The effects of the 'loop diuretics' bumetanide, furosemide and piretanide on fluid reabsorption from the rat proximal convoluted tubule have been tested to assess whether a Na(+)-Cl- cotransport mechanism can support fluid reabsorption from this nephron segment. Proximal convoluted tubules were perfused in vivo at 25 nl min-1 with chloride Ringer solutions containing [3H]inulin. Fluid reabsorptive rate (Jv) was 2.05 +/- 0.07 nl mm-1 min-1 (n = 115) from control chloride Ringer. Bumetanide at 10(-6) and 10(-5) M reduced Jv by about 40%. Bumetanide at 10(-7) M, furosemide (10(-4) and 10(-5) M) and piretanide (10(-4) and 10(-5) M) had no effect on Jv. At 10(-3) M, furosemide and piretanide reduced Jv by about 45%. The results show that luminal application of loop diuretics lowers fluid reabsorption from the proximal convoluted tubule and the order of potency for inhibiting reabsorption is bumetanide greater than furosemide identical to piretanide. The specificity of the loop diuretics at their effective concentrations needs to be confirmed, however, before attribution of a role for Na(+)-Cl- co-transport in the support of proximal tubule fluid reabsorption.

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