Zinc inhibition of chloride efflux from skeletal muscle ofRana pipiens and its modification by external pH and chloride activity |
| |
Authors: | Bruce C Spalding Patricia Taber John G Swift Paul Horowicz |
| |
Institution: | (1) Department of Physiology, School of Medicine and Dentistry, University of Rochester, 14642 Rochester, New York |
| |
Abstract: | Summary Efflux of36Cl– from frog sartorius muscles equilibrated in depolarizing solutions was measured. Cl– efflux consists of a component present at low pH and a pH-dependent component which increases as external pH increases. In depolarized muscles fromRana pipiens, the pH-dependent Cl– efflux has an apparent pK
a
near 6.4.The reduction of Cl– efflux by external Zn2+ was determined at different external pHs and chloride activities. The effect of external chloride activity on the pH-dependent Cl– efflux was also examined.At pH 6.5 and a membrane potential of –22 mV, increasing external Cl– activity from 0.108 to 0.28m decreased inhibition of the pH-dependent Cl– efflux at all activities of Zn2+. The Zn2+ activity needed to reduce Cl– efflux by half increased from 0.39×10–3 to 2.09×10–3
m. By contrast, external Cl– activity had no measurable effect on the apparent pK
a
of the pH-dependent efflux.At constant Cl– activity less than 0.21m, increasing external pH from 6.5 to 7.5 decreased inhibition by low Zn2+ activities with either a slight increase or no change in the Zn2+ activity producing half-inhibition. In other words, for relatively low Cl– activities, protection against inhibition of Cl– efflux by low Zn2+ activities was obtained by raising, not lowering, external pH; this is not what is expected if H+ and Zn2+ ions compete at the same site to produce inhibition of Cl– efflux. We conclude that Zn2+ and low pH inhibit Cl– efflux by separate and distinct mechanisms.By contrast, the protection against Zn2+ inhibition produced by high external Cl– activity (0.28m) was partially reversed by raising external pH from 6.5 to 7.5 at all Zn2+ activities. The half-inhibition Zn2+ activity decreased from 2.09×10–3 to 0.68×10–3
m.The results can be simulated quantitatively by a model in which single Cl– channel elements are in equilibrium with sextets of associated single-channel elements, each sextet having a conductance six times that of a single-channel element. The association into sextets is promoted by OH– or Cl– binding to a control site on the single-channel elements. Both the single Cl– channel element and the sextet of Cl– channel elements are closed when this same control site instead binds ZnOH+. The sextet has a much higher affinity for ZnOH+ than does the single Cl– channel element. |
| |
Keywords: | skeletal muscle Cl– efflux Cl– channel pH pOH zinc muscle membrane |
本文献已被 SpringerLink 等数据库收录! |
|