Glycogen metabolism in dog inner medullary collecting ducts

The regulation of glycogen degradation and synthesis in canine inner medullary collecting ducts (IMCD) was investigated using IMCD tubules suspensions prepared from dog papilla. A small but significant amount of glycogen was found in dog IMCD. Under aerobic condition and especially when no exogenous substrate is available, glycogen breakdown can support IMCD glycolysis for a short period of time. Increasing concentration of exogenous glucose but not lactate was able to reduce and even to suppress (20 mM glucose) the glycogen breakdown. A net synthesis of glycogen was observed only when the endogenous glycogen pool was previously partially or totally depleted. Under anaerobic condition, glycogenolysis was stimulated. The addition of up to 20 mM glucose now reduced but never suppressed this process. Glycogen metabolism responded to variation in the cells energy needs, since the net glycogen breakdown was diminished and glycogen synthesis increased when the cellular ATP turnover was reduced. The reverse effects were observed when the ATP turnover was increased. At all times, glycogen metabolism correlated well with changes in tissue glucose 6-phosphate concentration. The energy requirement of IMCD cells and the availability of alternative energy sources (active mitochondria, exogenous glucose) are therefore capable of eliciting an integrated and appropriate response of glycogen phosphorylase and synthase in IMCD tubules in suspension studied in vitro.