Identification of an oxygen-sensitive potassium channel in neonatal rat carotid body type I cells

Contribution of two-pore K+ channels to cardiac ventricular action potential revealed using human iPSC-derived cardiomyocytes

Two-pore K⁺ (K_2p) channels have been described in modulating background conductance as leak channels in different physiological systems. In the heart, the expression of K_2p channels is heterogeneous with equivocation regarding their functional role. Our objective was to determine the K_2p expression profile and their physiological and pathophysiological contribution to cardiac electrophysiology. Induced pluripotent stem cells (iPSCs) generated from humans were differentiated into cardiomyocytes (iPSC-CMs). mRNA was isolated from these cells, commercial iPSC-CM (iCells), control human heart ventricular tissue (cHVT), and ischemic (iHF) and nonischemic heart failure tissues (niHF). We detected 10 K_2p channels in the heart. Comparing quantitative PCR expression of K_2p channels between human heart tissue and iPSC-CMs revealed K_2p1.1, K_2p2.1, K_2p5.1, and K_2p17.1 to be higher expressed in cHVT, whereas K_2p3.1 and K_2p13.1 were higher in iPSC-CMs. Notably, K_2p17.1 was significantly lower in niHF tissues compared with cHVT. Action potential recordings in iCells after K_2p small interfering RNA knockdown revealed prolongations in action potential depolarization at 90% repolarization for K_2p2.1, K_2p3.1, K_2p6.1, and K_2p17.1. Here, we report the expression level of 10 human K_2p channels in iPSC-CMs and how they compared with cHVT. Importantly, our functional electrophysiological data in human iPSC-CMs revealed a prominent role in cardiac ventricular repolarization for four of these channels. Finally, we also identified K_2p17.1 as significantly reduced in niHF tissues and K_2p4.1 as reduced in niHF compared with iHF. Thus, we advance the notion that K_2p channels are emerging as novel players in cardiac ventricular electrophysiology that could also be remodeled in cardiac pathology and therefore contribute to arrhythmias.NEW & NOTEWORTHY Two-pore K⁺ (K_2p) channels are traditionally regarded as merely background leak channels in myriad physiological systems. Here, we describe the expression profile of K_2p channels in human-induced pluripotent stem cell-derived cardiomyocytes and outline a salient role in cardiac repolarization and pathology for multiple K_2p channels.

Postnatal development of carotid body glomus cell response to hypoxia

This study examines developmental changes in CB glomus cell depolarization, intracellular calcium ([Ca(2+)](i)) and the magnitude of an O(2)-sensitive background ionic conductance that may play roles in the postnatal increase in oxygen sensitivity of glomus cells isolated from rats of 1-3 days and 11-14 days postnatal age. Using fura-2 and perforated patch whole cell recordings, we simultaneously measured [Ca(2+)](i) and membrane potential (E(m)) during normoxia and hypoxia. Resting E(m) in normoxia was similar at both ages. Hypoxia caused a larger E(m) depolarization and correspondingly larger [Ca(2+)](i) response in glomus cells from 11- to 14-day-old rats compared to 1-3-day-old rats. E(m) and [Ca(2+)](i) responses to 40mM K(+) were identical between the two age groups. Under normoxic conditions both age groups had similar background conductances. Under anoxic conditions (at resting membrane potential) background K(+) conductance decreased significantly more in cells from 11- to 14-day-old rats compared to cells from 1- to 3-day-old rats. Glomus cells from newborns therefore have less O(2)-sensitive background K(+) conductance. These results support the hypothesis that postnatal maturation of glomus cell O(2) sensitivity involves developmental regulation of the expression and/or O(2)-sensitivity of background ionic conductances.

Postnatal development of carotid body glomus cell O2 sensitivity

In mammals, the main sensors of arterial oxygen level are the carotid chemoreceptors, which exhibit low sensitivity to hypoxia at birth and become more sensitive over the first few days or weeks of life. This postnatal increase in hypoxia sensitivity of the arterial chemoreceptors, termed "resetting", remains poorly understood. In the carotid body, hypoxia is transduced by glomus cells, which are secretory sensory neurons that respond to hypoxia at higher P(O2) levels than non-chemoreceptor cell types. Maturation or resetting of carotid body O2 sensitivity potentially involves numerous aspects of the O2 transduction cascade at the glomus cell level, including glomus cell neurotransmitter secretion, neuromodulator function, neurotransmitter receptor expression, glomus cell depolarization in response to hypoxia, [Ca2+]i responses to hypoxia, K+ and Ca2+ channel O2 sensitivity and K+ channel expression. However, although progress has been made in the understanding of carotid body development, the precise mechanisms underlying postnatal maturation of these numerous aspects of chemotransduction remain obscure.

Hemeoxygenase-2 as an O2 sensor in K+ channel-dependent chemotransduction

The physiological response of the carotid body is critically dependent upon oxygen-sensing by potassium channels expressed in glomus cells. One such channel is the large conductance, voltage- and calcium-dependent potassium channel, BK(Ca). Although it is well known that a decrease in oxygen evokes glomus cell depolarization, voltage-gated calcium entry, and transmitter release, the molecular identity of the upstream oxygen sensor has been the subject of some controversy for decades. Recently, we have demonstrated that hemeoxygenase-2 associates tightly with recombinant BK(Ca) and that activity of this enzyme confers oxygen sensitivity to the BK(Ca) channel complex. Similar observations were also made in native channels recorded from carotid body glomus cells, suggesting that hemoxygenase-2 functions as an oxygen sensor of native and recombinant BK(Ca) channels.

Mitochondria in health and disease: perspectives on a new mitochondrial biology

The integrity of mitochondrial function is fundamental to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. In this review, I consider recent developments in our knowledge of basic aspects of mitochondrial biology as an essential step in developing our understanding of the contributions of mitochondria to disease. The identification of novel mechanisms that govern mitochondrial biogenesis and replication, and the delicately poised signalling pathways that coordinate the mitochondrial and nuclear genomes are discussed. As fluorescence imaging has made the study of mitochondrial function within cells accessible, the application of that technology to the exploration of mitochondrial bioenergetics is reviewed. Mitochondrial calcium uptake plays a major role in influencing cell signalling and in the regulation of mitochondrial function, while excessive mitochondrial calcium accumulation has been extensively implicated in disease. Mitochondria are major producers of free radical species, possibly also of nitric oxide, and are also major targets of oxidative damage. Mechanisms of mitochondrial radical generation, targets of oxidative injury and the potential role of uncoupling proteins as regulators of radical generation are discussed. The role of mitochondria in apoptotic and necrotic cell death is seminal and is briefly reviewed. This background leads to a discussion of ways in which these processes combine to cause illness in the neurodegenerative diseases and in cardiac reperfusion injury. The demands of mitochondria and their complex integration into cell biology extends far beyond the provision of ATP, prompting a radical change in our perception of mitochondria and placing these organelles centre stage in many aspects of cell biology and medicine.

Biophysical properties and metabolic regulation of a TASK-like potassium channel in rat carotid body type 1 cells

The single channel properties of TASK-like oxygen-sensitive potassium channels were studied in rat carotid body type 1 cells. We observed channels with rapid bursting kinetics, active at resting membrane potentials. These channels were highly potassium selective with a slope conductance of 14-16 pS, values similar to those reported for TASK-1. In the absence of extracellular divalent cations, however, single channel conductance increased to 28 pS in a manner similar to that reported for TASK-3. After patch excision, channel activity ran down rapidly. Channel activity in inside-out patches was markedly increased by 2 and 5 mM ATP and by 2 mM ADP but not by 100 microM ADP or 1 mM AMP. In cell-attached patches, both cyanide and 2,4-dinitrophenol strongly inhibited channel activity. We conclude that 1) whilst the properties of this channel are consistent with it being a TASK-like potassium channel they do not precisely conform to those of either TASK-1 or TASK-3, 2) channel activity is highly dependent on cytosolic factors including ATP, and 3) changes in energy metabolism may play a role in regulating the activity of these background K+ channels.