Kinetics of noradrenaline released by sympathetic nerves.

At the skeletal neuromuscular junction the released neurotransmitter, acetylcholine, is eliminated within some milliseconds. This time course is known with great precision through the electrical response of target cells. At the sympathetic neuroeffector junction the fast electrical response is not mediated by noradrenaline but by a cotransmitter: ATP. The slow electrical response and the slow component of smooth muscle contraction are principally mediated by noradrenaline. These responses are two orders of magnitude slower than the electrical response to ATP. Therefore, great uncertainty remains regarding the kinetics of noradrenaline appearance and elimination. Here, the local noradrenaline concentration at the surface of the isolated rat tail artery was electrochemically monitored in real time using a carbon fibre electrode. We have shown that the time course of the neurogenically released noradrenaline is at least one order of magnitude faster than the resulting contraction. The kinetics of noradrenaline inactivation by neuronal reuptake were also precisely measured.