Published online before print July 27, 2006, doi: 10.1161/01.RES.0000238388.79295.4c
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Submitted on February 15, 2006
Revised on June 19, 2006
Accepted on July 18, 2006
Sustained Currents Through ASIC3 Ion Channels at the Modest pH Changes That Occur During Myocardial Ischemia
Junichi Yagi ;
From the Vollum Institute (J.Y., H.N.W., L.A.N., E.W.M.), Oregon Health & Sciences University, Portland; Department of Integrative Physiology (J.Y.), Kyorin University School of Medicine, Tokyo, Japan; and Department of Physiology and Biophysics (L.A.N.), Federal University of Minas Gerais, Belo Horizonte, Brazil.
* To whom correspondence should be addressed. E-mail: mccleske{at}ohsu.edu.
Acid-sensing ion channel 3 (ASIC3) is highly expressed on sensory neurons that innervate heart and skeletal muscle and, therefore, is proposed to detect lactic acidosis and to transduce angina and muscle ischemic pain. A difficulty with this idea is that ASIC3 rapidly desensitizes. How can a desensitizing ion channel mediate a persisting sensation such as angina? Here, we show that rat ASIC3 produces a sustained current within the limited range of extracellular pH (7.3 to 6.7) that occurs during cardiac and skeletal muscle ischemia; experiments use patch clamp on transfected cell lines and on fluorescently tagged sensory neurons that innervate rat heart. No such sustained current occurs with ASIC1a (either as homomers or 1a/3 heteromers), whereas ASIC2a/3 heteromers give much larger currents than ASIC3 homomers. The sustained current persists even over tens of minutes because it is caused by a region of pH where there is overlap between inactivation and activation of the channel. Lactate, an anaerobic metabolite, allows the current to activate at slightly more basic pH. Surprisingly, amiloride, which blocks ASICs when they are activated at lower pH, increases ASIC3 current evoked at pH 7.0. Cardiac sensory neurons exhibit a small, perfectly sustained current when pH changes from 7.4 to 7.0. At least some of this current is carried by ASICs because the current is increased by both Zn2+, an ASIC modulator, and amiloride. We suggest that this sustained mode is the most relevant form of ASIC3 gating for triggering angina and other ischemic pain.
Key words: angina • intermittent claudication • acid-sensing ion channels • lactic acidosis
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