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.