Journal of Applied Physiology, Vol 61, Issue 1 61-67, Copyright © 1986 by American Physiological Society
K+-induced alterations in airway muscle responsiveness to electrical field stimulation
C. Murlas, G. Ehring, J. Suszkiw and N. Sperelakis
We investigated possible pre- and postsynaptic effects of K+-induced
depolarization on ferret tracheal smooth muscle (TSM) responsiveness to
cholinergic stimulation. To assess electromechanical activity, cell
membrane potential (Em) and tension (Tm) were simultaneously recorded in
buffer containing 6, 12, 18, or 24 mM K+ before and after electrical field
stimulation (EFS) or exogenous acetylcholine (ACh). In 6 mM K+, Em was
-58.1 +/- 1.0 mV (mean +/- SE). In 12 mM K+, Em was depolarized to -52.3
+/- 0.9 mV, basal Tm did not change, and both excitatory junctional
potentials and contractile responses to EFS at short stimulus duration were
larger than in 6 mM K+. No such potentiation occurred at a higher K+,
although resting Em and Tm increased progressively above 12 mM K+. The
sensitivity of ferret TSM to exogenous ACh appeared unaffected by K+. To
determine whether the hyperresponsiveness in 12 mM K+ was due, in part, to
augmented ACh release from intramural airway nerves, experiments were done
using TSM preparations incubated with [3H]choline to measure [3H]ACh
release at rest and during EFS. Although resting [3H]ACh release increased
progressively in higher K+, release evoked by EFS was maximal in 12 mM K+
and declined in higher concentrations. We conclude that small elevations in
the extracellular K+ concentration augment responsiveness of the airways,
by increasing the release of ACh both at rest and during EFS from
intramural cholinergic nerve terminals. Larger increases in K+ appear to be
inhibitory, possibly due to voltage-dependent effects that occur both pre-
and postsynaptically.
