55.5 Saturday, Jan. 5 Does glucose metabolism limit rainbow cardiac performance in rainbow trout at high temperatures? RODNICK, KEN; Idaho State University email@example.com
Fundamental questions remain regarding the limits and regulation of cardiac function in fish challenged with elevated water temperature. While the use of glucose for energy metabolism - especially during hypoxia - is well established in the fish heart, it is unclear how increased metabolic needs of cardiomyocytes are supported. Our recent work suggests that increases in cardiac output to meet the fish's overall metabolic demands at high temperatures are met primarily through an increase in heart rate, because tachycardia is an unavoidable physiological response. For quiescent rainbow trout, an acute increase in water temperature to near lethal values (from 14 to 24°C at 2°C h-1) raised heart rate in a linear fashion (from 60 beats min-1 to 125 beats min-1, Q10= 2.1). However, in isolated, electrically-stimulated ventricle strips, the presence of exogenous glucose did not support or improve contractile function at elevated temperatures or heart rates. An increase in temperature from 14 to 24°C also did not affect the uptake (transmembrane transport and intracellular phosphorylation) of 2-deoxyglucose (Q10 = 1.1-1.2) by quiescent ventricle strips, providing evidence for a lack of thermal sensitivity over this range and a limited ability to support increased energy metabolism. We also explored the possibility that endogenous glycogen and glycogenolysis play significant roles in supporting contracting cardiomyocytes. In contrast to the uptake of extracellular glucose, the thermal sensitivities of ventricular glycogen phosphorylase (Q10 = 1.9-2.2) and acid alpha glucosidase (Q10 = 1.8) between 14 and 24°C suggest greater enzymatic capacities to increase intracellular glucose metabolism and an important function of glycogen for cardiac energy production in rainbow trout at elevated temperatures.