Some observations on the determination of extracellular fluid volume of jejunal tissue using [3H]inulin and [14C]inulin

The objective of the present study was to examine the nature of equilibration of [3H]-inulin and [14C]inulin in the jejunal tissue in vitro. Rings of everted hamster jejunum were incubated at 37 °C in Krebs Ringer bicarbonate solution containing 10 mM glucose, tracer amounts of [14C]inulin, and tracer amounts of [3H]inulin from one of the two lots (lot X and lot Y) tested. The incubations were carried out for 5, 10, 20, 30, 45, or 60 min. One lot of [3H]inulin (lot X) provided an estimate of the extracellular (EC) fluid volume which, at all periods of incubation, was comparable with that provided by the [14C]inulin. In contrast, the estimate of EC fluid volume obtained from the other lot of [3H]inulin (lot Y) was consistently higher than that obtained from the [14C]inulin and increased linearly with the period of incubation. Because, with [3H]inulin of lot Y, the calculated intracellular (IC) fluid volume decreased linearly with the period of incubation, and because this was not the case with the [14C]inulin or with the [3H]inulin of lot X, it would appear that [3H]inulin of lot Y failed to equilibrate within the tissue water, while the other lot of [3H]inulin (lot X), as well as [14C]inulin, did. Although the EC fluid volume obtained from [14C]inulin (as well as that from [3H]inulin of lot X) increased progressively, the rate of increase during the first 30 min was considerably higher than that during the last 30 min of incubation. The progressive increase in the [14C]inulin space, however, was accompanied by a corresponding increase in the total water content as well as in the calculated IC fluid content of the tissue. Since the IC fluid volume as percentage of total water content after 5 min of incubation was not significantly different from that after 60 min of incubation, it would appear that [14C]inulin equilibrates within the EC fluid after 5 min, and that the progressive increase in the calculated EC fluid volume is the result of a corresponding increase in the EC fluid (containing [14C]inulin) content of the tissue.