ISSN:
1432-2048
Keywords:
Action potential and pH transient
;
Chlorophyta
;
Eremosphaera
;
Plasma membrane (I/V-curves)
;
Potassium channel
;
Signal transfer
;
Voltage and current clamp
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract To characterize the assumed potassium channels in the plasma membrane of the green alga Eremosphaera viridis (Köhler et al. 1985), current-voltage (I/V)-curves under resting conditions and during an action-potential-like response (CAP) were constructed using voltage- and current-clamp techniques. Under resting conditions the I/V-curves of Eremosphaera showed a distinct upward bending when approaching zero mV, a nearly straight line in the medium part and a downward bending during strong hyperpolarization. Measurements in light and darkness frequently displayed a parallel shift of the I/V-curve in the middle part, indicating a current source which is slowed down by light-off. Using the voltage-clamp technique, N-shaped I/V-curves were sometimes observed. The potassium concentration outside influenced the downward-bending part of the I/V-curve whereas the tetraethylammonium cation, known to block potassium channels, reduced the upward-bending part in particular. A change in external pH, either to pH 7 or pH 3.1 from a standard pH 5.5, caused an increase in conductivity. Chemically induced action potentials were released in Eremosphaera under voltage-clamp conditions by light-off and there was both a current flow and an increase in conductivity during the CAP. Clamping the membrane potential at a value more negative than Nernst potential of potassium revealed an inward current, whereas clamping at a more-positive value revealed an outward current. The experiments demonstrate that there is no threshold potential in releasing a CAP. The I/V-curves performed under current clamp at the peak of CAP verify a previously found increased conductivity with hyper- or depolarization depending on the external potassium concentration. These experiments provide further evidence that in Eremosphaera potassium channels are involved in the CAP caused by a light-off signal. Additional experiments indicate that after light-off a transient acidification of the cytoplasm takes place in correlation with the CAP and the opening of potassium channels. A preliminary “battery model” is discussed to understand the role of potassium channels during a CAP in pH-regulation of the cytoplasm.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00446370
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