The effects of external Ca++ and Mg++ on the voltage sensitivity of desensitization in Electrophorus electroplaques.
Desensitization onset was studied in voltage-clamped Electrophorus electroplaques during prolonged exposure to bath-applied carbamylcholine. The time-course of desensitization was described by a first-order rate constant kappa obs, which increased exponentially with membrane hyperpolarization from -20 to -90 mV. When Ca++ was increased from 2 to 10 mM, the voltage sensitivity of kappa obs decreased; kappa obs decreased for voltages more negative than -40 mV, and increased slightly at voltages more positive than -40 mV. 10 mM Mg++ had a less pronounced effect and the voltage sensitivity of kappa obs was unchanged. The equilibrium level of desensitization, estimated from the carbamylcholine-dependent conductance which remained after desensitization was apparently complete, also increased with hyperpolarization.
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002217:Carbachol / Q000494:pharmacology; D004558:Electric Stimulation; D004593:Electrophorus / Q000502:physiology*; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects*; D009469:Neuromuscular Junction / Q000502:physiology*
D002217:Carbachol; D008274:Magnesium; D002118:Calcium