Simulation of coupling between chemical reactions and ion transport in brown adipose tissue using network thermodynamics.

Several cellular events associated with energy turnover in the mitochondria and at the Na+/K+ plasma membrane pump have been formulated in terms of network thermodynamics. The calorigenic role of the Na+/K+ pump is examined in terms of the relationship between the movement of sodium and potassium ions and the chemical reactions involved. In addition, attention is centered on the potential thermogenic role of three mitochondrial pathways involving proton fluxes--namely, one in which protons are transported from the matrix to the intermembrane space; a second in which protons are transferred back into the matrix in conjunction with the synthesis of ATP; and a third wherein protons re-enter the matrix without being coupled to any chemical reaction. (This latter pathway has been delineated by studies on isolated mitochondria and may be unique to brown fat.) At both sites (plasma membrane and mitochondria) the conversion of chemical energy to heat is considered.