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The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 513-527
Copyright ©2000 by the American Physiological Society
Department of Anatomy and Cell Biology, Wayne State University School of Medicine, Detroit, Michigan 48201
Pan, Zhuo-Hua
Differential Expression of High- and Two Types of
Low-Voltage-Activated Calcium Currents in Rod and Cone Bipolar Cells
of the Rat Retina. J. Neurophysiol. 83: 513-527, 2000. Whole cell voltage-clamp recordings were
performed to investigate voltage-activated Ca2+ currents in
acutely isolated retinal bipolar cells of rats. Two groups of
morphologically different bipolar cells were observed. Bipolar cells of
the first group, which represent the majority of isolated bipolar
cells, were immunoreactive to protein kinase C (PKC) and, therefore
likely to be rod bipolar cells. Bipolar cells of the second group,
which represent only a small population of isolated bipolar cells, did
not show PKC immunoreactivity and were likely to be cone bipolar cells.
The validity of morphological identification of bipolar cells was
further confirmed by the presence of GABAC responses in
these cells. Bipolar cells of both groups displayed
low-voltage-activated (LVA) Ca2+ currents with similar
voltage dependence of activation and steady-state inactivation.
However, the activation, inactivation, and deactivation kinetics of the
LVA Ca2+ currents between rod and cone bipolar cells
differed. Particularly, the LVA Ca2+ currents of rod
bipolar cells displayed both transient and sustained components. In
contrast, the LVA Ca2+ currents of cone bipolar cells were
mainly transient. In addition, the LVA Ca2+ channels of rod
bipolar cells were more permeable to Ba2+ than to
Ca2+, whereas those of cone bipolar cells were equally or
less permeable to Ba2+ than to Ca2+. The LVA
Ca2+ currents of both rod and cone bipolar cells were
antagonized by high concentrations of nimodipine with IC50
of 17 and 23 µM, respectively, but largely resistant to
Cd2+ and Ni2+. Bipolar cells of both groups
also displayed high-voltage-activated (HVA) Ca2+ currents.
The HVA Ca2+ currents were, at least in part, to be L-type
that were potentiated by BayK-8644 (1 µM) and largely antagonized by
low concentrations of nimodipine (5 µM). The L-type Ca2+
channels were almost exclusively located at the axon terminals of rod
bipolar cells but expressed at least in the cell soma of cone bipolar
cells. Results of this study indicate that rod and cone bipolar cells
of the mammalian retina differentially express at least two types of
LVA Ca2+ channels. Rod and cone bipolar cells also show
different spatial distribution of L-type Ca2+ channels.
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