Allosteric cooperativity during intestinal cotransport of sodium and chloride in freshwater prawns.

Coupled influxes of sodium and chloride across the mucosal border of a freshwater prawn intestine were sigmoidal functions of luminal ion concentrations, indicative of a cooperative allosteric transport process. This process had a higher affinity for Cl (KCl = 94 mM) than for Na (KNa = 155 mM), maximally transported twice as much cation as anion (JNa max = 1.6; JCl max = 0.75 mumol-cm-2-h-1), and exhibited identical Hill interaction indices for both ions nNa = 3.4; nCl = 3.5). The suggestion is made that this cooperative carrier mechanism may be regulatory, maintaining relatively constant luminal ion concentrations which, in turn may facilitate ion-dependent absorption of non-electrolytes.