No abstract

Effects of amiloride analogues on Na transport were studied in isolated skins of the frog Rana ridibunda. The pattern of structure-activity relationship of these compounds showed that both the -NH2 group at position 5 and Cl at position 6 of the pyrazine ring of the amiloride molecule were important for their biological activity. The paramount role of the groups at position 5 was further demonstrated by the striking properties of an analogue resulting from dimethylation of that -NH2 group. A stimulation of Na transport, opposite to the effect of amiloride itself, was observed in this instance. The increase in Na transport could already be seen at 10(-6) M and was equivalent to the measured increase in Na influx, reversible, dose-dependent, and additive to the natriferic action of oxytocin. Such characteristics resemble those reported with "external" agents like propranolol and La3+. Furthermore, mutual inhibition was observed between the stimulatory effects of this analogue and those of propranolol or La3+. These results suggest that the analogue may be considered as another "external" agent acting at sites of the external membrane distinct from those activated by cAMP but similar to the Ca sites described by Herrera and Curran (Herrera, F.C., Curran, P.F. 1963. J. Gen. Physiol. 46:999).

In a composite membrane with heterogeneous channels, prevention of net volume flow with hydrostatic pressure differences and/or impermeant osmotic solutes may induce positive isotope interaction (coupling of isotope flows) consequent to circulation of volume flow. The permeability coefficient for net flow will then exceen the tracer permeability coefficient. A permeant osmotic solute will induce either positive or negative isotope interaction, according to whether membrane heterogeneity is more marked for the test solute or the osmotic solute, respectively. Thus membrane heterogeneity may account for phenomena commonly attributed to "single file diffusion". For sufficiently small flows the general flux ratio relationship for homogeneous membranes will continue to apply.

Summary. The thermodynamic formulation of isotope interaction (coupling of abundant and tracer isotope flows) has been tested in a highly permselective anion exchange membrane in the absence of significant electroosmosis. A previous study of C1-permeation has now been extended to include permeation of I-, Acetate, and SO 2-in different bath concentrations, with the use of both electrical and chemical driving forces. The flux ratios were "abnormal" according to the usual criteria for simple passive flow, but were closely predicted by the theoretical expression incorporating the influence of isotope interaction. In the absence of coupled flows of other chemical species the extent of isotope interaction can be determined either from the flux ratio or from the measurement of a single unidirectional flux at two settings of the electrochemical potential difference. Direct evidence of negative isotope interaction was presented.Despite the common use of radioactive tracers in the study of biological transport processes, fundamental anomalies interfere with the interpretation of the data obtained by these means. Thus, it is recognized that the evaluation of permeability by the self-exchange of a tracer may give a very different result from that derived from measurements of net flow. Similarly, there are difficulties in the use of the "flux ratio" to evaluate the forces promoting net transport [4,11,12].The above problems have been analyzed in terms of a variety of models [4,11,12]. Kedem and Essig have provided a more general thermodynamic analysis. In this formulation both of the above anomalies are attributable

Different biological effects of Ag+ (10(-4) M) were found depending on its presence in the outer or the inner solution bathing the frog skin. A marked increase in the electrical conductance and an interference with the action of oxytocin and amiloride were found only when Ag+ was added to the outer solution. Results suggest that Ag+ affects several transport processes, in particular the permeability of the Na entry pathways.

It is often not possible to evaluate a permeability coefficient for net flow P from the small flows produced by physiological gradients of concentration or electrical potential. The common use of a tracer permeability coefficient P-x for this purpose, under the assumption that P-x = P, requires that the species be transported passively, and that there be no significant coupling between its flow and that of other chemical species, and between the flows of its tracer and abundant isotopes (isotope interaction). These conditions are often not satisfied. However, for passive transport in the absence of coupling of flows of different chemical species the measurement of tracer flow at two values of electrical potential difference evaluates (P-x/P) and thus P. In the presence of coupling of flows of different chemical species, although these measurements no longer evaluate P, they evaluate the partial conductance G. A graphical method of evaluating (p-x/P), P, and G is presented.