1. Isolated brown fat cells from hamster respond to added catecholamines with a temporary increase in respiratory rate and an extended lipolysis.2. From experiments with catecholamines and a and fl-blockers, the receptors of these cells are classified as 3. Norepinephrine induces a rapid increase in adenosine 3' :5'-monophosphate levels which parallels in time the stimulated respiration. Maximal adenosine 3' : 5'-monophosphate levels are reached within 1 -3 min and are followed by a continuous decline.4. Parallel to the catecholamine-induced respiration and lipolysis there is a pronounced drop in ATP levels. This energy depletion could be reversed by addition within 5 min after norepinephrine of the fl-blocker propranolol. 5. The nucleotide pattern in isolated hamster brown fat cells after norepinephrine addition was mimicked in experiments with isolated hamster brown fat mitochondria. From these experiments it is concluded that a high ratio of AMP and ADP over ATP decreases the respiratory rate when endogenous free fatty acids are oxidized.according to classical definition.Isolated brown adipocytes from hamster and rat have for many years been used as a tool in work concerning hormone-induced lipolysis and non-shivering thermogenesis. A characteristic feature of the hamster brown adipocytes prepared in this laboratory has been that the catecholamine-induced increase in cell respiration is of short duration compared to the lipolysis which is simultaneously induced [l]. From experiments with isoprenaline, norepinephrine and epinephrine the receptors of these cells were classified as being of P-type. This character was further demonstrated by the effect of propranolol, a specific fl-blocker which rapidly inhibited the respiration induced by isoprenaline, norepinephrine and epinephrine [2,3]. a-Blockers, such as phentolamine, thymoxamine and dibenzyline did not affect the respiration induced by the agonists tested, It was further demonstrated that only the L-form of these agonists was active as well as of propranolol.The catecholamine-mediated increase in respiratory rate demonstrated in isolated brown adipocytes has, for a long time, been considered to be mediated by cyclic AMP lipase resulting in fatty acid production. Evidence for such a lipase mechanism has been presented for adipose tissue from rat, man, and chicken [4-71. The effect of norepinephrine on respiratory rate in hamster brown adipocytes could be mimicked by the addition of free fatty acids [2]. However, after initial maximal stimulation, the respiratory burst could not be repeated by successive additions of catecholamines or free fatty acids [2]. It has further been demonstrated that the catecholamine-induced increase in respiratory rate was parallelled by a loosening of respiratory control [8], a drop in energy potential [ 8 ] and release of free fatty acids [1,3]. Data presented here correlate the shift in cyclic AMP, adenine nucleotides and free fatty acids to the respiratory burst induced by catecholamines or free fatty acids in hamster brown...

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1. Piericidin A inhibits the oxidative phosphorylation capacity of the NADH oxidase in submitochondrial systems a t concentrations of 30 pmoles/mg protein, that of the succinic oxidase at 6 nmoles/mg protein and the phosphorylation in the cytochrome oxidase region a t concentrations of 5 vmoles/mg protein.2. The energy dependent NAD' reductions are 10 times more sensitive to piericidin A than the NADH oxidase reaction. There is a correlation between the oxidation reduction state of the respiratory chain components, presumably of coenzyme Q and the extent of sensivity to the inhibitor.3. Piericidin A and guanidine compounds showed an additive or more than additive inhibitory effect when added together.4. p-Chloromercuribenzoate and piericidin have a synergistic effect when added simultaneously. 2,3-Dimercaptopropanol (BAL) could partly release the piericidin inhibition, in contrast to glutathione, indicating an involvement of proximal -SH-groups in the piericidin sensitive site.5. Some discrepancies between the piericidin action and the inhibitory action of rotenone and amytal are noted.6. A hypothesis is put forward for the possible interaction of a non-heme iron component, structurally arranged by S-bonds and coenzyme Q and the relevance of this chelate for the generation of a primary high energy complex.Piericidin A is an insecticidal metabolite of Streptomyces mobarensis, the chemical structure of which has been determined as a pyridine ring with methoxyl groups, nuclear methyl and hydroxyl groups and an isoprenoid-like side chain arranged in a coenzyme Q like manner [ 1 -31.It acts as a competitive inhibitor of mitochondrial reactions where coenzyme Q is involved [4,5]. This effect, suggested by a similarity in structure between the quinoid coenzyme and the insecticide, was demonstrated as a release of piericidin A inhibited succinic oxidase by the addition of coenzyme Q. It was further shown that NADH oxidase was sensitive to extremely low concentrations of piericidin A.The reduction of cytochrome b after addition of NADH or succinate is considerably delayed in the presence of piericidin A and coenzyme Q reduction is completely abolished (61.

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