Tyramine in the assessment of regional adrenergic function

Regional adrenergic function is difficult to assess in humans. Tyramine given through a microdialysis probe may be a useful tool in this regard. However, tyramine data is hard to interpret given the drug's complex mode of action. We characterized the response to tyramine, isoproterenol, and dopamine in adipose tissue with microdialysis probes in normal subjects. We measured glycerol concentrations to follow changes in lipolysis and monitored tissue perfusion with ethanol dilution. During perfusion with tyramine, dialysate glycerol concentration increased dose-dependently from 83 ± 8 μM at baseline to 181 ± 18 μM at 3.5 mM tyramine (p < 0.001) followed by a fall down to 121 ± 9 μM at 35 mM tyramine (p < 0.001). Propranolol almost completely blocked this response. A similar lipolytic response was not observed in isolated human adipocytes. Dopamine <35 μM did not replicate the tyramine-induced lipolysis; however, dopamine >35 μM potently inhibited lipolysis. We conclude that tyramine-induced lipolysis is explained by a pre-synaptic mechanism. Tyramine applied through a microdialysis probe in concentrations up to 3.5 mM can be used to assess pre- and post-synaptic mechanisms regulating lipid mobilization.