Pyrimidine synthesis in tissue culture

Abstract— Myelinated cerebellar tissue culture (organ culture) was used to assess the salvage and de novo pathways of pyrimidine synthesis in mammalian brain. Radioactive orotic acid and carbamyl aspartic acid were readily incorporated into UMP and into perchloric acid-insoluble RNA. The incorporation was effectively blocked by azauridine. Neither radioactive sodium bicarbonate or citrulline was incorporated into UMP or blocked by azauridine. [³H]Uridine, on the other hand, rapidly entered the cultures, was incorporated into UMP and perchloric acid-insoluble material, and was partially inhibited by azauridine. The failure to demonstrate activity of carbamyl phosphate synthetase suggests the potential importance of the salvage pathway and the likely dependence of the brain upon exogenous and endogenous pyrimidine precursors.     

Microelectrode Studies of Dog's Gastric Mucosa

In anesthetized dogs, the potentials in the mucous coat and gastic cells were measured with microelectrodes. In the secreting stomach, with isotonic saline in contact with the mucosal surface, the orientation of the initial change in potential difference (PD) was often the same as that of the liquid junction potential between gastric juice and saline (the microelectrode became negative to a reference electrode in the saline) but the magnitude of the change was never more than 11 mv. On the basis of this finding an explanation is offered for the observation that in the secreting stomach replacing isotonic saline with isotonic HCl as the bathing fluid on the mucosal surface, results in a change in the serosal to mucosal PD of only 19 mv, which is 40% less than the liquid junction potential between gastric juice and saline. In the surface epithelial cells of both resting and secreting stomach, multiple levels of potentials were found. For the secreting stomach, the resistance between the interstitial fluid of the pit region and the fluid on the mucosal surface was 55 ohm cm², determined as the change in PD per unit of applied current across stomach. The implications of these findings are discussed with reference to the separate site theory of HCl formation.