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Synergistic activation of G protein-gated inwardly rectifying potassium channels by the betagamma subunits of G proteins and Na(+) and Mg(2+) ions.

Petit-Jacques J, Sui JL, Logothetis DE - J. Gen. Physiol. (1999)

Bottom Line: Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins.The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation.At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) subunits resulted in severalfold stimulated levels of channel activity.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Mount Sinai School of Medicine of the New York University, New York, New York 10029, USA.

ABSTRACT
Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins. The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation. Formation (via hydrolysis of ATP) of endogenous PIP(2) or application of exogenous PIP(2) increases the mean open time of GIRK channels and sensitizes them to gating by internal Na(+) ions. In the present study, we show that the activity of ATP- or PIP(2)-modified channels could also be stimulated by intracellular Mg(2+) ions. In addition, Mg(2+) ions reduced the single-channel conductance of GIRK channels, independently of their gating ability. Both Na(+) and Mg(2+) ions exert their gating effects independently of each other or of the activation by the G(betagamma) subunits. At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) subunits resulted in severalfold stimulated levels of channel activity. Changes in ionic concentrations and/or G protein subunits in the local environment of these K(+) channels could provide a rapid amplification mechanism for generation of graded activity, thereby adjusting the level of excitability of the cells.

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Concentration dependence of GIRK channel activation on internal Mg2+ ions. Normalized activity (NPo) of GIRK1/GIRK4 channels is plotted for different Mg2+ concentrations. Inside-out patches were exposed to 2.5 μM PIP2 before and during application of Mg2+. Responses were expressed in fold increase above the activity in the presence of PIP2 and were normalized to those recorded in the presence of 1 mM Mg2+. Vertical bars represent SEM. The responses obtained at the low concentrations of Mg2+ tested (<1 mM) were significantly higher than those with PIP2 alone (P < 0.05, paired t test). The holding potential was at −80 mV. 5 μM ACh was present in the pipette. *Significant differences from 1 mM Mg2+ (P < 0.01; paired t test).
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Figure 4: Concentration dependence of GIRK channel activation on internal Mg2+ ions. Normalized activity (NPo) of GIRK1/GIRK4 channels is plotted for different Mg2+ concentrations. Inside-out patches were exposed to 2.5 μM PIP2 before and during application of Mg2+. Responses were expressed in fold increase above the activity in the presence of PIP2 and were normalized to those recorded in the presence of 1 mM Mg2+. Vertical bars represent SEM. The responses obtained at the low concentrations of Mg2+ tested (<1 mM) were significantly higher than those with PIP2 alone (P < 0.05, paired t test). The holding potential was at −80 mV. 5 μM ACh was present in the pipette. *Significant differences from 1 mM Mg2+ (P < 0.01; paired t test).

Mentions: In experiments shown in Fig. 7, where exogenous PIP2 was applied throughout the experiment (i.e., Fig. 7A and Fig. C), occasional applications of the same ion as a function of time in the experiment were used as control to ascertain that the synergistic effects described were not due to a time-dependent accumulation of PIP2 in the membrane patch. Similar precautions were taken in the experiments shown in Fig. 4. Experiments used to generate the data shown in these two figures were never longer than 14 min (usually 10–13 min). Na+ and/or Mg2+ ions were applied for 30 s.


Synergistic activation of G protein-gated inwardly rectifying potassium channels by the betagamma subunits of G proteins and Na(+) and Mg(2+) ions.

Petit-Jacques J, Sui JL, Logothetis DE - J. Gen. Physiol. (1999)

Concentration dependence of GIRK channel activation on internal Mg2+ ions. Normalized activity (NPo) of GIRK1/GIRK4 channels is plotted for different Mg2+ concentrations. Inside-out patches were exposed to 2.5 μM PIP2 before and during application of Mg2+. Responses were expressed in fold increase above the activity in the presence of PIP2 and were normalized to those recorded in the presence of 1 mM Mg2+. Vertical bars represent SEM. The responses obtained at the low concentrations of Mg2+ tested (<1 mM) were significantly higher than those with PIP2 alone (P < 0.05, paired t test). The holding potential was at −80 mV. 5 μM ACh was present in the pipette. *Significant differences from 1 mM Mg2+ (P < 0.01; paired t test).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230539&req=5

Figure 4: Concentration dependence of GIRK channel activation on internal Mg2+ ions. Normalized activity (NPo) of GIRK1/GIRK4 channels is plotted for different Mg2+ concentrations. Inside-out patches were exposed to 2.5 μM PIP2 before and during application of Mg2+. Responses were expressed in fold increase above the activity in the presence of PIP2 and were normalized to those recorded in the presence of 1 mM Mg2+. Vertical bars represent SEM. The responses obtained at the low concentrations of Mg2+ tested (<1 mM) were significantly higher than those with PIP2 alone (P < 0.05, paired t test). The holding potential was at −80 mV. 5 μM ACh was present in the pipette. *Significant differences from 1 mM Mg2+ (P < 0.01; paired t test).
Mentions: In experiments shown in Fig. 7, where exogenous PIP2 was applied throughout the experiment (i.e., Fig. 7A and Fig. C), occasional applications of the same ion as a function of time in the experiment were used as control to ascertain that the synergistic effects described were not due to a time-dependent accumulation of PIP2 in the membrane patch. Similar precautions were taken in the experiments shown in Fig. 4. Experiments used to generate the data shown in these two figures were never longer than 14 min (usually 10–13 min). Na+ and/or Mg2+ ions were applied for 30 s.

Bottom Line: Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins.The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation.At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) subunits resulted in severalfold stimulated levels of channel activity.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Mount Sinai School of Medicine of the New York University, New York, New York 10029, USA.

ABSTRACT
Native and recombinant G protein-gated inwardly rectifying potassium (GIRK) channels are directly activated by the betagamma subunits of GTP-binding (G) proteins. The presence of phosphatidylinositol-bis-phosphate (PIP(2)) is required for G protein activation. Formation (via hydrolysis of ATP) of endogenous PIP(2) or application of exogenous PIP(2) increases the mean open time of GIRK channels and sensitizes them to gating by internal Na(+) ions. In the present study, we show that the activity of ATP- or PIP(2)-modified channels could also be stimulated by intracellular Mg(2+) ions. In addition, Mg(2+) ions reduced the single-channel conductance of GIRK channels, independently of their gating ability. Both Na(+) and Mg(2+) ions exert their gating effects independently of each other or of the activation by the G(betagamma) subunits. At high levels of PIP(2), synergistic interactions among Na(+), Mg(2+), and G(betagamma) subunits resulted in severalfold stimulated levels of channel activity. Changes in ionic concentrations and/or G protein subunits in the local environment of these K(+) channels could provide a rapid amplification mechanism for generation of graded activity, thereby adjusting the level of excitability of the cells.

Show MeSH
Related in: MedlinePlus