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. 2001 Oct 15;536(Pt 2):409-20.
doi: 10.1111/j.1469-7793.2001.0409c.xd.

The effect of external divalent cations on spontaneous non-selective cation channel currents in rabbit portal vein myocytes

A P Albert  1 , W A Large
Affiliations

The effect of external divalent cations on spontaneous non-selective cation channel currents in rabbit portal vein myocytes

A P Albert et al. J Physiol. .

Abstract

1. The effects of external divalent cations on spontaneous single non-selective cation channel currents were studied in outside-out patches from rabbit portal vein smooth muscle cells in K+-free conditions. 2. In an external medium containing 1.5 mM Ca2+ (Ca2+o) the majority of spontaneous channel currents had a unitary conductance of 23 pS, reversal potential (Vr) of +10 mV and a low open probability (Po) at negative patch potentials. Some channels opened to a lower conductance state of about 13 pS suggesting that the cation channels have two conductance states. Open time and burst duration distributions could both be described by two exponentials with time constants of about of 1 ms and 7 ms for open times and 3 ms and 16 ms for burst durations. 3. In 0 Ca2+o the majority of spontaneous cation channels had a unitary conductance of 13 pS and Vr was shifted to +4 mV. Moreover the longer open time and longer burst duration time constants were both reduced to approximately half the values in 1.5 mM Ca2+o. 4. Compared to 0 Ca2+o the single channel currents in 3 microM and 100 microM Ca2+o had a 5- to 6-fold increase in Po which was accompanied by increases in both open times and burst durations. In 3 microM and 100 microM Ca2+o the unitary conductance of the single channel currents was between 22 and 26 pS. 5. At positive membrane potentials the single channel currents had an increased Po compared to negative potentials which was associated with increased open times and burst durations but these values were similar in 3 microM, 100 microM and 1.5 mM Ca2+o. 6. In 1.5 mM Sr2+o and 1.5 mM Ba2+o channels opened to the higher conductance state of about 22-25 pS and had a 3- to 7-fold greater Po than in 0 Ca2+o. 7. In conclusion, external divalent cations have marked effects on the unitary conductance and kinetic behaviour of non-selective cation channels in rabbit portal vein smooth muscle cells.

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Figures

Figure 1
Figure 1. Effect of 0 Cao2+ on spontaneous single cation currents
A(i), spontaneous single inward cation currents recorded from an outside-out patch at −50 mV in 1.5 mm Cao2+. The continuous line represents the closed level (C) and the dashed lines represent open levels. The dashed line marked ‘L’ represents the level of a lower conductance state and ‘H’ the higher conductance state. Note the flickering between closed and open levels denotes bursting behaviour; A(ii), effect of 0 Cao2+ on the single cation currents recorded from the same patch as shown in A(i). B (i), single cation channel current amplitude histogram for the events shown in A(i); B (ii), single channel current amplitude histogram for the channels shown in A(ii). Both histograms could be fitted by three Gaussian curves with different proportions of peaks at approximately −0.7, −1.2 and −2 pA. C, current-voltage (I–V) relationships created from pooled main peak amplitude data for single cation currents recorded in 1.5 mm (•) and 0 (○) Cao2+.
Figure 2
Figure 2. The effect of 0 Cao2+ on open time and burst duration distributions
A, open time distributions of the single channel currents shown in Fig. 1A. In 1.5 mm and 0 Cao2+ the distributions could be fitted by the sum of two exponentials with time constants Oτ1 and Oτ2. Changing from 1.5 mm to 0 Cao2+ reduced Oτ1 from 1.4 ms to 0.7 ms and Oτ2 from 7 ms to 4 ms. B, closed time distributions of the single channel currents shown in Fig. 1A could be described by two exponentials with markedly different time constants of approximately 2 ms and 200 ms. The faster time constant (Cτ1) was used to calculate Tcrit (see Methods). Changing from 1.5 mm to 0 Cao2+ did not significantly alter any of these values. C, burst duration distributions of the single channel currents shown in Fig. 1A could both be fitted by the sum of two exponentials with time constants Bτ1 and Bτ2. Changing from 1.5 mm to 0 Cao2+ did not alter Bτ1 whereas Bτ2 was reduced from 16 ms to 6 ms.
Figure 3
Figure 3. Effect of reducing Cao2+ concentration from 1.5 mm to 3 μm on single cation channel current activity
A, effect of reducing Cao2+ from 1.5 mm to 3 μm on single channel currents recorded from the same patch. Note the increased channel activity and longer openings in 3 μm Cao2+. B, effect of changing Cao2+ from 1.5 mm to 3 μm on the open time distribution of the single cation channels shown in Fig. 3A. Both open time distributions could be described by the sum of two time constants, Oτ1 and Oτ2. Note that reducing Cao2+ to 3 μm did not alter Oτ1 but it did increase Oτ2 from 7 ms to 19 ms. C, effect of reducing Cao2+ from 1.5 mm to 3 μm on the burst duration distributions of the single channel currents shown in Fig. 3A. The burst duration distributions could be described by two time constants, Bτ1 and Bτ2. In 3 μm Cao2+ Bτ1 was not changed whereas Bτ2 was increased from 14 ms to 46 ms.
Figure 4
Figure 4. Single cation channel activity in 100 μm Cao2+
A, effect of reducing Cao2+ from 1.5 mm to 100 μm on channel currents recorded from the same patch. In 100 μm Cao2+ the channel currents showed significantly longer bursts of channel activity. B, effect of reducing Cao2+ from 1.5 mm to 100 μm on the open time distributions of the single channel currents shown in Fig. 4A. Changing from 1.5 mm to 100 μm Cao2+ did not alter Oτ1 or Oτ2. C, burst duration distributions of the single channel currents shown in Fig. 4A. Reducing Cao2+ from 1.5 mm to 100 μm did not change Bτ1 but increased the Bτ2 from 16 ms to 52 ms.
Figure 5
Figure 5. Effect of membrane potential on single channel currents in different Cao2+ concentrations
A, B and C, effects of different Cao2+ concentrations on single channel currents recorded from three different patches at −50 mV and +40 mV. Note that single channel currents behave similarly at +40 mV in 3 μm (A), 100 μm (B) and 1.5 mm (C) Cao2+.
Figure 6
Figure 6. The effect of replacing of 1.5 mm Cao2+ by 1.5 mm Sro2+ on the properties of single cation channel currents
A, effects of replacing 1.5 mm Cao2+ by 1.5 mm Sro2+ on single channel currents recorded from the same patch at −50 mV. B, I–V relationship of the single channel currents shown in A in 1.5 mm Sro2+ had an unitary conductance of 25 pS and an extrapolated Vr of +9 mV. C, the open time distribution of the single channel currents in Sro2+ shown in A could be described by two exponentials with time constants of 1.2 ms (Oτ1) and 7 ms (Oτ2). D, the burst duration distribution of the single channel currents in Sro2+ shown in A could be described by two exponentials with time constants of 2.5 ms (Bτ1) and 31 ms (Bτ2).
Figure 7
Figure 7. The effect of replacing 1.5 mm Cao2+ by 1.5 mm Bao2+ on the properties of single cation channel currents
A, effects of replacing 1.5 mm Cao2+ by 1.5 mm Bao2+ on single cation currents recorded from the same patch at −50 mV. B, I–V relationship of the single cation currents shown in A in 1.5 mm Bao2+ had a unitary conductance of 20 pS and an extrapolated Vr of +11 mV. C, the open time distribution of the single channel currents shown in A could be described by two exponentials with time constants of 1.4 ms (Oτ1) and 7 ms (Oτ2). D, the burst duration distribution of the single channel currents shown in A could be described by two exponentials with time constants of 2.6 ms (Bτ1) and 19 ms (Bτ2)
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References

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