Separation of ionic currents in the somatic membrane of frog sensory neurons

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Abstract
Electrical properties of isolated frog primary afferent neurons were examined by suction pipette technique, which combines internal perfusion with current or voltage clamp using a switching circuit with a single electrode. When K+ in the external and internal solutions was totally replaced with Cs+, extremely prolonged Ca spikes, lasting for 5 to 10 sec, and Na spikes, having a short plateau phase of 10 to 15 msec, were observed in Na+-free and Ca2+-free solutions, respectively. Under voltage clamp, Ca2+ current (I Ca) appeared at around −30 mV and maximum peak current was elicited at about 0 mV. With increasing test pulses to the positive side,I Ca became smaller and flattened but did not reverse. Increases of [Ca] o induced a hyperbolic increase ofI Ca and also shifted itsI-V curve along the voltage axis to the more positive direction. Internal perfusion of F− blockedI Ca time-dependently. The Ca channel was permeable to foreign divalent cations in the sequence ofI Ca>I Ba>I Sr≫I Mn>I Zn. Organic Ca-blockers equally depressed the divalent cation currents dose- and time-dependently without shifting theI-V relationships, while inorganic blockers suppressed these currents dose-dependently and the inhibition appeared much stronger in the order ofI Ba=I Sr>I Ca>I Mn=I Zn.