HINDLEG MUSCLE ENERGY AND SUBSTRATE BALANCES IN COLD-EXPOSED RATS

Rats chronically cannulated in the carotid artery and the muscular bra nch of the femoral vein were subjected to a cold (4 degrees C) environ ment for up to 2 h, The changes in blood how (measured with Sc-46 micr ospheres) and arterio-venous differences in the concentrations of gluc ose, lactate, triacylglycerols and amino acids allowed the estimation of substrate (and energy) balances across the hindleg, Mean glucose up take 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 tria cylglycerols gave place to a massive release following exposure, The m easurement 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. Thr ee distinct phases were observed in hindleg substrate utilization, (1) The onset of shivering, with the use of glucose/glycogen and an incre ase in lactate efflux, Lipid oxidation was practically zero (respirato ry quotient near 1), but the uptake of triacylglycerols from the blood remained unchanged, (2) A substrate-energy shift, with drastically de creased 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-home ostasis setting for prolonged cold-exposure, with maintenance of muscl e energy and heat production based on triacylglycerol utilization and efflux from the hindleg (muscle plus skin and subcutaneous adipose mas ses) contributing energy to help sustain heat production by the core o rgans and surrounding brown adipose tissue.