| Abstract: | Respiratory rates of brown adipocytes were stimulated by 10 microM phenylephrine, the magnitude of this stimulation being about only one-third that evoked by 1 microM isoproterenol, or norepinephrine. The smaller thermogenic response to phenylephrine did not reflect limited substrate availability as shown by the finding that respiration with phenylephrine plus butyrate was not greater than that with phenylephrine or butyrate alone. These data also imply that the thermogenic response to the alpha-agonist does not involve a significant degree of mitochondrial uncoupling. Examination of in vivo changes in brown adipocyte membrane potentials indicated that phenylephrine and isoproterenol can each elicit multiphasic responses (i.e. depolarization followed by hyperpolarization and a second depolarization), although the magnitude and duration of each phase differed for the two agonists. The initial depolarization occurring within seconds after norepinephrine injection appears to result primarily from activation of alpha-adrenergic receptors, with a small contribution from the beta-pathway. Similarly, the subsequent hyperpolarization and second depolarization primarily reflect events associated with the activated beta-adrenergic pathway, with a modest contribution from alpha-mediated events. |
