A highly-selective chloride microelectrode based on a mercuracarborand anion carrier

Abstract

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABAA receptors. Measurement of intracellular chloride activity, , using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl− ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular may be up to 4 times more abundant than Cl− (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl− – sensing ISM. Previously, a sensitive and highly-selective Cl− sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of aCl (0.1 mM to 100 mM), is highly selective for Cl− over other biological anions or inhibitors of Cl− transport, and has a 10% to 90% settling time of 3 sec. Importantly, over the physiological range of aCl (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular aCl, and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.

The Pump-Leak/Donnan ion homeostasis strategies of skeletal muscle fibers and neurons

“…By 1200-APs in SM-CD ( Figure 2A ), [Na + ] I rises almost 25 mM above steady-state whereas post-one-AP, [Na + ] I increases <0.05 mM. For contraction-inhibited SMFs (current or voltage-clamped), a new 4-microelectrode technique enables simultaneous monitoring of multiple [ion] i (Heiny et al 2019; DiFranco et al 2019; for measurably-large perturbations, an AP train (not a single AP) would be needed).…”

“…Recently-developed cell-physiological techniques (Heiny et al 2019; DiFranco et al 2019) could make it possible to obtain concurrent SMF [ion] i data (plus V m (t)) along the lines of Figure 2A , while for whole tissue in humans, MRI advances are allowing for the non-invasive probing of ion homeostatic parameters in healthy, injured and diseased subjects (e.g., Hammon et al 2015; Dahlmann et al 2016, Gerhalter et al 2017, 2019; Zhang et al 2020). The accessible theoretical framework process presented here for SMF ion homeostasis as a P-L/D process should help guide investigative directions and facilitate interpretation of new data streams.…”

“…For ∼2 min, [Cl - ] i and Vol cell undershoot, then converge on steady-state. Though ΔVol cell is small [Cl - ] i almost doubles; new techniques (DiFranco et al 2019) would make this measurable. SM-CD’s very small P Na ensures that the transiently-large I pump is not undermined by a concurrently large I Naleak .…”

Potentiometric Sensing

“…DiFranco et al reported on a chloride-selective microelectrode using [9]-mercuracarborand-3 as an ionophore. 191 The fabricated Cl − -selective microelectrode demonstrated a linear response in the concentration range of 10 −3 −10 −1 M with a slope of 54.3 mV/decade. The sensor was characterized with an improved selectivity against bicarbonate ion (log K Cl − ,HCO 3 − Pot = −2.0).…”

“…DiFranco et al reported on a chloride-selective microelectrode using [9]-mercuracarborand-3 as an ionophore . The fabricated Cl – -selective microelectrode demonstrated a linear response in the concentration range of 10 –3 –10 –1 M with a slope of 54.3 mV/decade.…”