalent short circuit current ( = transepithelial voltage divided by transepithelial resistance, which corresponds to the rate of chloride reabsorption) with a half maximal inhibition concentration of 3 X 10(-7) mol/l. This effect was accompanied by a hyperpolarization of the luminal and basolateral membrane from -78 to -81 mV and from -72 to -81 mV, respectively. A similar hyperpolarization of both membrane voltages was also observed in medullary TAL segments of the mouse. Torasemide, added to the basolateral perfusate of cortical TAL segments of the rabbit, also inhibited the equivalent short circuit current. However, 3 X 10(-5) mol/l were necessary for a half maximal inhibition. The fall in the equivalent short circuit current was accompanied by a significant increase in transepithelial resistance from 34 to 38 omega cm2, by an increase in the fractional resistance of the basolateral membrane, and by a hyperpolarization mainly of the basolateral membrane. Again, similar results were obtained in the medullary TAL segment of the mouse. The strong inhibitory effect of torasemide from the lumen side can be explained by an interference with the Na+ 2Cl-K+ carrier in the luminal membrane. In fact, torasemide apparently is structurally related to furosemide.(ABSTRACT TRUNCATED AT 250 WORDS)