Local Control Model of Excitation–Contraction Coupling in Skeletal Muscle |
| |
| Authors: | Michael D Stern Gonzalo Pizarro and Eduardo Ríos |
| |
| Affiliation: | Michael D. Stern, Gonzalo Pizarro, and Eduardo Ríos |
| |
| Abstract: | The voltage-activated H+ selective conductance of rat alveolar epithelial cells was studied using
whole-cell and excised-patch voltage-clamp techniques. The effects of substituting deuterium oxide, D2O, for water, H2O, on both the conductance and the pH dependence of gating were explored. D+ was able to permeate
proton channels, but with a conductance only about 50% that of H+. The conductance in D2O was reduced more
than could be accounted for by bulk solvent isotope effects (i.e., the lower mobility of D+ than H+), suggesting
that D+ interacts specifically with the channel during permeation. Evidently the H+ or D+ current is not diffusion
limited, and the H+ channel does not behave like a water-filled pore. This result indirectly strengthens the hypothesis that H+ (or D+) and not OH? is the ionic species carrying current. The voltage dependence of H+ channel
gating characteristically is sensitive to pHo and pHi and was regulated by pDo and pDi in an analogous manner,
shifting 40 mV/U change in the pD gradient. The time constant of H+ current activation was about three times
slower (τact was larger) in D2O than in H2O. The size of the isotope effect is consistent with deuterium isotope effects for proton abstraction reactions, suggesting that H+ channel activation requires deprotonation of the channel. In contrast, deactivation (τtail) was slowed only by a factor ≤1.5 in D2O. The results are interpreted within the
context of a model for the regulation of H+ channel gating by mutually exclusive protonation at internal and external sites (Cherny, V.V., V.S. Markin, and T.E. DeCoursey. 1995. J. Gen. Physiol. 105:861–896). Most of the kinetic
effects of D2O can be explained if the pK
a of the external regulatory site is ～0.5 pH U higher in D2O. |
| |
| Keywords: | ion channels proton transport pH pneumocytes membrane transport |
|
|
