Physiologically-Induced Loose Coupling of Brown-Adipose-Tissue Mitochondria Correlated to Endogenous Fatty Acids and Adenosine Phosphates

Abstract

Loose-coupling of young guinea-pig brown-adipose-tissue mitochondria, expressed as ADP-released respiratory control has been studied as a function of the time of exposure of the animals to a cold environment (+6 °C). The acceptor control was found to decrease with time of cold exposure. The response to 0.1 mM ADP was characterized by a slowly accelerating respiratory rate followed by a slow transition into resting state. A second addition of ADP produced a normal transition from state 4 to state 3 irrespective of the time of cold exposure. Thus, coupling tightened following the first addition of ADP. This mitochondrial respiratory pattern was fully developed after exposing the animals to cold for about 4 days, but reverted to the one exhibited by control animals where GDP, GTP, or ATP was included in the medium, or where the tissue had been obtained from cold-exposed animals returned to a thermoneutral environment (>21 °C). At the tissue level, the observed alterations in mitochondrial respiratory patterns as induced by cold exposure could be related neither to changes in the contents of non-esterified fatty acids nor of adenosine phosphates. The contents of non-esterified fatty acids of freshly prepared brown-adipose-tissue mitochondria were 39.9 ± 30.2 nequiv/mg protein for cold-exposed animals and 57.5 ± 30.9 for controls. Centrifugation of the mitochondria through a composite ficoll-serum albumin gradient reduced these amounts to 4.3 ± 2.5 and 11.0 ± 3.5 nequiv/mg protein, respectively. Elicitation of respiratory control in mitochondria so treated again required supplements of serum albumin and high-energy phosphates. Cold exposure resulted in significant decreases in mitochondrial ATP and inorganic phosphate, and a significant increase in the AMP. Preincubation of the mitochondria from cold-exposed animals with ADP caused a reversion of the mitochondrial AMP/ATP ratio, concomitant with the tightening of coupling. These results suggest that changes in the relative concentrations of adenosine phosphates within the mitochondria of brown adipose tissue may be part of a mechanism regulating the coupling of respiration to phosphorylation in these particles.