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Correlation between G protein activation and reblocking kinetics of Ca2+ channel currents in rat sensory neurons
1Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030.
Abstract:
Membrane depolarization relieves the G protein-mediated inhibition or block of high threshold Ca2+ channel currents. We found that the net rate of reblocking depended on the extent of G protein activation. With low intracellular concentrations of GTP gamma S reblocking rates resembled inactivation rates; with higher concentrations reblocking rates increased progressively. Reblocking kinetics were fit with a sum of two exponential functions having time constants (in ms) tau F greater than or equal to 10 and tau S greater than or equal to 30. Unblock during depolarization was fit by a single exponential function with time constant tau A similar to tau F. A model was developed in which unblocking followed dissociation of a blocking molecule, possibly the G protein itself, from Ca2+ channels, and reblocking occurred at rates that depended on the concentration of the blocking molecule. The time course of Ca2+ entry and thus presynaptic Ca2+ levels can be regulated by both the concentration of the G-protein-dependent blocking particle and membrane potential.
Insights
Membrane depolarization rapidly unblocks high-threshold Ca2+ channels. Reblocking rates depend on G protein activation, with higher activation leading to faster reblocking. This reveals a key mechanism for regulating Ca2+ entry.
Area of Science:
- Neuroscience
- Molecular Biology
- Cell Physiology
Background:
- G protein-mediated signaling regulates ion channel function.
- High-threshold Ca2+ channels are crucial for neurotransmitter release.
- Membrane potential influences channel gating and G protein modulation.
Purpose of the Study:
- To investigate the kinetics of G protein-mediated block and unblock of high-threshold Ca2+ channels.
- To determine how G protein activation levels affect Ca2+ channel reblocking rates.
- To develop a model explaining the regulation of Ca2+ channel activity by G proteins and membrane potential.
Main Methods:
- Electrophysiological recordings of high-threshold Ca2+ channels.
- Application of GTP gamma S to modulate G protein activation.
- Kinetic analysis of channel unblocking and reblocking.
- Development of a mathematical model for channel gating.
Main Results:
- Membrane depolarization rapidly unblocks Ca2+ channels, relieving G protein-mediated inhibition.
- Reblocking rates are dependent on the concentration of activated G proteins.
- Reblocking kinetics are described by a sum of two exponential functions, while unblocking follows a single exponential.
- A model was proposed where unblocking involves G protein dissociation and reblocking depends on G protein concentration.
Conclusions:
- G protein-mediated inhibition of Ca2+ channels is dynamically regulated by membrane potential and G protein activation levels.
- The concentration of the G-protein-dependent blocking particle and membrane potential are key regulators of Ca2+ entry.
- This mechanism provides insight into the precise control of presynaptic Ca2+ levels and synaptic transmission.