AJP - Renal Physiology, Vol 250, Issue 6 1107-F1118, Copyright © 1986 by American Physiological Society
Role of topology in bioenergetics of sodium transport in complex epithelia
E. G. Huf and D. C. Mikulecky
There has been some controversy as to whether in sodium-transporting
epithelia (e.g., frog skin, toad urinary bladder) the Na-O2 ratio is
independent of the net rate of transepithelial sodium transport or remains
constant over a wide range of transport rates. This computer simulation
study shows that both views are defensible, depending on whether one wishes
to exclude or include "static head" energy in the calculations. This energy
arises from intramembrane sodium recirculation, as shown here when applying
a multicompartment epithelial membrane model. Assuming reasonable kinetic
parameters of a transport model whose sodium transport rate is varied over
a wide range by a step-by-step simulated amiloride action, the computations
have shown the following. When static head energy (O2 consumption) is
excluded from the calculations, the Na-O2 ratio is constant over a wide
range of transepithelial sodium flux, up to the tested value of 20 neq X
cm-2 X min-1, a "normal" value found in frog skin. The Na-O2 ratios were
19.4 and 28.7 in low- and high-sodium models, respectively. If the static
head energy is included in the calculations, the Na-O2 ratios increase with
increasing transport rate from zero values to values up to 15.7. These data
are in good agreement with laboratory results, as are derived data on
phenomenological coefficients and thermodynamic coupling coefficients (LNa
= 80, 128; LNa,r = 4.4; Lr = 0.27, 0.58; q = 0.50, 0.90, depending on the
chosen model parameters).
