Sodium Movements Across the Vascularly Perfused Anuran Small Intestine and Colon

Abstract

SUMMARYValues of unidirectional Na fluxes measured across the vascularly perfused small intestine of three anuran species are higher than those found in other preparations in vitro. In Rana ridibunda and R. pipiens no net movement of Na across the small intestine can be detected. In contrast, the unidirectional fluxes of Na across the colon of R. ridibunda and R. pipiens are lower than across the small intestine and a significant net absorption is found. Apparent loading and unloading pools for Na within the small intestine, as measured with tracer Na under standard experimental conditions, consist largely ofextracellular Na presumably within the bulk phase of the lumen. The size of these pools can be greatly reduced by the rapid addition to or removal from the lumen of tracer. The loading pool appears to occupy not more than about 9% of the total tissue water, equivalent to about 20% of the extracellular water of the tissue. The washout of tracer Na from preloaded small intestine into the vascular bed is bi-exponential and appears from a pool, or pools, of apparent greater size than that of the loading pool. The results show that Na can move very rapidly across the small intestine and suggest that a high proportion of this movement occurs possibly via paracellular shunt pathways.

INFLUENCE OF VASCULAR AND LUMEN FLOW ON SODIUM MOVEMENTS ACROSS ANURAN INTESTINE IN VITRO

“…The salient details for establishing vascular perfusion of the whole anuran small intestine are given by Boyd et al (1975). A modification to this procedure (Parsons & Wade, 1982) is to perfuse a short segment of small intestine (about 5 cm) which, in all the experiments described here, was taken from a region just distal to the duodenum. Ligation of all vasculature supplying the small intestine allows the colon alone to be perfused.…”

“…Use of one or two Na isotopes (24Na alone, or with 22Na) allowed unidirectional Na fluxes to be measured singly or simultaneously (see Parsons & Wade, 1982). This involved switching from tracer-free perfusate, held in one reservoir, to tracer-containing perfusate, held in another, and pumping in single pass through the cis side of the preparation, collecting fractions of effluent appearing on the trans side until isotopic equilibrium had been achieved (usually at least 30 min).…”

“…Measurement of extracellular space Theory. A non-destructive, non-steady-state method for measuring the extracellular space within the wall of the small intestine was used (Parsons & Wade, 1981). Following the introduction of an extracellular marker, ['4C]sucrose, into the vascular bed during an experiment, the initial changes in concentration of the marker in the venous effluent are followed.…”

“…We recently described some observations on Na movements across the vascularly perfused small intestine and colon of the frog (Parsons & Wade, 1982). It was shown that Na moves rapidly in either direction across the small intestine at rates equal in magnitude, but up to fifteen times higher than across other preparations of isolated intestine with vascular flow absent.…”

“…Increasing the vascular flow rate increases both of the unidirectional fluxes of Na across the intestine and these increases may be linked to an increase in the apparent extracellular fluid volume of the intestinal wall. Some of this work has been reported to the Physiological Society (Wade, 1981). In these studies a preparation of vascularly perfused intestine of the frog was used.…”