© 1997 American Heart Association, Inc.
Articles |
Conductances and Selective Permeability of Connexin43 Gap Junction Channels Examined in Neonatal Rat Heart Cells
From the Department of Physiology (V.V., R.W.), University of Bern (Switzerland) and the Department of Physiology (F.F.B.), University of Rochester (NY).
Abstract Myocytes from neonatal rat hearts were used to
assess the conductive properties of gap junction channels by means of
the dual voltage-clamp method. The experiments were carried out on
three types (groups) of preparations: (1) induced cell pairs, (2)
preformed cell pairs with few gap junction channels (1 to 3 channels),
and (3) preformed cell pairs with many channels (100 to 200 channels)
after treatment with uncoupling agents such as SKF-525A (75
µmol/L), heptanol (3 mmol/L), and arachidonic
acid (100 µmol/L). In group 1, the first opening of a newly
formed channel was slow (20 to 65 ms) and occurred 7 to 25 minutes
after physical cell contact. The rate of channel insertion was 1.3
channels/min. Associated with a junctional voltage gradient
(Vj), the channels revealed multiple conductances, a main
open state [
j(main state)], several substates
[j(substates)], and a residual state
[j(residual state)]. On rare occasions, the channels
closed completely. The same phenomena were observed in groups 2 and 3.
The existence of j(residual state) provides an
explanation for the incomplete inactivation of the junctional current
(Ij) at large values of Vj in cell pairs with
many gap junction channels. The values of j(main state)
and j(residual state) gained from groups 1, 2, and 3
turned out to be comparable and hence were pooled. The fit of the data
to a Gaussian distribution revealed a narrow single peak for both
conductances. The values of j were dependent on the
composition of the pipette solution. Solutions were as follows: (1) KCl
solution, j(main state)=96 pS and
j(residual state)=23 pS; (2) Cs+
aspartate- solution, j(main state)=61 pS
and j(residual state)=12 pS; and (3)
tetraethylammonium+
aspartate- solution, j(main state)=19 pS
and j(residual state)=3 pS. The respective
j(main state)-to-j(residual state) ratios
were 4.2, 5.1, and 6.3. This indicates that the residual state
restricts ion permeation more efficiently than does the main state.
Transitions of Ij between open states (main open state,
substates, and residual state) were fast (<2 ms), and transitions
involving the closed state and an open state were slow (15 to 65 ms).
This implies the existence of two gating mechanisms. The residual state
may be regarded as the ground state of electrical gating controlled by
Vj; the closed state, as the ground state of chemical
gating.
Key Words: neonatal rat heart • single-channel conductance • gap junction • connexin43 • selective permeability
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