Parameter Estimation for Models of NH4+ Transport by the Renal Na‐K‐2Cl Cotransporter

Abstract

An optimization algorithm was used to compute parameter values for kinetic models of NH4+ transport by the renal Na‐K‐2Cl cotransporter (NKCC2). The optimization method finds model off‐binding and translocation rate constants by minimizing the difference between model unidirectional fluxes and published data on NH4+‐ dependent 86Rb+ uptake by the NKCC2 cotransporter in transfected Xenopus oocytes. By using different symmetry assumptions, which reduces the number of parameters, five different models were optimized. The computed parameters for each model yielded fluxes consistent with the experimental curves, and the fittings of the five models exhibited residual errors that were not significantly different from each other. However, a systematic exploration of the parameter space revealed that, in some models, different parameter sets yield essentially identical fits to the data. With the identified parameters, the NKCC2 models show K+/NH4+ exchange, and enhancement of Na+ uptake by NH4+ binding when external [K+] is low. Further, for the A, B, and F isoforms, the optimization method identified parameter sets that differ only in off‐binding affinities, a result that is consistent with a published mutagenesis analysis of the molecular and structural bases for differences in 86Rb+ uptake between isoforms.This research was supported in part by NIH grants SC1GM084744 and DK042091.