M. Imai authored a 1977 study comparing bumetanide and furosemide in isolated thick ascending limbs of rabbit and rat kidneys. Perfusing the isolated renal tubules allowed direct effects on the epithelium to be studied apart from whole-animal changes in circulation or hormone output.[ref]
Applying the compounds inside the tubule reversibly reduced the lumen-positive electric potential difference. Bumetanide also reduced the outward movement of labelled chloride; the dose-response comparison found greater potency than furosemide under the tested conditions.[ref]
Effect of bumetanide and furosemide on the thick ascending limb of Henle's loop of rabbits and rats perfused in vitro by M. Imai. Abstract: luminal application, chloride tracer and dose-response observations
Pairing electrical and tracer measurements matters. An electric potential difference is a collective result of ion transport; tracking labelled chloride adds a chemically specific readout. Reversibility after changing the perfusate helps distinguish an acute transport effect from permanent destruction of the preparation.
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