HINDLEG MUSCLE ENERGY AND SUBSTRATE BALANCES IN COLD-EXPOSED RATS

Rats chronically cannulated in the carotid artery and the muscular branch of the femoral vein were subjected to a cold (4 degrees C) environment for up to 2 h, The changes in blood how (measured with Sc-46 microspheres) and arterio-venous differences in the concentrations of glucose, lactate, triacylglycerols and amino acids allowed the estimation of substrate (and energy) balances across the hindleg, Mean glucose uptake was 0.28 mu mol min(-1), mean lactate release was 0.33 mu mol min(-1) and the free fatty acid basal release of 0.31 mu mol min(-1) was practically zero upon exposure to the cold; the initial uptake of triacylglycerols gave place to a massive release following exposure, The measurement of P-O2, P-CO2 and pH also allowed the estimation of oxygen, CO2 and bicarbonate balances and respiratory quotient changes across the hindleg, The contribution of amino acids to the energy balance of the hindleg was assumed to be low. These data were used to determine the sources of energy used to maintain muscle shivering with time. Three distinct phases were observed in hindleg substrate utilization, (1) The onset of shivering, with the use of glucose/glycogen and an increase in lactate efflux, Lipid oxidation was practically zero (respiratory quotient near 1), but the uptake of triacylglycerols from the blood remained unchanged, (2) A substrate-energy shift, with drastically decreased use of glucose/glycogen, and of lactate efflux; utilization of triacylglycerol as practically the sole source of energy (respiratory quotient approximately 0.7); decreasing uptake of triacylglycerol and increased tissue lipid mobilization, (3) The onset of a new heat-homeostasis setting for prolonged cold-exposure, with maintenance of muscle energy and heat production based on triacylglycerol utilization and efflux from the hindleg (muscle plus skin and subcutaneous adipose masses) contributing energy to help sustain heat production by the core organs and surrounding brown adipose tissue.