Extraneuronal uptake of noradrenaline in human tissue (uptake2).

The neurotransmitter, noradrenaline, is inactivated by active transport out of the synaptic cleft, either back into the adrenergic neurons (uptake1) or into extraneuronal cells (uptake2). Although experimental models clearly demonstrate uptake2 in various animal tissues, there are few reports of studies that demonstrate uptake2 in human tissue. Furthermore, uptake2 has never been described in human heart tissue. We demonstrated a reduction of 3H-noradrenaline uptake in human atrial and venous tissue and in rat atrial and ventricular tissue by O-methyl isoprenaline-a known inhibitor of uptake2. These results indicate that uptake2, as has been described in previous experimental models, does exist in humans and further, the results emphasize the importance of uptake2, since about 50% of exogenous noradrenaline is inactivated by uptake2 in the rat heart.