Intracellular Action of Spermine on Neuronal Ca²⁺ and K⁺ Currents

Abstract

Intra- and extracellular effects of the polyamine spermine on electrical activity and membrane currents of identified neurons in the abdominal ganglion of Aplysia californica were studied under current- and voltage-clamp conditions. Ionophoretic injection of spermine reduced the amplitude of action potentials and altered their time course as well as spontaneous discharge activity. Investigation of membrane currents showed that intracellular spermine suppressed the total outward current but increased the inward rectifier current. After separation of ion currents it was found that the voltage-activated, delayed K+ outward current and the Ca2+ inward current were reduced by intracellular spermine in a dose- and voltage-dependent manner. The block of the K+ current can be described by a voltage-dependent reaction, where one spermine molecule binds to one channel. The binding constant Kb, at zero voltage, and the effective valency, z delta, had values of 176/M and 0.41 for cell R-15, 223/M and 0.64 for cell L-11, and 137/M and 0.42 for cell L-3. Apparently, more than one spermine cation is needed to block one Ca2+ channel, since the coefficient n, which absorbs the molecularity and cooperativity of the reaction, had non-integral values between 1.34 and 2.22. The binding constant Kb and the effective valency z delta had values of 265/M and 0.64 for cell R-15, 82/M and 0.56 for cell L-4, and 263/M and 0.51 for cell L-6. Intracellular spermine also blocked the Ca(2+)-activated K+ current induced by ionophoretic Ca(2+)-injections, but increased the current at prolonged times after spermine injection.(ABSTRACT TRUNCATED AT 250 WORDS)