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The Ca2+ channel beta3 subunit differentially modulates G-protein sensitivity of alpha1A and alpha1B Ca2+ channels
1Department of Pharmacology and Molecular Toxicology, Program in Neuroscience, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
Abstract:
We have shown previously that the Ca2+ channel beta3 subunit is capable of modulating tonic G-protein inhibition of alpha1A and alpha1B Ca2+ channels expressed in oocytes. Here we determine the modulatory effect of the Ca2+ channel beta3 subunit on M2 muscarinic receptor-activated G-protein inhibition and whether the beta3 subunit modulates the G-protein sensitivity of alpha1A and alpha1B currents equivalently. To compare the relative inhibition by muscarinic activation, we have used successive ACh applications to remove the large tonic inhibition of these channels. We show that the resulting rebound potentiation results entirely from the loss of tonic G-protein inhibition; although the currents are temporarily relieved of tonic inhibition, they are still capable of undergoing inhibition through the muscarinic pathway. Using this rebound protocol, we demonstrate that the inhibition of peak current amplitude produced by M2 receptor activation is similar for alpha1A and alpha1B calcium currents. However, the contribution of the voltage-dependent component of inhibition, characterized by reduced inhibition at very depolarized voltage steps and the relief of inhibition by depolarizing prepulses, was slightly greater for the alpha1B current than for the alpha1A current. After co-expression of the beta3 subunit, the sensitivity to M2 receptor-induced G-protein inhibition was reduced for both alpha1A and alpha1B currents; however, the reduction was significantly greater for alpha1A currents. Additionally, the difference in the voltage dependence of inhibition of alpha1A and alpha1B currents was heightened after co-expression of the Ca2+ channel beta3 subunit. Such differential modulation of sensitivity to G-protein modulation may be important for fine tuning release in neurons that contain both of these Ca2+ channels.
Insights
The beta3 subunit differentially modulates G-protein inhibition of calcium channels, reducing sensitivity more for alpha1A than alpha1B currents. This fine-tunes neuronal release by impacting calcium channel function.
Area of Science:
- Neuroscience
- Molecular Biology
- Ion Channel Physiology
Background:
- G-protein coupled receptors regulate ion channel activity.
- Calcium channel beta subunits are known to modulate channel function.
- Previous work indicated beta3 subunit modulates tonic G-protein inhibition of alpha1A and alpha1B calcium channels.
Purpose of the Study:
- To investigate the modulatory effect of the calcium channel beta3 subunit on M2 muscarinic receptor-activated G-protein inhibition.
- To determine if the beta3 subunit equivalently modulates the G-protein sensitivity of alpha1A and alpha1B calcium currents.
- To compare the voltage-dependent components of G-protein inhibition between alpha1A and alpha1B currents.
Main Methods:
- Expression of alpha1A and alpha1B calcium channels and M2 muscarinic receptors in oocytes.
- Application of acetylcholine (ACh) to activate M2 receptors and induce G-protein inhibition.
- Utilizing a rebound potentiation protocol to assess tonic and receptor-activated inhibition.
- Co-expression of the beta3 subunit to evaluate its modulatory effects.
Main Results:
- M2 receptor activation caused similar inhibition of peak current amplitude for both alpha1A and alpha1B calcium currents.
- The voltage-dependent component of inhibition was slightly greater for alpha1B than alpha1A currents.
- Co-expression of the beta3 subunit reduced G-protein inhibition sensitivity for both channel types, but significantly more for alpha1A currents.
- The beta3 subunit heightened the difference in voltage dependence of inhibition between alpha1A and alpha1B currents.
Conclusions:
- The beta3 subunit differentially modulates G-protein inhibition of alpha1A and alpha1B calcium channels.
- This differential modulation may be crucial for fine-tuning neurotransmitter release in neurons expressing both channel subtypes.
- The findings highlight the role of beta subunits in regulating neuronal excitability and signaling.