Endomorphins potentiate acid‐sensing ion channel currents and enhance the lactic acid‐mediated increase in arterial blood pressure: effects amplified in hindlimb ischaemia

Abstract

Chronic muscle ischaemia leads to accumulation of lactic acid and other inflammatory mediators with a subsequent drop in interstitial pH. Acid-sensing ion channels (ASICs), expressed in thin muscle afferents, sense the decrease in pH and evoke a pressor reflex known to increase mean arterial pressure. The naturally occurring endomorphins are also released by primary afferents under ischaemic conditions. We examined whether high affinity mu opioid receptor (MOR) agonists, endomorphin-1 (E-1) and -2 (E-2), modulate ASIC currents and the lactic acid-mediated pressor reflex. In rat dorsal root ganglion (DRG) neurons, exposure to E-2 in acidic solutions significantly potentiated ASIC currents when compared to acidic solutions alone. The potentiation was significantly greater in DRG neurons isolated from rats whose femoral arteries were ligated for 72 h. Sustained ASIC current potentiation was also observed in neurons pretreated with pertussis toxin, an uncoupler of G proteins and MOR. The endomorphin-mediated potentiation was a result of a leftward shift of the activation curve to higher pH values and a slight shift of the inactivation curve to lower pH values. Intra-arterial co-administration of lactic acid and E-2 led to a significantly greater pressor reflex than lactic acid alone in the presence of naloxone. Finally, E-2 effects were inhibited by pretreatment with the ASIC3 blocker APETx2 and enhanced by pretreatment with the ASIC1a blocker psalmotoxin-1. These findings have uncovered a novel role of endomorphins by which the opioids can enhance the lactic acid-mediated reflex increase in arterial pressure that is MOR stimulation-independent and APETx2-sensitive.

Opioid-Mediated Modulation of Acid-Sensing Ion Channel Currents in Adult Rat Sensory Neurons

“…L-Cell Transfection. L-cells (American Type Culture Collection, Manassas, VA) were cotransfected via electroporation with either ASIC1a or ASIC3 cDNA plasmids as described elsewhere (Farrag et al, 2017). Briefly, once cells reached 70%-80% confluency, they were incubated in trypsin for 3-5 minutes, and enzymatic activity was stopped with Dulbecco's modified Eagle's medium (supplemented with 10% FBS, 1% penicillin-streptomycin, 1% glutamine, and 10% nonessential amino acids).…”

“…The electrophysiologic recordings were both analyzed and generated with Igor Pro (Wave-Metrics). The opioid-mediated potentiation of the sustained (I S ) and peak (I P ) ASIC currents were determined as described elsewhere (Farrag et al, 2017); they are illustrated in Fig. 1B in a cell during exposure to external pH 6.0 alone or in the presence of pH 6.0 1 Oxy (10 mM).…”

“…Although DRG neurons express primarily ASIC1 and ASIC3 isoforms, their stoichiometry is not known. Moreover, under ischemic conditions the expression levels of both isoforms have been reported to be altered (Liu et al, 2010;Walder et al, 2010;Farrag et al, 2017). Both variants exhibit a high degree of H 1 ion sensitivity, and their threshold for activation occurs when the extracellular pH reaches 7.2 (Gründer and Pusch, 2015;Kellenberger and Schild, 2015).…”

“…Endogenous opioid peptides are known to be released at ischemic or inflammatory sites (Mousa et al, 2002). Previously, we found that the high-affinity mu opioid receptor (MOR) peptides, endomorphin 1 (E-1) and endomorphin 2 (E-2), potentiated sustained ASIC3 currents in the absence of MOR stimulation and G protein signaling (Farrag et al, 2017). Thus, the present study expands on these findings and determines whether the clinically employed opioids fentanyl (Fen), remifentanil (Rem), and oxycodone (Oxy) would also potentiate ASIC3 currents.…”

The exercise pressor reflex and active O 2 transport in peripheral arterial disease

“…This is due, in part, to a greater responsiveness of these fibers to controlled levels of external input, such as capsaicin (Tsuchimochi et al, 2010) and muscle stretch (Kempf et al, 2018). Additionally, oxidative stress and stimulation of acid-sensing ion channels (by changes in pH balance) have been reported to play significant roles in evoking the exaggerated pressor response in both human and animal models of PAD (Tsuchimochi et al, 2011;Muller et al, 2012;Farrag et al, 2017;Harms et al, 2017;Xing et al, 2018). Therefore, the initial decrease in muscle oxygenation observed in our study, and others (Ledermann et al, 2006), would likely contribute to the exaggerated stimulation of these Group III and IV afferents via the aforementioned mechanisms.…”