Modulation of Neuronal Voltage-Activated Calcium and Sodium Channels by Polyamines and pH

Abstract

The endogenous polyamines spermine, spermidine and putrescine are present at high concentrations inside neurons and can be released into the extracellular space where they have been shown to modulate ion channels. Here, we have examined polyamine modulation of voltage-activated Ca2+ channels (VACCs) and voltage-activated Na+ channels (VANCs) in rat superior cervical ganglion neurons using whole-cell voltage-clamp at physiological divalent concentrations. Polyamines inhibited VACCs in a concentration-dependent manner with IC50s for spermine, spermidine, and putrescine of 4.7 ± 0.7, 11.2 ± 1.4, and 90 ± 36 mM, respectively. Polyamines caused inhibition by shifting the VACC half-activation voltage (V0.5) to depolarized potentials and by reducing total VACC permeability. The shift was described by Gouy-Chapman-Stern theory with a surface charge density of 0.120 ± 0.005 e- nm-2 and a surface potential of -19 mV. Attenuation of spermidine and spermine inhibition of VACC at decreased pH was explained by H+ titration of surface charge. Polyamine-mediated effects also decreased at elevated pH due to the inhibitors having lower valence and being less effective at screening surface charge. Polyamines affected VANC currents indirectly by reducing TTX inhibition of VANCs at high pH. This may reflect surface charge induced decreases in the local TTX concentration or polyamine-TTX interactions. In conclusion, polyamines inhibit neuronal VACCs via complex interactions with extracellular H+ and Ca. Many of the observed effects can be explained by a model incorporating polyamine binding, H+ binding and surface charge screening.