Sodium and glucose transport across the in vitro perfused midgut of the blue crab, Callinectes sapidus Rathbun

Abstract

The principal objectives of the present study are: (1) to investigate sodium transport across the midgut as well as the hindgut of Callinectes sapidus and the functioning of these two regions of the gut in osmotic and ionic regulation of the animal, and (2) to elucidate the mechanism of glucose absorption across the midgut of C. sapidus and to assess its role in nutrient absorption. The transport processes across the gut tissues were studied with an in vitro perfusion technique. I demonstrated a net mucosal to serosal sodium transport across the midgut of C. sapidus. This net flux was within the same order of magnitude as the fluxes of isolated gut tissues of many other animal species. The flux displayed saturation kinetics and was ouabain-sensitive. The flux did not vary with environmental salinity, thus providing no evidence for an active regulatory role of the midgut in sodium balance. A reduction of passive permeability with salinity, however, was suggested. There was no measurable net sodium transport across the hindgut of the animal. The midgut of C. sapidus was also capable of net transmural glucose absorption. The net flux was considerably less than the fluxes reported in mammalian intestine. The unidirectional mucosal to serosal glucose flux was depressed by metabolic inhibitors, the absence of mucosal Na+, and the presence of mucosal phlorizin or serosal ouabain. The flux was also decreased by the presence of mucosal D-galactose, but not D-fructose or L-glucose. The net glucose flux followed Michaelis-Menten kinetics. The mucosal glucose uptake by the midgut was composed of two processes: a sodium-dependent, saturable component, and a sodium-indenpendent, non-saturable counterpart. The kinetic characteristics of the uptake suggest that the midgut does not play a major role in total nutrient absorption. Nutrients taken up by the midgut may be largely utilized to provide energy for other physiological functions served by this tissue. Thin layer chromatographic analyses showed that whereas most glucose appeared as phosphorylated forms upon entering the midgut, free glucose was translocated across the tissue. These results suggest that the mucosal and serosal membrane transport processes are coupled to phosphorylation-dephosphorylation mechanisms, respectively. An alternative hypothesis is the presence of a high-affinity glucose carrier in the serosal border of the midgut.