Calcium dependence of voltage sensitivity in adenosine 3',5'-cyclic phosphate-stimulated sodium current in Pleurobranchaea.
1. Ionophoretic injection of cyclic AMP into a voltage-clamped molluscan neurone caused a transient slow inward current (Isi) whose amplitude was enhanced by depolarization. Na+-replaced salines abolished the current, placing it with cyclic AMP-stimulated Na+ currents of other gastropod species. 2. Isi amplitude was suppressed by extracellular Ca2+. The amplitude increased up to 4-fold at holding potentials of -50 mV in nominally Ca2+-free saline. Ion substitutions showed that Ca2+ suppressed Isi more effectively than Mg2+, Co2+, Cd2+, Mn2+, Ba2+ or Sr2+. 3. Voltage sensitivity of Isi was abolished by low-Ca2+ salines, by the Ca2+ current blocker Co2+ and by substitution of Ba2+ or Sr2+ as Ca2+ channel current carriers. In such salines Isi showed no appreciable change in amplitude at holding potentials between -70 and -25 mV. 4. Intracellular injection of the Ca2+ chelator EGTA both augmented the amplitude of the current and its duration. EGTA injection failed to suppress the Ca2+-dependent voltage sensitivity of Isi. Intracellular injection of concentrated 3-N-(morpholino) propanesulphonic acid (MOPS) pH buffer to inhibit secondary, Ca2+-dependent intracellular acidification also failed to suppress the voltage sensitivity, as did injections of a mixed EGTA and MOPS solution. 5. While the data indicate a requirement for extracellular Ca2+ in conferring voltage sensitivity, they do not support a role for an intracellular action. An extracellular binding site for Ca2+ could mediate the voltage sensitivity, either by local depolarization-dependent changes in extracellular Ca2+ concentration or through direct voltage-sensitive block of the Isi channel.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002413:Cations, Divalent / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology; D005724:Ganglia / Q000502:physiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects*; D008974:Mollusca; D009025:Morpholines / Q000494:pharmacology; D009474:Neurons / Q000502:physiology*; D012964:Sodium / Q000502:physiology*; D013997:Time Factors
D002413:Cations, Divalent; D007473:Ion Channels; D009025:Morpholines; C008550:morpholinopropane sulfonic acid; D004533:Egtazic Acid; D012964:Sodium; D000242:Cyclic AMP; D002118:Calcium