Low‐conductance intercellular coupling between mouse chromaffin cells in situ

Abstract

Patch‐clamp experiments were used to compare membrane properties of mouse chromaffin cells in thin tissue slices and of isolated cells in primary culture. The mean membrane input resistance (Rin) and membrane capacitance were 3.1 ± 0.5 GΩ and 9.1 ± 0.5 pF in situ and 9.9 ± 1.8 GΩ and 8.2 ± 0.2 pF in isolated cells. Spike‐like currents were observed on top of the calcium currents during depolarizations in thirty out of forty‐nine cells in situ. They were not seen in isolated cells nor after addition of Cd2+ (100 μM) and TTX (10 μM) to the perfusate of the slices. The mean Rin of cells which displayed current spikes (2.3 ± 0.18 GΩ) was significantly smaller than that of cells lacking spikes (3.9 ± 0.25 GΩ). It is suggested that the current spikes represent intercellular currents which result from action potential firing in neighbouring cells during the depolarization of the patch‐clamped cell. Investigation of capacitative currents induced by square voltage pulses showed a slow component in twenty‐four out of twenty‐seven cells in situ. It is concluded that a large fraction of mouse chromaffin cells in situ are electrically coupled. From the slow capacitative currents and the amplitude of the intercellular current spikes a junctional conductance between chromaffin cells of below 1 nS was deduced. This junctional conductance appears to be too low to support spreading of electrical activity in cases where a single cell is stimulated by an action potential. However, the junctional conductance could allow longer depolarizations of one cell or simultaneous firing of several cells to trigger electrical activity in neighbouring cells.

Electrical Coupling in Sustentacular Cells of the Mouse Olfactory Epithelium

“…This range of values is considerably larger than the R j measured between pairs of cardiac myocytes where R j may be as low as 2-5 M⍀ (Sugiura et al 1990;Weingart and Maurer 1987), but is similar to the value of R j measured in chromaffin cells in tissue slices of rat adrenal gland (Ͻ1 G⍀) (Moser 1998). Just as we found in SCs, chromaffin cells in situ also failed to show dye-coupling (Moser 1998), suggesting that R j needs to be in the tens of M⍀ before dyes with molecular weights of several hundred Dalton can diffuse across gap junctions at a sufficient rate to accumulate in neighboring cells, assuming the connexins involved are nonselective and form homotypic channels. The range of deduced R j values in SCs suggests that electrical coupling between SCs is not uniform, which may reflect, in part, ongoing dynamic control of electrical coupling by intracellular and extracellular factors (Contreras et al 2002).…”

“…This is because R j will be in series with the capacitance of the coupled cells and will therefore contribute a component to the I C recorded in the patched cell. If I C decays with the sum of 2 exponentials, then the dominant fast component (I Cfast ) will reflect charging of the capacitance of the patched cell, whereas the minor slow component (I Cslow ) will represent charging of part of the membrane capacitance of one or more of the coupled cells (Moser 1998). Therefore the peak of I Cslow will be proportional to the sum of the junctional conductances in the patched cell.…”

Cell coupling in mouse pancreatic β-cells measured in intact islets of Langerhans

“…By analogy to a similar patch-clamp study on the cell coupling between chromaffin cells within adrenal slices (Moser 1998), we conclude that the gap junctional conductance between b-cells in situ is too low to allow voltage clamping of the neighbouring cells. We estimate that only approximately 30 per cent of the voltage change imposed on the voltage-clamped cell spreads into the neighbours (see figure 2).…”

“…However, depolarization of the cell to more positive voltages occasionally caused secondary current waveforms that were superimposed on the voltage-clamp Ca 2C currents recorded (figure 1b, red curve in (ii)). By analogy to what has been described in chromaffin cells (Moser 1998), we attribute these secondary waveforms to the firing of an action potential in neighbouring cells. During the depolarization of the voltage-clamped cell, current spread into the electrically coupled neighbours results in the concurrent depolarization of these cells.…”

“…As discussed elsewhere (Moser 1998), the transient capacitive current of an isolated cell should consist only of a single rapid component (reflecting the redistribution of charge during the depolarization) that relaxes within less than 1 ms. Usually, this current is cancelled out by the capacitance compensation network of the patch-clamp amplifier (EPC-9, Heka Electronics, Germany). In the presence of electrical coupling, however, the transient capacitive current should consist of at least two components with widely different kinetics: a fast component caused by the recharging of the membrane of the patched cell and a slow component resulting from the charging of the membranes of cells coupled to it.…”