Limit-cycle oscillations and tubuloglomerular feedback regulation of distal sodium delivery

A mathematical model was used to evaluate the potential effects of limit-cy cle oscillations (LCO) on tubuloglomerular feedback (TGF) regulation of flu id and sodium delivery to the distal tubule. In accordance with linear syst ems theory, simulations of steady-state responses to infinitesimal perturba tions in single-nephron glomerular filtration rate (SNGFR) show that TGF re gulatory ability (assessed as TGF compensation) increases with TGF gain mag nitude gamma when gamma is less than the critical value gamma(c), the value at which LCO emerge in tubular fluid flow and NaCl concentration at the ma cula densa. When gamma > gamma(c) and LCO are present, TGF compensation is reduced for both infinitesimal and finite perturbations in SNGFR, relative to the compensation that could be achieved in the absence of LCO. Maximal T GF compensation occurs when gamma approximate to gamma(c). Even in the abse nce of perturbations, LCO increase time-averaged sodium delivery to the dis tal tubule, while fluid delivery is little changed. These effects of LCO ar e consequences of nonlinear elements in the TGF system. Because increased d istal sodium delivery may increase the rate of sodium excretion, these simu lations suggest that LCO enhance sodium excretion.