CCA-1, EGL-19 and EXP-2 currents shape action potentials in the Caenorhabditis elegans pharynx.
The pharynx of Caenorhabditis elegans is a tubular muscle controlled by its own set of neurons. We developed a technique to voltage clamp the pharyngeal muscle and demonstrate by analyzing mutants that the pharyngeal action potential is regulated by three major voltage-gated currents, conducted by a T-type calcium channel CCA-1, an L-type calcium channel EGL-19 and a potassium channel EXP-2. We show that CCA-1 exhibits T-type calcium channel properties: activation at -40 mV and rapid inactivation. Our results suggest that CCA-1's role is to accelerate the action potential upstroke in the pharyngeal muscle in response to excitatory inputs. Similarly to other L-type channels, EGL-19 activates at high voltages and inactivates slowly; thus it may maintain the plateau phase of the action potential. EXP-2 is a potassium channel of the kV family that shows inward rectifier properties when expressed in Xenopus laevis oocytes. We show that endogenous EXP-2 is not a true inward rectifier--it conducts large outward currents at potentials up to +20 mV and is therefore well suited to trigger rapid repolarization at the end of the action potential plateau phase. Our results suggest that EXP-2 is a potassium channel with unusual properties that uses a hyperpolarization threshold to activate a regenerative hyperpolarizing current.
Shtonda
Boris
B
Department of Molecular Biology, University of Texas Southwestern Medical Center, 6000 Harry Hines Blvd., Dallas, TX 75390-9148, USA. boris@eatworms.swmed.edu
Avery
Leon
L
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D017173:Caenorhabditis elegans / Q000502:physiology*; D029742:Caenorhabditis elegans Proteins / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D020747:Calcium Channels, T-Type / Q000502:physiology*; D009124:Muscle Proteins / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010614:Pharynx / Q000187:drug effects / Q000502:physiology; D015221:Potassium Channels / Q000502:physiology*
D029742:Caenorhabditis elegans Proteins; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020747:Calcium Channels, T-Type; C109164:Egl-19 protein, C elegans; D009124:Muscle Proteins; D015221:Potassium Channels; C403529:exp-2 protein, C elegans; D009543:Nifedipine