Postnatal development of sympathetic innervation of rat brown adipose tissue reevaluated with a method allowing for monitoring flavoprotein redox state.

1. A reflection spectrometric method was developed which allowed the simultaneous measurement of flavoprotein absorption and fluorescence on an in vitro preparation of brown adipose tissue. 2. From their spectral characteristics and from the effects of substrates and a metabolic inhibitor (amytal) it was shown that the absorption and fluorescence signals are associated with different flavoproteins. 3. The fluorescence signal is mainly due to changes in the redox state of NADH dehydrogenase, and the absorption signal to changes in redox state of he flavoproteins in the acyl-CoA dehydrogenase pathway. 4. The results suggest that changes in the flavoprotein redox state in response to electrical nerve stimulation, exogenous norepinephrine and substrate addition reflect changes in the metabolic activity of the tissue. These responses were studied in the postnatal period. 5. The amplitude of the tissue response to either nerve stimulation or norepinephrine administration is already maximal at birth and decreases in animals 50 days old. The frequency of nerve stimulation of the concentration of norepinephrine required to produce a half maximum response is significantly higher for the new-born as compared to 13 day and 50 day old animals. 6. For small stimulation intensities a steady state oxidation of the NADH dehydrogenase concomitant with a steady state reduction of the flavoproteins in the acyl-CoA dehydrogenase pathway was recorded. 7. It is concluded that in rats less than 12 hours old, brown adipose tissue is functionally innervated although previous histochemical studies had failed to detect nerve terminals containing catecholamines at this early age.