Limits...
Calcium-induced transitions between the spontaneous miniature outward and the transient outward currents in retinal amacrine cells.

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

Bottom Line: An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude.A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx.Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to).

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
Spontaneous miniature outward currents (SMOCs) occur in a subset of retinal amacrine cells at membrane potentials between -60 and -40 mV. At more depolarized potentials, a transient outward current (I(to)) appears and SMOCs disappear. Both SMOCs and the I(to) are K(+) currents carried by BK channels. They both arise from Ca(2+) influx through high voltage-activated (HVA) Ca(2+) channels, which stimulates release of internal Ca(2+) from caffeine- and ryanodine-sensitive stores. An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude. This reduction showed a temporal dependence: the effect being greater in the latter part of a voltage step. Thus, Ca(2+) influx, although required to generate SMOCs, also produced a negative modulation of their amplitudes. Increasing Ca(2+) influx also led to a decline in the first latency to SMOC occurrence. A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx. Therefore, low levels of Ca(2+) influx, arising from low levels of activation of the HVA Ca(2+) channels, produce randomly occurring SMOCs within the range of -60 to -40 mV. Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to). Based on their characteristics, SMOCs in retinal neurons may function as synaptic noise suppressors at quiescent glutamatergic synapses.

Show MeSH

Related in: MedlinePlus

Ca2+ influx induced transition between SMOCs and the Ito. Cells were bathed in 6 mM Co21/1.8 mM Ca2+ Ringer's and currents were elicited by depolarizing steps to 30 mV. SMOCs superimposed on a voltage-induced outward current of ∼1900 pA. Traces were from a representative cell in 0.9 (A), 1.8 (B), 2.7 (C), and 3.6 (D) mM [Ca2+]. Note the gradual disappearance of SMOCs and the concomitant appearance of a transient outward K+ current (Ito).
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2311395&req=5

fig5: Ca2+ influx induced transition between SMOCs and the Ito. Cells were bathed in 6 mM Co21/1.8 mM Ca2+ Ringer's and currents were elicited by depolarizing steps to 30 mV. SMOCs superimposed on a voltage-induced outward current of ∼1900 pA. Traces were from a representative cell in 0.9 (A), 1.8 (B), 2.7 (C), and 3.6 (D) mM [Ca2+]. Note the gradual disappearance of SMOCs and the concomitant appearance of a transient outward K+ current (Ito).

Mentions: The results presented thus far suggest a Ca2+ influx dependence of SMOCs, whereby increasing influx lead to a temporally dependent reduction in peak amplitude and first latency of SMOC occurrence. An extreme consequence of these effects is that at high levels of Ca2+ influx individual SMOCs disappear, with the concomitant appearance of the transient outward K+ current (Ito). This occurs in a gradual fashion as influx increased (Fig. 5). Cells were bathed in 6 mM Co2+ Ringer's containing varying amounts of [Ca2+] (0.9, 1.8, 2.7, 3.6, and 4.5 mM) and pulsed to 30 mV. Fig. 5 A is a trace from a representative cell in 0.9 mM [Ca2+]. Individual SMOCs are clearly evident. However, increasing bath [Ca2+] caused a gradual disappearance of individual SMOCs along with the appearance of the Ito. In this cell, at 3.6 mM [Ca2+], individual SMOCs completely disappear, leaving the Ito (Fig. 5 D). In a total of eight cells studied at 30 mV, three cells displayed a total disappearance of SMOCs and generated the Ito by 3.6 mM [Ca2+], whereas in the rest SMOCs disappeared at 4.5 mM [Ca2+]. Thus, increasing levels of Ca2+ influx into the cells causes a gradual disappearance of discrete SMOCs along with the appearance of the Ito.


Calcium-induced transitions between the spontaneous miniature outward and the transient outward currents in retinal amacrine cells.

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

Ca2+ influx induced transition between SMOCs and the Ito. Cells were bathed in 6 mM Co21/1.8 mM Ca2+ Ringer's and currents were elicited by depolarizing steps to 30 mV. SMOCs superimposed on a voltage-induced outward current of ∼1900 pA. Traces were from a representative cell in 0.9 (A), 1.8 (B), 2.7 (C), and 3.6 (D) mM [Ca2+]. Note the gradual disappearance of SMOCs and the concomitant appearance of a transient outward K+ current (Ito).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2311395&req=5

fig5: Ca2+ influx induced transition between SMOCs and the Ito. Cells were bathed in 6 mM Co21/1.8 mM Ca2+ Ringer's and currents were elicited by depolarizing steps to 30 mV. SMOCs superimposed on a voltage-induced outward current of ∼1900 pA. Traces were from a representative cell in 0.9 (A), 1.8 (B), 2.7 (C), and 3.6 (D) mM [Ca2+]. Note the gradual disappearance of SMOCs and the concomitant appearance of a transient outward K+ current (Ito).
Mentions: The results presented thus far suggest a Ca2+ influx dependence of SMOCs, whereby increasing influx lead to a temporally dependent reduction in peak amplitude and first latency of SMOC occurrence. An extreme consequence of these effects is that at high levels of Ca2+ influx individual SMOCs disappear, with the concomitant appearance of the transient outward K+ current (Ito). This occurs in a gradual fashion as influx increased (Fig. 5). Cells were bathed in 6 mM Co2+ Ringer's containing varying amounts of [Ca2+] (0.9, 1.8, 2.7, 3.6, and 4.5 mM) and pulsed to 30 mV. Fig. 5 A is a trace from a representative cell in 0.9 mM [Ca2+]. Individual SMOCs are clearly evident. However, increasing bath [Ca2+] caused a gradual disappearance of individual SMOCs along with the appearance of the Ito. In this cell, at 3.6 mM [Ca2+], individual SMOCs completely disappear, leaving the Ito (Fig. 5 D). In a total of eight cells studied at 30 mV, three cells displayed a total disappearance of SMOCs and generated the Ito by 3.6 mM [Ca2+], whereas in the rest SMOCs disappeared at 4.5 mM [Ca2+]. Thus, increasing levels of Ca2+ influx into the cells causes a gradual disappearance of discrete SMOCs along with the appearance of the Ito.

Bottom Line: An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude.A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx.Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to).

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
Spontaneous miniature outward currents (SMOCs) occur in a subset of retinal amacrine cells at membrane potentials between -60 and -40 mV. At more depolarized potentials, a transient outward current (I(to)) appears and SMOCs disappear. Both SMOCs and the I(to) are K(+) currents carried by BK channels. They both arise from Ca(2+) influx through high voltage-activated (HVA) Ca(2+) channels, which stimulates release of internal Ca(2+) from caffeine- and ryanodine-sensitive stores. An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude. This reduction showed a temporal dependence: the effect being greater in the latter part of a voltage step. Thus, Ca(2+) influx, although required to generate SMOCs, also produced a negative modulation of their amplitudes. Increasing Ca(2+) influx also led to a decline in the first latency to SMOC occurrence. A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx. Therefore, low levels of Ca(2+) influx, arising from low levels of activation of the HVA Ca(2+) channels, produce randomly occurring SMOCs within the range of -60 to -40 mV. Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to). Based on their characteristics, SMOCs in retinal neurons may function as synaptic noise suppressors at quiescent glutamatergic synapses.

Show MeSH
Related in: MedlinePlus