The heterogeneity of ion channels in chromaffin granule membranes

Abstract

Abbreviations used: BLM -black lipid membrane technique, I -single-channel current amplitude, K CG -large conductance potassium channel, P o -open-probability; Upotential, U rev -reversal potential, γ -ion conductance, Abstract: Chromaffin granules are involved in catecholamine synthesis and traffic in the adrenal glands. The transporting membrane proteins of chromaffin granules play an important role in the ion homeostasis of these organelles. In this study, we characterized components of the electrogenic 86 Rb + flux observed in isolated chromaffin granules. In order to study single channel activity, chromaffin granules from the bovine adrenal medulla were incorporated into planar lipid bilayers. Four types of cationic channel were found, each with a different conductance. The unitary conductances of the potassium channels are 360 ± 10 pS, 220 ± 8 pS, 152 ± 8 pS and 13 ± 3 pS in a gradient of 450/150 mM KCl, pH 7.0. A multiconductance potassium channel with a conductivity of 110 ± 8 pS and 31 ± 4 pS was also found. With the exception of the 13 pS conductance channel, all are activated by depolarizing voltages. One type of chloride channel was also found. It has a unitary conductance of about 250 pS in a gradient of 500/150 mM KCl, pH 7.0.

Chromogranin B (CHGB) is dimorphic and responsible for dominant anion channels delivered to cell surface via regulated secretion

“…The measured reversal potential of ~ −62 mV (at the crossing point of traces b and c in Figure 1B ) yielded a low limit for the permeation ratio of Cl − vs. Na + , P Cl /P Na > 25, if permeation of other ions is too small to consider. Under such conditions, chloride ions carried >95% of the increased currents that accompanied granule release, suggesting that cation channels in DCSG membranes are of very low abundance if existent ( Thevenod et al, 2000 ; Hordejuk et al, 2006 ). After ~5 min, the current increase reached a plateau (time point b in Figure 1A ), probably due to depletion of RRGs.…”

“…Previously, a large-conductance Ca 2+ -activated K + -channel (BK), a few other K + channels and a 250 pS Cl − channel (450/150 mM Cl − ) were assigned to isolated chromaffin granules (Hordejuk et al, 2006). However, no significant permeability of cations other than proton was observed in isolated chromaffin granules four decades ago (Johnson 10.3389/fnmol.2023.1205516 Frontiers in Molecular Neuroscience 17 frontiersin.org and Scarpa, 1976;Johnson et al, 1982).…”

“…Secretogranin II (SgII, also called chromogranin C [CgC or CHGC]) was found to undergo liquid–liquid phase separation in vitro and likely contribute to the regulation of granule sizes ( Takeuchi and Hosaka, 2008 ; Lin et al, 2022 ). Chromogranins A & B (CHGA & CHGB), used to be called CgA and CgB (SgI), respectively, were proposed to participate in protein aggregation-induced membrane budding in TGN ( Tooze, 1998 ; Hordejuk et al, 2006 ; Bearrows et al, 2019 ). Their roles in biogenesis are nonessential because of strong compensatory effects in Chga/Chgb double knockout mice ( Dominguez et al, 2018 ).…”

“…The granular phase separation thus requires proper ionic homeostasis, pH regulation and osmolality balance across granular membranes ( Jiang and Yadav, 2022 ). Permeation of granular membranes to proton and Cl − is known ( Johnson and Scarpa, 1976 ; Thevenod, 2002 ), but genetic identity of the Cl − conductors remains unknown ( Nobile et al, 2000 ; Kelly et al, 2005 ; Hordejuk et al, 2006 ; Stolting et al, 2014 ; Comini et al, 2022 ).…”

Stilbene derivatives inhibit the activity of the inner mitochondrial membrane chloride channels

“…Measurements of the channel activity in the brain and skeletal muscle mitochondria were performed essentially as previously described [30][31][32][33]. Briefly, BLMs were formed in a 250 µm diameter hole drilled in a Delrin cup (Warner Instrument, CT) which separated two chambers (cis and trans, each of 1 ml internal volume).…”