Nicotinic transmission and drugs in anesthesia : neuromuscular blocking agents and propofol : consequences for carotid body function

Abstract

The carotid body is the global oxygen sensor of the human body. Acute hypoxia elicits instant hyperventilation mediated from the carotid body type 1 cells, where nicotinic transmission is a key component in oxygen sensing and signalling. Neuromuscular blocking agents (NMBAs) reduce this acute hypoxic ventilatory response in humans, and the depression seems to originate from impaired oxygen sensing and signalling in the carotid body. Notably, the carotid bodies are situated outside the blood-brain barrier, and thus accessible for NMBAs. The general anesthetic agent propofol is a potent respiratory depressant and reduces the ventilatory response to hypoxia; however, the site of action for this depression is still not known.

The overall aim of this thesis was to investigate whether NMBAs and propofol impair nicotinic transmission in the carotid body, and furthermore to characterize the pharmacological properties of NMBAs at neuronal nicotinic acetylcholine receptors (nAChRs).

In order to achieve this we used an isolated carotid body preparation for electrophysiological registrations of the afferent carotid sinus nerve activity in response to either step reductions in P02 or nicotine administration. In addition, mRNA for human muscle (α1β1εδ) and neuronal (α3β2, α3β4, α4β and α7) and nAChR subtypes were expressed in Xenopus oocytes and studied with a two-electrode voltage clamp set-up, the OpusXpress™.

We demonstrate that atracurium and vecuronium reduce the nicotine-induced carotid sinus nerve activity, in a concentration-dependent manner. Equipotent concentrations of NMBAs attenuate the nicotine-induced carotid sinus nerve activity to the same degree. The inhibition is dependent of the nicotine dose, thus suggesting a competitive mechanism of block. Propofol impairs carotid body chemosensitivity to various reductions in P02 in a dose-dependent manner. Furthermore, propofol reduces nicotine-induced chemoreceptor activity, most likely by an inhibition of nAChRs in the carotid body. Clinically used non-depolarizing NMBAs inhibit neuronal nAChRs, both by competitive and non-competitive mechanisms, but no receptor activation was seen. Succinylcholine does not activate neuronal nAChRs in concentrations up to 1 mM, and is furthermore a weak antagonist at these subtypes.

We conclude that both non-depolarizing NMBAs and propofol reduce nicotinic transmission in the carotid body, and furthermore that non-depolarizing NMBAs in contrast to depolarizing NMBAs inhibit neuronal nAChRs in a clinically relevant concentration range. This provides a molecular explanation for the reduced hypoxic ventilatory response in humans during residual effects of non-depolarizing NMBAs and propofol. The finding of a distinct action of non-depolarizing NMBAs on the neuronal nAChR subtypes, while succinylcholine had very low affinity to these subtypes, provides interesting insights into the molecular background for neuromuscular transmission.