A mechanism for hormonal activation of lipolysis and respiration in free brown fat cells

Free brown fat cells were obtained by digestion of minced brown adipose tissue with bacterial collagenase. Electron micrographs have shown many mitochondria with tightly packed cristae in free brown fat cells. Lipolysis was activated in free brown fat cells was lipolysis accompanied by a marked stimulation of respiration. Insulin inhibits the catecholamine-induced respiration in the complete absence of glucose. Catecholamines, theophylline and dibutyryl 3′,5′-AMP all appear to activate lipolysis and respiration by a process dependent upon energy derived from mitochondrial oxidative phosphorylation. Uncoupling agents such asm-chlorocarbonyl cyanide phenylhydrazone stimulated respiration. The addition of oligomycin prior to that of catecholamine inhibited the increase in respiration to a much greater extent than if it was added after the catecholamine. The increase in respiration due to lipolytic agents appears to be the result of increased free fatty acid release mediated through activation of lipolysis by cyclic 3′,5′-AMP, since the addition of octanoate or palmitate to brown fat cells mimicked the effects of lipolytic agents on respiration. The activation of energy metabolism may be the result of a large increase in the energy-dependent cyclic transport of K+, as a result of alterations in the mitochondrial membrane by free fatty acids. This hypothesis is based on the following findings. The activation of respiration by fatty acids or lipolytic agents was dependent upon the presence of K+ in the medium. The addition of K+, Rb+ or Cs+, but not NH4 +, to fat cells isolated and incubated in K+-free buffer restored the ability of lipolytic agents to increase respiration. Valinomycin, an antibiotic which stimulates uptake of K+ by mitochondria and increases the permeability of lipid membranes to K+, was a potent stimulator of respiration in free brown fat cells in the presence of K+. The stimulation of respiration by theophylline or valinomycin was blocked by nigericin which also increases the K+ permeability of membranes but blocks the uptake of K+ by mitochondria. The lipolytic action of theophylline was virtually unaffected by K+ or nigericin.