Regulation of the Phosphoinositide Cascade by Polyamines in Brain

The endogenous polyamines spermidine and spermine enhanced guanosine 5′‐ O ‐(3‐thiotriphosphate) (GTP‐γ‐S)‐stimulated phosphoinositide turnover with EC 50 values of 100 ± 30 and 50 ± 15 µ M , respectively, whereas the synthetic polyamines N,N ′‐bis(3‐aminopropyl)‐1,3‐propanediamine and ‐ethylenediamine inhibited GTP‐γ‐S‐stimulated phosphoinositide turnover, with maximal inhibition at 1 m M . Kinetic analysis of GTP‐γ‐S‐stimulated phosphoinositide turnover in the absence and presence of spermidine showed that the K m for GTP‐γ‐S was not changed (1,303 ± 270 and 1,069 ± 214 n M , respectively), whereas the V max was increased by 206% (1,566 ± 141 and 4,792 ± 84 cpm, respectively), indicating that spermidine and GTP‐γ‐S acted at different sites. Spermidine also enhanced Ca 2+ ‐stimulated phosphoinositide turnover in the absence of GTP‐γ‐S by decreasing the Ca 2+ requirement of the phosphoinositide‐specific phospholipase C. Arcaine and agmatine, polyamine antagonists at the NMDA receptor complex, did not block the effects of spermidine on GTP‐γ‐S‐ and Ca 2+ ‐induced phosphoinositide turnover, suggesting that the spermidine effects are not mediated through these specific polyamine sites. Furthermore, spermidine increased the level of [ 3 H]phosphatidylinositol 4‐phosphate (EC 50 = 120 ± 10 µ M ), without affecting significantly the levels of [ 3 H]phosphatidylinositol and [ 3 H]phosphatidylinositol 4,5‐bisphosphate. Collectively these data indicate that the enhanced phosphoinositide turnover induced by spermidine in the presence of GTP‐γ‐S or Ca 2+ is mediated through multiple levels of the phosphoinositide turnover cascade.