Does AMP-activated Protein Kinase Couple Hypoxic Inhibition of Oxidative Phosphorylation to Carotid Body Excitation?

Abstract

The carotid bodies are the primary peripheral chemoreceptors. They respond to a fall in blood pO2, a rise in blood pCO2 and consequent fall in pH by releasing neurotransmitters. These increase the firing frequency of the carotid sinus nerves which then correct the pattern of breathing via an action at the brainstem. It is now generally accepted that the type 1 or glomus cells are the chemosensory element within the carotid body. However, the precise mechanism by which a fall in pO2 excites the neurotransmitter rich type 1 cells has been the subject of hearty debate for decades now. It has been known for many years that agents that inhibit mitochondrial function excite the carotid body (Heymans et al., 1931; Krylov and Anichkov, 1968). These observations led to work which suggested that O2-sensing in the carotid body was mediated by an aspect of mitochondrial function (Mills and Jobsis, 1972). More recently it has been demonstrated that hypoxia and mitochondrial inhibitors excite carotid body type 1 cells via inhibition of membrane K currents, causing depolarization and voltage-gated calcium entry (Peers, 1990; Buckler and Vaughan-Jones, 1994; Barbe et al., 2002; Wyatt and Buckler, 2004). However, the mechanism by which inhibition of oxidative phosphorylation couples to K channel closure remains unknown. In this article we present our preliminary findings indicating that the ‘metabolic fuel gauge’, AMP-activated protein kinase (AMPK), may be the missing link in the hypoxic chemotransduction pathway. It is known that any small decrease in the cellular ATP/ADP ratio, such as would be seen with hypoxic inhibition of oxidative phosphorylation, is translated into an increase in the AMP/ATP ratio via the adenylate kinase reaction. Adenylate kinase converts 2 molecules of ADP to ATP + AMP in an attempt to maintain ATP levels. The increased AMP/ATP ratio leads to subsequent activation of the enzyme AMPkinase (Hardie, 2004). Whilst the majority of work on AMPK has focused on its role in energy metabolism, recent data has indicated that AMP-kinase can affect