Evidence for protein kinase involvement in the 5-HT-[Ca2+ ]i -pannexin-1 signalling pathway in type II glial cells of the rat carotid body.

Abstract

What is the central question of this study? The mammalian carotid body (CB) is a peripheral chemoreceptor organ involved in O2 and CO2 /H+ homeostasis. Recent studies suggest that 5-HT, released from CB receptor cells, can stimulate adjacent glial-like typeII cells, leading to an increase in intracellular Ca2+ (Δ[Ca2+ ]i ) and activation of ATP-permeable pannexin-1 (Panx-1) channels. The aim of this study was to elucidate the role of protein kinases in the 5-HT-[Ca2+ ]i -Panx-1 signalling pathway. What is the main finding and its importance? Src family kinase and protein kinaseA, acting downstream from Δ[Ca2+ ]i , played central roles in 5-HT-mediated Panx-1 channel activation. This provides new insight into mechanisms regulating CB excitation, especially in pathophysiological conditions. Chemoreceptor (typeI) cells of the rodent carotid body (CB) synthesize and release several neurotransmitters/neuromodulators, including 5-hydroxytryptamine (5-HT), implicated in enhanced CB excitation after exposure to chronic intermittent hypoxia, e.g. sleep apnoea. However, recent studies suggest that 5-HT can robustly stimulate adjacent glial-like typeII cells via ketanserin-sensitive 5-HT2 receptors, leading to intracellular Ca2+ elevation (Δ[Ca2+ ]i ) and activation of ATP-permeable pannexin-1 (Panx-1) channels. Using dissociated rat CB cultures, we investigated the role of protein kinases in the intracellular signalling pathways in typeII cells. In isolated typeII cells, 5-HT activated a Panx-1-like inward current (I5-HT ) that was reversibly inhibited by the Src family kinase inhibitor PP2 (1μm), but not by its inactive analogue, PP3 (1μm). Moreover, I5-HT was reversibly inhibited (>90%) by H89 (1μm), a protein kinaseA blocker, whereas the protein kinaseC blocker GF109203X (2μm) was largely ineffective. In contrast, the P2Y2R agonist UTP (100μm) activated Panx-1-like currents that were reversibly inhibited (∼60%) by either H89 or GF109203X. Using fura-2 spectrofluorimetry, the 5-HT-induced Δ[Ca2+ ]i was unaffected by PP2, H89 and GF109293X, suggesting that the kinases acted downstream of the Ca2+ rise. Given that intracellular Ca2+ chelation was previously shown to block receptor-mediated Panx-1 current activation in typeII cells, these data suggest that CB neuromodulators use overlapping, but not necessarily identical, signalling pathways to activate Panx-1 channels and release ATP, a CB excitatory neurotransmitter. In conclusion, these studies provide new mechanistic insight into 5-HT signalling in the CB that has pathophysiological relevance.