Journal of Neurophysiology, Vol 71, Issue 5 1661-1671, Copyright © 1994 by APS
Modulation of transient outward potassium current by GTP, calcium, and glutamate in horizontal cells of the Xenopus retina
A. Akopian and P. Witkovsky
Department of Ophthalmology, New York University Medical Center, New York 10016.
1. Membrane currents of luminosity horizontal cells (L-HCs) and chromatic
horizontal cells (C-HCs) isolated from the Xenopus retina were
characterized using the whole-cell patch-clamp technique. 2. The
current-voltage curve for the L-HC had a characteristic negative slope
conductance in the voltage range of -30 to -10 mV that was not evident in
the C-HC. 3. A transient outward 4-aminopyridine-sensitive potassium
current (A-current) was the most prominent current in C-HCs but was also
present in L-HCs. A-current characteristics in the two horizontal cell (HC)
classes were closely similar. Its threshold of activation was above -45 mV.
The half-voltage of inactivation was close to -70 mV. The decay of the
A-current was fit by a single exponential with time constants of 30 and 40
ms at depolarizing voltage steps to -10 and +30 mV, respectively. 4. The
voltage for 50% A-current inactivation shifted toward negative potentials
shortly after we established the whole-cell configuration. This shift was
changed to more positive potentials by internal application of guanosine
5'-triphosphate, resulting in a significant overlap of A-current activation
and inactivation functions near -40 mV, which is well within the normal
operating range of the HC. 5. Internal application of the G-protein
activator GTP gamma S shifted the voltage-dependent inactivation of the
A-current toward positive potentials by +15 mV. In contrast, GDP beta S
shifted the inactivation curve by about -10 mV, similar to what was
observed in untreated cells. 6. GTP and GTP gamma S increased the rate of
recovery from inactivation and slowed down the rate of inactivation of the
A-current enabled by a depolarizing prepulse. 7. Glutamate superfused in
the bath solution significantly accelerated the rate of inactivation of
A-current induced by depolarizing prepulses. The rate of A-current recovery
from inactivation, however, was not affected by glutamate. 8. Removal of
calcium from the bath solution reversibly decreased the amplitude of the
A-current without a significant shift in its threshold of activation.
