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Fluorescence staining characteristics of "large vacuoles," i.e. vacuoles ranging up to almost cell size, were studied in suckling rats and pigs. In the distal epithelium of the small intestine of suckling rat, yellow autofluorescence and accumulation of orally administered FITC-dextran were observed in the supranuclear vacuole. In both species the weakly basic amino dye acridine orange (AO) stained the nuclei at neutral pH bright yellow-green and the transport and digestive vacuoles bright red or orange. It is concluded that trapping and accumulation of the dye (red shift) were due to the acidity of the vacuolar interior. Assessment of the vacuolar pH in rat enterocytes is in agreement with published data on lysosomal pH values. Acidic buffers, lysosomotropic and destructive agents, or illumination with bright light induced irreversible fading of AO-stained vacuoles; the color of the porcine transport vacuoles was the most labile. This fading was used to differentiate vacuoles from other structures, e.g., vacuolar inclusion bodies and goblet cells. In suckling rat, staining characteristics of the gut epithelium changed on Days 19 and 20 of postnatal age. Detection of acidity in the distal (digestive) vacuoles supports the lysosome-like nature of their function. They appear to constitute an auxiliary, intracellular digestive system for the young animal. However, the function of acidity in the non-digestive transport vacuoles of newborn pig is unclear.

A panel of orally administered lectins (100 mg/kg b.w.) of different binding specificities was tested for suppression of voluntary food consumption in prefasted rats. PHA isolectins (Phaseolus vulgaris) and RPA-I (Robinia pseudoacacia) were found to exert a marked and significant effect, but two other gut-binding lectins, i.e. SBA (Glycine max) and WGA (Triticum vulgar) and several non-binding lectins were ineffective. In cannulated rats PHA infused into the duodenum induced food suppression, i.e. binding of the lectin to the mouth or stomach was unnecessary. Suppression of food consumption lasted through the whole nocturnal feeding period, control (BSA) and experimental (PHA) curves of cumulative food consumption showed a V-like divergence. Suppression by PHA or RPA-I showed very similar time courses, but a long-lasting inhibition of gastric emptying was only observed in the RPA-treated animals. Intraperitoneally administered lectins suppressed food consumption much more effectively than the oral ones, whereas Galanthus nivalis agglutinin (ONA) had little or no effect. It is concluded that lectins can be used as effective tools for the modulation of food consumption and gastric emptying in experimental animals.