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Functional and morphological properties of the in vitro frog gastric mucosa were studied during and after exposure to very hypotonic (approximately 25 mOs M) solutions. Within 20 min the acid secretory rate decreased to zero, but it returned to normal levels after isotonic fluids had been restored. The potential difference (PD) dropped within the first minutes after the exposure to hypotonic solutions, and became inverted. Following the return of isotonic conditions the PD increased to levels higher than in the controls. The electrical resistance increased about 10-fold during the hypotonic period, but decreased to near normal values when isotonic conditions were restored. By light and electron microscopy the cells of the hypotonic mucosae appeared greatly swollen, and the alterations were assessed by morphometric methods. The gland lumina were almost obliterated, and the lamina propria was reduced to about 63% of its former volume. After the return to isotonic conditions normal morphology was restores. It is conceivable that the great increase in resistance duroing the hypotonic period was caused by the occlusion of the gland lumina. Quantitative analyses of the Na, K, and Cl tissue concentrations indicated a large loss of these ions during the hypotonic state. Presumably the epithelial cells in the hypotonic mucosae avoid bursting by rapidly letting large numbers of ions exit, which results in a celulular osmolarity close to that of the bathing fluids.

Four enzymes were studied in the rat colon during perinatal development, namely, carbonic anhydrase, beta-D-galactosidase, and alkaline and acid pphosphatase. Quantitative analyses of tissue extracts revealed a peak in carbonic anhydrase activity 10 days after birth and histochemically most of the enzyme was found at the surface of the colonic mucosa. A similar localization was found for beta-D-galactosidase, which displayed a peak of activity in the 1-day-old rats. Alkaline phosphatase activity reached its maximum 5 days after birth, and by electron-microscopical histochemistry this enzyme was traced to the microvillous border, supranuclear vesicles and tubules, meconium bodies and the lateral surfaces of the principal cells. Acid hosphatase activity was also found in the microvillous border, in the supranuclear vacuoles and meconium bodies and in the Golgi apparatus. In the infant rat colon all these enzymes had higher activities and sometimes a different localization than in the adult rat, which indicates functional differences between the young and the adult animals. An example is the absorption of proteins. Peroxidase and insulin, given as enemas, were absorbed in the infant rat colon, but this capacity was much reduced after weaning.