Heptanol-induced decrease in cardiac gap junctional conductance is mediated by a decrease in the fluidity of membranous cholesterol-rich domains |
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Authors: | E M Lars Bastiaanse Habo J Jongsma Arnoud van der Laarse Brenda R Takens-Kwak |
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Institution: | (1) Department of Cardiology, University Hospital, Leiden, the Netherlands;(2) Department of Physiology, Academic Medical Center, University of Amsterdam, the Netherlands;(3) Present address: Department of Medical Physiology and Sportsmedicine, University of Utrecht, Vondelaan 24, 3521 gg, Utrecht, the Netherlands |
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Abstract: | To assess whether alterations in membrane fluidity of neonatal rat heart cells modulate gap junctional conductance (g
j
), we compared the effects of 2mm 1-heptanol and 20 μm 2-(methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)-octanoate (A2C) in a combined fluorescence anisotropy and electrophysiological study. Both substances decreased fluorescence steady-state
anisotropy (rss), as assessed with the fluorescent probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) by 9.6±1.1% (mean
±sem,n=5) and 9.8±0.6% (n=5), respectively, i.e., both substances increased bulk membrane fluidity. Double whole-cell voltage-clamp experiments showed
that 2mm heptanol uncoupled cell pairs completely (n=6), whereas 20 μm A2C, which increased bulk membrane fluidity to the same extent, did not affect coupling at all (n=5).
Since gap junction channels are embedded in relatively cholesterol-rich domains of the membrane, we specifically assessed
the fluidity of the cholesterol-rich domains with dehydroergosterol (DHE). Using DHE, heptanol increased rss by 14.9±3.0% (n=5), i.e., decreased cholesterol domain fluidity, whereas A2C had no effect on rss (−0.4±6.7%,n=5).
Following an increase of cellular “cholesterol” content (by loading the cells with DHE), 2mm heptanol did not uncouple cell pairs completely:g
j
decreased by 80±20% (range 41–95%,n=5). The decrease ing
j
was most probably due to a decrease in the open probability of the gap junction channels, because the unitary conductances
of the channels were not changed nor was the number of channels comprising the gap junction. The sensitivity of non-junctional
membrane channels to heptanol was unaltered in cholesterol-enriched myocytes.
These results indicate that the fluidity of cholesterol-rich domains is of importance to gap junctional coupling, and that
heptanol decreasesg
j
by decreasing the fluidity of cholesterol-rich domains, rather than by increasing the bulk membrane fluidity. |
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Keywords: | Fluorescence anisotropy Voltage clamp Membrane fluidity Gap junctional conductance 1-heptanol A2C |
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