Actions of n-alcohols on nicotinic acetylcholine receptor channels in cultured rat myotubes.
1. The actions of the n-alcohols from pentanol to dodecanol on nicotinic acetylcholine receptor (nAChR) channels were investigated by recording single ACh-activated channel activity from inside-out membrane patches isolated from cultured rat myotubes. Alcohols were applied to the cytoplasmic side of the membrane; aqueous concentrations ranged from 11.7 mM-pentanol to 0.02 mM-dodecanol. 2. The intermediate-chain alcohols (pentanol to octanol) caused channel currents to fluctuate between the fully open and closed state level so that openings occurred in bursts interrupted by brief gaps. Closed time distributions were fitted well with two exponential components, the fast component representing the closures within a burst. The number of gaps within a burst was dependent on alcohol concentration whereas gap duration was independent of concentration but increased with increasing chain length of the alcohol up to octanol. 3. Nonanol and decanol reduced the mean duration of bursts of openings but did not cause an increase in the number of short closed intervals within a burst. Beyond decanol there was a decline in the ability of the n-alcohols to affect channel function. A saturated solution of undecanol (0.07 mM) reduced the mean open time by 33 +/- 17%, whereas a saturated solution of dodecanol had no significant effect. 4. The current integral per burst was reduced by all the n-alcohols between pentanol and undecanol. The IC50S were as follows: hexanol, 0.53 +/- 0.14 mM; heptanol, 0.097 +/- 0.02 mM; octanol, 0.04 mM and nonanol, 0.16 +/- 0.035 mM. 5. The results were analysed in terms of an open channel block model with a long-lived closed-blocked state beyond the blocked state. Over the range of concentrations tested this describes the effects of all the n-alcohols (C5 to C12) on channel gating reasonably well. 6. Blocking rate constants (k+B) for pentanol through to nonanol were calculated to be between 2.8 and 5.7 X 10(6) M-1 S-1. These values are based on the assumption that the concentration of the alcohols at their site(s) of action was equal to the aqueous concentration applied to the membrane. 7. Equilibrium dissociation constants (KD), calculated from the blocking and unblocking rate constants (KD = k-B/k+B), decreased with increasing chain length from 8 mM for pentanol to 0.15 mM for octanol. The standard free energy per methylene group for adsorption to the site of action was calculated to be about -3.3 kJ mol-1.(ABSTRACT TRUNCATED AT 400 WORDS)
Murrell
R D
RD
Physiological Laboratory, University of Cambridge.
Braun
M S
MS
Haydon
D A
DA
D020003:1-Octanol; D000438:Alcohols / Q000494:pharmacology*; D000818:Animals; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D019850:Heptanol; D000441:Hexanols / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects*; D009132:Muscles / Q000187:drug effects*; D000442:Octanols / Q000494:pharmacology; D000439:Pentanols / Q000494:pharmacology; D051381:Rats; D011978:Receptors, Nicotinic / Q000378:metabolism*; D013997:Time Factors
D000438:Alcohols; D000441:Hexanols; D000442:Octanols; D000439:Pentanols; D011978:Receptors, Nicotinic; C036260:1-hexanol; D019850:Heptanol; C024999:n-pentanol; D020003:1-Octanol