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Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells.

Mitra P, Slaughter MM - J. Gen. Physiol. (2002)

Bottom Line: Ca(2+) permeation through the kainic acid receptor had no effect.Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect.This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, School of Medicine, State University of New York at Buffalo, Buffalo, NY 14214, USA.

ABSTRACT
A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

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SMOCs are K+ currents based on ion substitution experiments. (A) SMOC amplitude decreases with increase in [K+]ext. SMOCs were generated at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's containing 2.5 mM [K+]ext (A1). Increasing the [K+]ext to 25 mM lowered SMOCs amplitudes (A2). SMOCs in A1 were superimposed on a baseline current of 120 pA, and those in A2 on a current of 75 pA. (B) Average data from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. The solid line is a linear fit to the data. (C) SMOCs generated at −55 mV in normal Ringer's (C1; Ek = −95 mV) reversed direction in high K+ Ringer's (C2; Ek = 0 mV). SMOCs were the only observed current in normal Ringer's at this voltage. The arrow in C2 indicates the point at which superfusion of high K+ Ringer's was started.
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fig5: SMOCs are K+ currents based on ion substitution experiments. (A) SMOC amplitude decreases with increase in [K+]ext. SMOCs were generated at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's containing 2.5 mM [K+]ext (A1). Increasing the [K+]ext to 25 mM lowered SMOCs amplitudes (A2). SMOCs in A1 were superimposed on a baseline current of 120 pA, and those in A2 on a current of 75 pA. (B) Average data from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. The solid line is a linear fit to the data. (C) SMOCs generated at −55 mV in normal Ringer's (C1; Ek = −95 mV) reversed direction in high K+ Ringer's (C2; Ek = 0 mV). SMOCs were the only observed current in normal Ringer's at this voltage. The arrow in C2 indicates the point at which superfusion of high K+ Ringer's was started.

Mentions: In muscle cells, spontaneous inward Cl− fluxes have also been reported, termed spontaneous transient inward currents (STICs) (Wang et al., 1992; Janssen and Sims, 1994; Henmi et al., 1996). Experiments to determine the ion selectivity of spontaneous events in retinal neurons are summarized in Fig. 5. EK was varied while ECl was kept constant by equimolar substitution of NaCl by KCl. Fig. 5 A shows representative SMOCs with 2.5 mM [K+]ext and with 25 mM [K+]ext at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's. The mean SMOC amplitude in this cell was 78 ± 4 pA in 2.5 mM [K+]ext, and 36 ± 1 pA in 25 mM [K+]ext. Mean SMOC amplitude decreased as the [K+]ext increased. Fig. 5 B plots mean SMOC amplitude versus log [K+]ext. The data points are average values obtained from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. SMOCs had mean amplitudes (in pA) of 169.69 ± 4.4 (2.5 mM [K+]ext), 117.01 ± 2.7 (10 mM [K+]ext), 89.79 ± 2.2 (17.5 mM [K+]ext), 83.86 ± 4 (25 mM [K+]ext), and 48.92 ± 1.8 (37.5 mM [K+]ext). A linear fit provides a slope of −106.69 under these conditions; corresponding to an average conductance of 1830 pS or ∼16 BK channels at the peak of the SMOC (see discussion).


Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells.

Mitra P, Slaughter MM - J. Gen. Physiol. (2002)

SMOCs are K+ currents based on ion substitution experiments. (A) SMOC amplitude decreases with increase in [K+]ext. SMOCs were generated at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's containing 2.5 mM [K+]ext (A1). Increasing the [K+]ext to 25 mM lowered SMOCs amplitudes (A2). SMOCs in A1 were superimposed on a baseline current of 120 pA, and those in A2 on a current of 75 pA. (B) Average data from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. The solid line is a linear fit to the data. (C) SMOCs generated at −55 mV in normal Ringer's (C1; Ek = −95 mV) reversed direction in high K+ Ringer's (C2; Ek = 0 mV). SMOCs were the only observed current in normal Ringer's at this voltage. The arrow in C2 indicates the point at which superfusion of high K+ Ringer's was started.
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Related In: Results  -  Collection

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fig5: SMOCs are K+ currents based on ion substitution experiments. (A) SMOC amplitude decreases with increase in [K+]ext. SMOCs were generated at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's containing 2.5 mM [K+]ext (A1). Increasing the [K+]ext to 25 mM lowered SMOCs amplitudes (A2). SMOCs in A1 were superimposed on a baseline current of 120 pA, and those in A2 on a current of 75 pA. (B) Average data from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. The solid line is a linear fit to the data. (C) SMOCs generated at −55 mV in normal Ringer's (C1; Ek = −95 mV) reversed direction in high K+ Ringer's (C2; Ek = 0 mV). SMOCs were the only observed current in normal Ringer's at this voltage. The arrow in C2 indicates the point at which superfusion of high K+ Ringer's was started.
Mentions: In muscle cells, spontaneous inward Cl− fluxes have also been reported, termed spontaneous transient inward currents (STICs) (Wang et al., 1992; Janssen and Sims, 1994; Henmi et al., 1996). Experiments to determine the ion selectivity of spontaneous events in retinal neurons are summarized in Fig. 5. EK was varied while ECl was kept constant by equimolar substitution of NaCl by KCl. Fig. 5 A shows representative SMOCs with 2.5 mM [K+]ext and with 25 mM [K+]ext at 20 mV in 6 mM Co2+/1.8 mM Ca2+ Ringer's. The mean SMOC amplitude in this cell was 78 ± 4 pA in 2.5 mM [K+]ext, and 36 ± 1 pA in 25 mM [K+]ext. Mean SMOC amplitude decreased as the [K+]ext increased. Fig. 5 B plots mean SMOC amplitude versus log [K+]ext. The data points are average values obtained from four cells in which SMOCs were generated at −10 mV by the addition of 6 mM Co2+/1.8 mM Ca2+ Ringer's with various K+ concentrations. SMOCs had mean amplitudes (in pA) of 169.69 ± 4.4 (2.5 mM [K+]ext), 117.01 ± 2.7 (10 mM [K+]ext), 89.79 ± 2.2 (17.5 mM [K+]ext), 83.86 ± 4 (25 mM [K+]ext), and 48.92 ± 1.8 (37.5 mM [K+]ext). A linear fit provides a slope of −106.69 under these conditions; corresponding to an average conductance of 1830 pS or ∼16 BK channels at the peak of the SMOC (see discussion).

Bottom Line: Ca(2+) permeation through the kainic acid receptor had no effect.Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect.This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, School of Medicine, State University of New York at Buffalo, Buffalo, NY 14214, USA.

ABSTRACT
A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

Show MeSH
Related in: MedlinePlus