Studies on intestinal absorption by single-injection technique and continuous measurement of portal vein blood electrolyte concentration and hematocrit in the alert rat.

Whole blood and ultrafiltrate conductivity in the portal vein (as a measure of hematocrit and total electrolyte concentration, Cel, respectively) and arterial pressure of alert rats were measured continuously. Single intraduodenal injections of solutions iso- and hyperosmotic to blood (0.5 or 1 per cent of body weight) produced characteristic changes which were compared with those after the application of water. Whereas H2O and isosmotic passively absorbed substances (sorbose and urea) caused a very variable Cel drop, isosmotic actively absorbed nutrients elicited individually constant but interindividually different (glucose greater than alanine++ greater than arginine) changes due to solute coupled electrolyte free water transport. This was taken as evidence of variable paracellular shunt permeability playing a role in passive, but not in active absorption. The magnitude of Cel changes was related to absorption rate, which was confirmed by behaviour with hypertonic solutions, where osmotic activity in the gut was lost the sooner, the more rapid absorption rate was. Shunt permeability was temporarily blocked by arginine. Hct changes immediately after injections indicated fluid loss from portal vein blood, which could be evaluated in the case of mannitol. The thickness of the absorptive layer, obtained from latency of Cel change after urea, delivered values not exceeding 0,51 mm for the unstirred layer. The latencies after glucose and alanine were usually not much greater than after urea. Cel rise after hypertonic solutions had the same latency as Cel drop after water. Small arterial pressure changes after nutrient solutions, mostly absent after injections of water and NaCl, indicated circulatory effects originating in the gut in association with the former.