We investigated the mechanism of action of a novel 'high ceiling' diuretic, M17055, in in vivo clearance studies with anesthetized dogs during water diuresis and in vitro microperfusion studies of isolated rabbit renal tubules. In the clearance study, intravenous infusion of M17055 (1 mg/kg per h) decreased free water clearance and increased urinary excretion of Na+ and Cl- to a greater extent than did a maximum dose of furosemide (30 mg/kg per h). With the maximum dose of furosemide, an additional dose of M17055 or hydrochlorothiazide resulted in additional suppression of free water clearance. These results indicate that M17055 has some additional mechanisms of action in the distal nephron. In isolated rabbit cortical thick ascending limb of Henle's loop, M17055 applied to the lumen decreased the lumen positive transepithelial voltage at concentrations over 10(-6) M and suppressed the lumen-to-bath 36Cl- flux at 10(-5) M. In the connecting tubule, M17055 added to the lumen suppressed lumen negative transepithelial voltage in a concentration-dependent manner in a range from 10(-4) to 10(-3) M. The effect of M17055 on transepithelial voltage was also observed in the distal convoluted tubule and cortical collecting duct. Moreover, 10(-3) M of M17055 in the lumen significantly decreased the lumen-to-bath 22Na+ flux in the cortical collecting duct. From these observations, it appears that M17055 acts not only on the thick ascending limb of Henle's loop but also on the distal segments via inhibition of electrogenic Na+ transport.