Juxtamedullary and superficial nephron phosphate reabsorption in the cat

The cat kidney possesses discrete venous drainage systems for superficial and deep (juxtamedullary) portions of the renal cortex. Arteriovenous (A-V) extraction and micropuncture studies may thus be used to evaluate PO4 transport in the two nephron populations. Determinations of A-V extraction showed whole kidney percentage of fractional PO4 excretion (%FEPO4) was 29.04 +/- 2.01%, percentage of fractional delivery of PO4 (%FDPO4) from the superficial nephrons was 42.94 +/- 2.42%, and that from deep nephrons was 22.35 +/- 2.05%. The %FDPO4 from superficial nephrons was significantly greater than %FEPO, in urine (P < 0.001) and than %FDPO4 from deep nephrons (P < 0.001). Micropuncture studies showed %FDPO4 from late distal tubules was 41.63 +/- 3.61%, while %FEPO4 in urine was 30.24 +/- 2.22%. Infusion of acetazolamide increased both %FEPO4 in urine and %FDPO4 from deep nephrons (A-V extraction studies), but had no effect on %FDPO4 from either superficial nephrons (A-V extraction) or from late distal tubules (micropuncture), thus abolishing the differences between superficial and deep compartments. Nephron heterogeneity apparently exists for superficial and juxtamedullary PO4 reabsorption in the cat. Acetazolamide increase %FEPO4 in urine primarily by reducing deep nephron PO4 reabsorption.