Tracer flux ratios: A phenomenological approach

Abstract

The kinetic behavior of tracer flows across epithelial membranes is examined and attention is called to the condition under which unidirectional tracer flows may be described by first order rate equations. It is shown that the first order nature of the tracer rate equation when combined with simple thermodynamic constraints on tracer flow yields a relation between the ratio of the unidirectional rate coefficients and thermodynamic driving forces. The form of this relation is examined for the case of simple diffusion and in the presence of coupled process.

Mechanism of epithelial lithium transport. Evidence for basolateral Na:Na and Na:Li exchange.

“…The simplest explanation for this behavior is that a portion of the tracer flux proceeded via an obligatory, sodium-sodium exchange mechanism. In exchange flow, acceleration of the "uphill" unidirectional flux is produced by counterflow between the abundant and tracer species of the ion (8,9) .2 Support for this hypothesis is provided by Fig. 2, in which the amiloridesensitive sodium fluxes, D,Jma and, Jms, are plotted vs. the amiloride-sensitive short-circuit current.…”

“…3 were fitted by a straight line with unity slope ; however, the intercept for the fluxes measured into a nominally sodium-free mucosal bath is near zero, whereas that for 3 mM mucosal sodium is^-1 AEq/5 .2 cm2 -h. The relation between dJma and AJsc is thus consistent with the notion that the S-to-M cellular sodium flux consists of two components: a conductive flow that gives rise to the reverse current, and a nonconductive exchange component that is dependent on the presence of mucosal sodium . The anomalous flux ratio that is characteristic of an exchange transport mechanism is a direct result of a counterflow interaction between the tracer and abundant isotopes of the transported species (8,9). Hence, counterflow, i.e., the driving of the net flow of one cation with the free energy in the gradient of another cation, is the most direct test for obligatory cation exchange.…”

“…The definitive criterion for the existence of a cation exchanger is the demonstration of thermodynamic coupling : the donation of energy from the electrochemical potential gradient of one ion to the net flow of another. This coupling is conveniently measured by the flux ratio, which is a function of the total effective thermodynamic force on a transported substance (8,9). Table III shows that the application ofa transmural lithium gradient reduces the S-to-M sodium flux and increases the M-to-S sodium flux, i.e ., the transcellular sodium flux ratio goes from unity in the absence of a lithium gradient to a value of^-30 in the presence of a lithium gradient .…”

Basolateral potassium channel in turtle colon. Evidence for single-file ion flow.

“…2 serves to emphasize the important point that for simple diffusional flow the rate coefficients should be identical (since Vm, is zero) and independent of the magnitude or direction of the transmural potassium gradient (9, which suggests that some nonconjugate force was coupled to tracer flow, which either facilitated tracer flow in the M-to-S direction or retarded tracer flow in the S-to-M direction, or both. In the absence of metabolic coupling and other significant ion gradients, these results suggest positive coupling between the flows of labeled and unlabeled potassium (9,11). A graphic demonstration of this positive coupling is provided by Fig.…”

Potassium transport across the frog retinal pigment epithelium

“…These coefficients, which have units of (cm2hr) -1, are defined as the unidirectional tracer flux per total amount of tracer on the "hot" side (Dawson, 1977). The unidirectional fluxes in Table 1A, which have units of nM/cm 2 hr, are obtained by multiplying the rate coefficients by the amount of abundant species (2 mM x 1.8 ml or 3.6 gM).…”