Abstract
Sarcoplasmic reticulum (SR) K$^+$ channels from canine diaphragm
were studied upon fusion of longitudinal and junctional membrane
vesicles into planar lipid bilayers (PLB). The large-conductance
cation selective channel (gamma(max) = 250 pS; Km = 33 mM) displays
long-lasting open events which are much more frequent at positive
than at negative voltages. A major subconducting state about 45\%
of the fully-open state current amplitude was occasionally observed
at all voltages. The voltage-dependence of the open probability displays
a sigmoid relationship that was fitted by the Boltzmann equation
and expressed in terms of thermodynamic parameters, namely the free
energy (delta Gi) and the effective gating charge (Zs): delta Gi
= 0.27 kcal/mol and Zs = -1.19 in 250 mM potassium gluconate (K-gluconate).
Kinetic analyses also confirmed the voltage-dependent gating behavior
of this channel, and indicate the implication of at least two open
and three closed states. The diaphragm SR K$^+$ channel shares
several biophysical properties with the cardiac isoform: g = 180
pS, delta Gi = 0.75 kcal/mol, Zs = -1.45 in 150 mM K-gluconate, and
a similar sigmoid P(o)/voltage relationship. Little is known about
the regulation of the diaphragm and cardiac SR K$^+$ channels.
The conductance and gating of these channels were not influenced
by physiological concentrations of Ca$^2+$ (0.1 microM-1 mM)
or Mg2+ (0.25-1 mM), as well as by cGMP (25-100 microM), lemakalim
(1-100 microM), glyburide (up to 10 microM) or charybdotoxin (45-200
nM), added either to the cis or to the trans chamber. The apparent
lack of biochemical or pharmacological modulation of these channels
implies that they are not related to any of the well characterized
surface membrane K$^+$ channels. On the other hand, their voltage
sensitivity strongly suggests that their activity could be modulated
by putative changes in SR membrane potential that might occur during
calcium fluxes.
- 8624367
- adenosine
- animals,
- channels,
- chloride
- comparative
- diaphragm,
- dogs,
- factors,
- gov't,
- myocardium,
- non-u.s.
- potassium
- research
- reticulum,
- sarcoplasmic
- study,
- support,
- thermodynamics,
- time
- triphosphate,
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