ISSN:
1432-1424
Keywords:
K+ channel
;
Ca2+ channel
;
selectivity
;
permeation
;
plant
;
Vicia faba
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary The whole-cell patch-clamp method has been used to measure Ca2+ influx through otherwise K+-selective channels in the plasma membrane surrounding protoplasts from guard cells of Vicia faba. These channels are activated by membrane hyperpolarization. The resulting K+ influx contributes to the increase in guard cell turgor which causes stomatal opening during the regulation of leaf-air gas exchange. We find that after opening the K+ channels by hyperpolarization, depolarization of the membrane results in tail current at voltages where there is no electrochemical force to drive K+ inward through the channels. Tail current remains when the reversal potential for permeant ions other than Ca2+ is more negative than or equal to the K+ equilibrium potential (−47 mV), indicating that the current is due to Ca2+ influx through the K+ channels prior to their closure. Decreasing internal [Ca2+] (Ca i ) from 200 to 2 nm or increasing the external [Ca2+] (Ca o ) from 1 to 10 mm increases the amplitude of tail current and shifts the observed reversal potential to more positive values. Such increases in the electrochemical force driving Ca2+ influx also decrease the amplitude of time-activated current, indicating that Ca2+ permeation is slower than K+ permeation, and so causes a partial block. Increasing Ca o also (i) causes a positive shift in the voltage dependence of current, presumably by decreasing the membrane surface potential, and (ii) results in a U-shaped current-voltage relationship with peak inward current ca. −160 mV, indicating that the Ca2− block is voltage dependent and suggesting that the cation binding site is within the electric field of the membrane. K+ channels in Zea mays guard cells also appear to have a Ca i -, and Ca o -dependent ability to mediate Ca2+ influx. We suggest that the inwardly rectiying K+ channels are part of a regulatory mechanism for Ca i . Changes in Ca o and (associated) changes in Ca i regulate a variety of intracellular processes and ion fluxes, including the K+ and anion fluxes associated with stomatal aperture change.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00231883
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