The modulation by endogenous polyamine spermine on acetylcholine (ACh)-induced currents was evaluated using a outside-out patch and whole-cell voltage-clamp recordings from cultured Xenopus muscle cells. Spermine potentiated the ACh-induced whole-cell currents at micromolar concentrations but is less effective at millimolar concentrations at a holding potential of -60 mV. It produced a voltage-dependent potentiation of ACh-induced whole-cell currents; the degree of potentiation by 100 μM spermine was 18.3 ± 1.7% at a holding potential of -60 mV while 108.2 ± 2.9% increase at +60 mV. Arcaine, a putative competitive antagonist at the polyamine-binding site on the N-methyl-d-aspartate (NMDA) receptor-channel complex, reduced the spermine enhancement of ACh-induced whole-cell currents. Glycine (10 μM) had no effect on the potentiation of ACh-induced whole-cell currents by spermine. In single-channel recordings, spermine (1 μM to 1 mM) increased the opening frequency of both low- and high-conductance ACh channels and at higher concentrations (≥ 100 μM) it produced, in addition, a voltage-dependent decrease in channel amplitude and average open time of both types of ACh channels that limits its enhancing action. It is likely that these two actions of spermine, due to differences in concentration- and voltage-dependence, are mediated by different binding sites on the nicotinic ACh receptor-channel complex of the muscle cells.
All Science Journal Classification (ASJC) codes