Hemoxygenase-2 Is an Oxygen Sensor for a Calcium-Sensitive Potassium Channel

Abstract

Modulation of calcium-sensitive potassium (BK) channels by oxygen is important in several mammalian tissues, and in the carotid body it is crucial to respiratory control. However, the identity of the oxygen sensor remains unknown. We demonstrate that hemoxygenase-2 (HO-2) is part of the BK channel complex and enhances channel activity in normoxia. Knockdown of HO-2 expression reduced channel activity, and carbon monoxide, a product of HO-2 activity, rescued this loss of function. Inhibition of BK channels by hypoxia was dependent on HO-2 expression and was augmented by HO-2 stimulation. Furthermore, carotid body cells demonstrated HO-2-dependent hypoxic BK channel inhibition, which indicates that HO-2 is an oxygen sensor that controls channel activity during oxygen deprivation.

The carbon monoxide donor, CORM-2, is an antagonist of ATP-gated, human P2X4 receptors

“…CO gas solutions were generated by bubbling CO gas (99.5%) (BOC gases, Guilford, UK) through the bath solution under positive pressure for at least 2 h. This saturated solution (0.93 mM) was diluted 1:4, to obtain a 20% CO gas solution which was used within 2 h. CO applications were also made using a well-characterised CO donor molecule, tricarbonyldichlororuthenium (II) dimer [Ru(CO 3 )Cl 2 ] 2 , also known as CORM-2 [27,32,36]. Stock solutions of CORM-2 were made up in DMSO at 100 mM and stored at −20°C for up to 1 month.…”

“…Stock solutions of CORM-2 were made up in DMSO at 100 mM and stored at −20°C for up to 1 month. Working solutions were made up daily at the concentrations described in the text and figures, and RuCl 2 (DMSO) 4 +0.1% DMSO was used as the breakdown product control and was synthesised as described by Williams et al [27].…”

“…There are many proposed cellular targets of CO including soluble guanylate cyclase (sGC) [24] and mitogen-activated protein kinases (for review, see [25]). Recent experiments have also shown CO to play important roles in cardiovascular function [26] and O 2 sensing [27,28] via an activation of large-conductance, calcium-activated potassium channel (BK Ca ). Other ion channels have also been shown to be modulated by CO such as L-type calcium channels (Ca v 1.2) [29], TREK-1 [30] and ENaC channels [31].…”

Chemoreception in the context of the general biology of ROS

“…In a recent work, Williams et al (2004;see Kemp et al, 2006) working with HEK cells stably transfected to express maxi K ␣ and ␤ subunits and constitutively expressing HO-2 have shown that both components (the channel subunits and the enzyme) coprecipitate. Additionally, they showed that a chemical donor of CO dramatically increased the activity of the channel in inside-out patches, and that activation was mimicked while recording in normoxic conditions (P O 2 around 150 mmHg) by co-application to the patches the substrates of HO-2 (NADPH and heme), but was absent in hypoxic conditions (P O 2 of 10-15 mmHg).…”

“…We include this section in our article by two reasons: first, because heme oxygenases (HO; specifically the isoform HO-2) has been suggested to participate in oxygen sensing (Williams et al, 2004;Kemp et al, 2006), and second, because HO on degrading free heme groups generate biliverdin which is enzymatically reduced to bilirubin, being bilirubin an scavenger of ROS of apparently a major quantitave importance in some (or all) mammalian cells (Baranano et al, 2002;Sedlak and Snyder, 2004;Ryter et al, 2006).…”