Metabolic consequences of hypercapnia in the rainbow trout, Salmo gairdneri: beta-adrenergic effects

The metabolic consequences of external hypercapnia (1% CO2) were assessed in rainbow trout (Salmo gairdneri) in the presence or absence of circulating levels of the beta adrenoceptor antagonist, propranolol. External hypercapnia caused a severe extracellular respiratory acidosis and a less pronounced reduction of hepatic intracellular pH (pHi). pHi was restored to prehypercapnic values after 48 hr of continuous hypercapnia due to elevation of bicarbonate levels. In the presence of propranolol, hypercapnia elicited a pronounced activation of pyruvate kinase (PyK) (measured at both low and high phosphoenolpyruvate (PEP) concentrations) and inactivation of both total glycogen phosphorylase (GPase) and glycogen phosphorylase a (GPase a). In the absence of propranolol, the changes in enzyme activities were significantly reduced (low PEP PyK activity) or totally absent (GPase inactivation). These results suggest that beta adrenoceptor-mediated phenomena offset disruptive effects of hypercapnia on PyK and GPase activities and may be important in the control of gluconeogenesis and glycogenolysis during this acid-base disturbance. The adrenergic effects were not related to modification of hepatic intracellular acid-base status. Hypercapnia induced a rapid depletion of liver glycogen and concomitant hyperglycemia. These effects were not prevented by pretreating fish with propranolol and appeared to be unrelated to changes in GPase a activity. These results suggest that factors other than adrenergic activation of GPase a are involved in the enhancement of liver glycogenolysis.