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G-proteins modulate amiloride-sensitive sodium channels
J K Bubien1, R S Jope, D G Warnock
1Department of Medicine, University of Alabama at Birmingham 35294.
Abstract:
We examined the regulation of an amiloride-sensitive sodium conductance expressed in human B lymphoid cells. This conductance was activated by two independent pathways, one involving cyclic adenylyl monophosphate (cAMP)-dependent protein kinase and the other involving a pertussis toxin-sensitive G-protein. Cholera toxin, presumably by increasing cellular cAMP, and pertussis toxin, which ADP-ribosylates certain GTP-binding proteins, both independently increased the amiloride-sensitive sodium conductance. Simultaneous treatment with both toxins, however, failed to increase the sodium conductance, implying that a single set of sodium channels was being affected by both toxins. In cells preactivated with pertussis toxin, 8-chlorophenylthio-cAMP inhibited the activated sodium conductance back to the basal level. Thus, cyclic AMP-dependent pathways can either activate or inhibit amiloride-sensitive sodium channels, depending upon the activation state of a pertussis toxin-sensitive GTP-binding protein. These findings support a hypothesis for the regulation of amiloride-sensitive sodium channels which incorporates the independent effects of cholera and pertussis toxins, and in which cyclic AMP can play a dual role in the regulation of channel activity.
Insights
Human B lymphoid cells
Area of Science:
- Cellular physiology
- Ion channel regulation
- Signal transduction
Background:
- Amiloride-sensitive sodium channels are crucial for cellular function.
- Their regulation in human B lymphoid cells is not fully understood.
- G-protein coupled receptors and cyclic AMP pathways are known modulators of ion channels.
Purpose of the Study:
- To investigate the regulatory mechanisms of amiloride-sensitive sodium conductance in human B lymphoid cells.
- To elucidate the roles of cyclic adenylyl monophosphate (cAMP) and pertussis toxin-sensitive G-proteins in channel activity.
- To determine if these pathways act independently or converge on the same sodium channels.
Main Methods:
- Utilized human B lymphoid cells.
- Applied cholera toxin to increase cellular cAMP levels.
- Used pertussis toxin to ADP-ribosylate GTP-binding proteins.
- Administered 8-chlorophenylthio-cAMP to modulate cAMP levels.
- Measured amiloride-sensitive sodium conductance.
Main Results:
- Both cholera toxin and pertussis toxin independently increased amiloride-sensitive sodium conductance.
- Simultaneous treatment with both toxins did not result in additive increases, suggesting a shared regulatory target.
- Pre-activation with pertussis toxin followed by cAMP stimulation inhibited the conductance, indicating a dual role for cAMP.
- Pertussis toxin-sensitive G-proteins can modulate cAMP-mediated regulation of sodium channels.
Conclusions:
- Amiloride-sensitive sodium channels in human B lymphoid cells are regulated by at least two independent pathways: one involving cAMP-dependent protein kinase and another involving a pertussis toxin-sensitive G-protein.
- These pathways converge on the same sodium channels.
- Cyclic AMP can exert both activating and inhibitory effects on channel activity, depending on the state of G-protein activation.