In vitro continuous amperometry with a diamond microelectrode coupled with video microscopy for simultaneously monitoring endogenous norepinephrine and its effect on the contractile response of a rat mesenteric artery.
Continuous amperometry with a diamond microelectrode and video microscopy were used to record (in vitro) endogenous norepinephrine release simultaneously with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat. Norepinephrine (NE) is a vasoconstricting neurotransmitter released from sympathetic nerves that innervate the smooth muscle cell layers surrounding arteries and veins. Using these two techniques along with several drugs, the NE released at sympathetic neuroeffector junctions nearby the microelectrode was measured as an oxidation current. Key to the amperometric measurement was the use of a diamond microelectrode because of the response sensitivity, reproducibility, and stability it provided. NE release was elicited by electrical stimulation at frequencies between 1 and 60 Hz, with a maximum response seen at 20 Hz. Confirmation that the oxidation current was, in fact, associated with endogenous NE came from the results of several drugs. Tetrodotoxin (TTX, 0.3 microM), a voltage-dependent sodium channel antagonist that blocks nerve conduction, abolished both the oxidation current and the arterial constriction. The alpha(2)-adrenergic autoreceptor antagonist, yohimbine (1.0 microM), caused an increase in the oxidation current and the corresponding constriction. The addition of cocaine (10 microM), an antagonist that inhibits neuronal NE reuptake, caused both the oxidation current and the contractile response to increase. These results, combined with the fact that the hydrodynamic voltammetric E(1/2) for endogenous NE was identical to that for a standard solution, confirmed that the oxidation current was due to NE and that this compound caused, at least in part, the contractile response. The results demonstrate that continuous amperometric monitoring of NE with a diamond microelectrode and video imaging of vascular tone allow real time local measurement of the temporal relationship between nerve-stimulated NE release and arterial constriction.