Intestinal electrophysiology and transmural ion transport in freshwater prawns

Abstract

A steady-state transmural potential difference of 1.63 +/- 0.26 mV (mean +/- SE, lumen negative) is recorded across the isolated, perfused intestine of the freshwater prawn, Macrobrachium rosenbergii, when saline approximating hemolymph in composition is present on both surfaces of the preparation. The magnitude of this potential is a hyperbolic function of bilateral K concentration. It is abolished by metabolic poinsons (NaN3 and iodoacetate) or ouabain and appears to result largely from the combined net transmural fluxes of Na, Cl, and K. Net fluxes of Na (1,49 +/- 0;30 mu mol . cm-2 . h-1) and Cl (0.72 +/- 0.22) are absorptive, whereas that of K (0.47 +/- 0.11) IS SECRETORY. Bilateral absence of Na abolishes net K secretion, whereas bilateral addition of ouabain (0.5 mM) eliminates net Na absorption. One-third of net K secretion appears to be transcellular and coupled to the net cellular transfer of Na in the opposite direction. The remaining component of K secretion can be attributed to paracellular cation flow responsive to transmural PD, Transmural diffusion potentials generated in the presence and absence of metabolic poisons provide the following passive permeability properties of the tissue: PK:PNa:PCl = 5.1:1.1:0.5. The symmetrical nature of these diffusion potentials implies the occurrence of a single rate-limiting barrier to ion flow with the above characteristics--probably the cell junction (septate desmosome) and paracellular channel. A model for transepithelial ion transport is presented where transmural potential difference is largely a result of an apparent 9 Na/1 K basolateral cationic exchange.