Resazurin dye is an in vivo sensor of kidney tubular function

Glomerular filtration rate (GFR) is the main functional index of kidney health and disease. Currently, no methods are available to directly measure tubular mass and function. Here, we report a serendipitous finding that the in vitro cell viability dye resazurin can be used in mice as an exogenous sensor of tubular function. Intravenously injected resazurin exhibited significant plasma protein binding and was found to mainly undergo tubular secretion. Mechanistic studies showed that the blue-colored, weakly fluorescent resazurin is taken up by tubular cells through organic anion transporters, followed by conversion to a highly fluorescent, pink-colored resorufin by mitochondrial and cytosolic reductases, converted to an orange-colored β-d-glucuronide with subsequent efflux into the urine. Here we report a simple method in which the intravenous injection of resazurin is followed by the measurement of fluorescent metabolites in the urine, providing a sensitive readout of tubular function. Three mouse models of acute kidney injury (rhabdomyolysis, bilateral ischemia-reperfusion injury, and cisplatin nephrotoxicity) were tested and the resazurin-based method was able to sensitively detect the loss of tubular function much earlier than the increase in serum creatinine levels. Strikingly, in mice with unilateral ischemia-reperfusion injury and genetic mutation-linked kidney hypoplasia (oligosyndactylism, a genetic model for congenital kidney hypoplasia), the resazurin-based method was able to detect loss of tubular mass and function despite normal GFR levels. Collectively, our findings establish the preclinical utility of resazurin as a sensitive exogenous marker of tubular function and support future examination in larger animals for potential clinical translation.