Spermine is an endogenous polyamine involved in cellular metabolism found in all eukaryotic cells. Spermine is required for eukaryotic cell growth and protein synthesis. Spermine functions as physiological blockers of inward rectifier K+ channels [1].

Inward rectifier K+ channels mediate the K+ conductance at resting potential in many types of cell. Inward rectifier K+ channels play an important role in controlling excitability [1].

Spermine blocked cloned inward rectifier K+ channels IRK1 with the IC50 of 31 nM clamped at 50 mV membrane potential [1]. Physiological concentrations of free spermine (10 μM) mediated strong rectification of IRK1 inward rectifier K+ channels even in the absence of free Mg2+ and in IRK1 mutant channels that have no endogenous rectification [1].

High dose level of spermine induced a great number of changes, such as emaciation, aggressiveness, convulsions and paralysis of the hind legs in Wistar rats. Spermine considerably decreased growth, food intake and water intake [3].

References:
[1] Fakler B, Brndle U, Glowatzki E, et al.  Strong voltage-dependent inward rectification of inward rectifier K+ channels is caused by intracellular spermine[J]. Cell, 1995, 80(1): 149-154.
[2] Til H P, Falke H E, Prinsen M K, et al.  Acute and subacute toxicity of tyramine, spermidine, spermine, putrescine and cadaverine in rats[J]. Food and Chemical Toxicology, 1997, 35(3): 337-348.