2-(Dimethylaminostyryl)-1-methyl pyridinium, a fluorescent indicator of mitochondrial membrane potential, is accumulated up to 400-fold by intact, isolated brown adipocytes from cold-adapted guinea pigs. The indicator can be optically localized within the in situ mitochondria. Ionophores which would be predicted to lower or increase the in situ mitochondrial membrane potential lead to decreased and increased uptake, respectively, of the radio-labelled indicator. Noradrenaline induces a 15 mV decrease in the sum of mitochondrial and plasma membrane potentials. The results indicate that the mitochondria within isolated brown adipocytes maintain a substantial membrane potential in the resting state, rather than being artifactually depolarized.
