Kinetics of the co-transport of phenylalanine and sodium ions in the guinea-pig small intestine. II. Sodium fluxes and flux ratios.

The validity of a general non-compulsory model for the description of the co-transport of sodium and phenylalanine in the small intestinal mucosa of the guinea-pig has been examined by measuring the influx of sodium and the flux ratios. The simultaneous influxes of sodium and phenylalanine have been determined by incubating intestinal tissues at a fixed sodium and variable phenylalanine concentrations. The experiment was repeated at a number of sodium concentrations, and a straight-line relationship between the fluxes was always obtained. The slope of this line, the flux ratio, is dependent on the sodium concentration; the function is biphasic inasmuch as it rises with sodium concentration at low external sodium, but decreases when the sodium concentration is raised above a level of about 70 mM. This response is compatible with the model under examination. No saturable component of sodium influx could be detected in the absence of phenylalanine, but the values of this influx corresponded with those predicted from the flux ratio experiment. In the presence of phenylalanine, sodium influx could be resolved into saturable and non-saturable components, and the Kt for sodium influx via the saturable mechanism agreed with that predicted from the constants derived from measurements of phenylalanine influx. The maximal velocity for sodium influx was similar to the maximal velocity for phenylalaline influx. Equations were derived to examine the behaviour of sodium influx as a function of the external phenylalanine concentration, and a Kt for phenylalanine was deduced which agrees closely with that obtained by studying phenylalanine fluxes directly. These results provide evidence in favour the applicability, in this species, of a model involving no compulsory pathway for the formation of a ternary complex between the carrier, a sodium ion and the phenylalanine molecule. Examination of the literature suggests that in other species different models may describe more accurately this co-transport mechanism.