SIMILARITIES AND DIFFERENCES IN OXYNTICOPEPTIC CELL ULTRASTRUCTURE OFONE MARINE TELEOST, GADUS-MORHUA AND ONE FRESH-WATER TELEOST, ONCORHYNCHUS-MYKISS, DURING BASAL AND HISTAMINE-STIMULATED PHASES OF ACID-SECRETION

Oxynticopeptic cells (OC) from rainbow trout, Oncorhynchus mykiss, and Atlantic cod, Gadus morhua, were studied during basal acid secretion and during stimulation with histamine. An in vitro method for measurin g acid secretion from isolated stomach mucosa was used. The OC ultrast ructure was studied using electron microscopy. During basal acid secre tion, the OC had short apical microvilli, an extensive tubulovesicular network, large electron-dense vesicles and many mitochondria. Stimula tion of acid secretion with histamine and ultrastructural changes in t he OC occurred simultaneously. The acid stimulation promoted large inc reases in the length of microvilli, a decrease in the size of the tubu lovesicular network and an increase in the diameter of the glandular l umen in cod. Large vacuoles were found to characterize the histamine-s timulated OC. These were located close to the apical membrane and some times opened into the lumen. During stimulation, electron-dense vesicl es were often observed close to the apical membrane and the large vacu oles. Sometimes these vesicles were observed inside the glandular lume n and/or the large vacuoles. It is concluded that the ultrastructure o f the OC in the saltwater cod and the freshwater trout show similar ch aracteristics as OC in other animal groups both during basal and high rates of acid secretion. In addition, vacuoles were found in both spec ies OC during basal rates of acid secretion and they significantly inc reased in size during high rates of acid secretion. We propose that th e OC vacuole have a part in the osmoregulation, since the increase was tenfold in cod but less then onefold in trout, this difference was st atistically significant.