ALPHA-TRINOSITOL BLOCKS NEUROPEPTIDE Y-INDUCED INOSITOLPHOSPHATE FORMATION IN CEREBRAL VESSELS

Neuropeptide Y (NPY) induces contraction of guinea-pig basilar arteries via activation of Y-1 receptors. This contraction is blocked by D-myo-inositol-1,2,6-triphosphate (alpha-trinositol). Previous binding studies have shown that alpha-trinositol has no effect at Y-1 or Y-2 binding sites thus the antagonistic effect should occur at the level of a second messenger. We have examined the effects of NPY on the formation of inositol phosphates (IP) and have looked for an antagonistic effect of alpha-trinositol. NPY (10(-9)-3 x 10(-7)M) induced strong concentration-dependent contraction of basilar arteries from you ng guinea-pigs (weight 200-250 g) (E(max): 76.4 +/- 11.1%) but not of arteries from old guinea-pigs (weight > 500 g) (E(max): 2.8 +/- 1.5%). [Pro(34)]NPY and PYY induced contraction of similar magnitude and potency, whereas NPY13-36 had only a weak effect. This demonstrates an effect via the Y-1 type of NPY receptor. The contraction induced by NPY was blocked by alpha-trinositol (p < 0.05). LiCl (2 x 10(-4)M), used to inhibit IP breakdown, had no effect on the contraction induced by NPY. NPY (10(-10)-10(-8)M) increased the formation of IP in cerebral vessels from young guineapigs from 357 +/- 48 cpm/mg w.w. to 900 +/- 233 cpm/mg w.w. However, there was no alteration in IP formation in cerebral vessels from old guinea-pigs (NPY 10(-9)-10(-7)M). In the presence of alpha-trinositol (10(-8)-10(-6)M) the NPY induced stimulation of IP formation was totally abolished. alpha-trinositol at a concentration of 10(-8)M but not 10(-7)M and 10(-6)M significantly increased the formation of IP. Fractionation of the total IP into IP3, IP2 and IP1 showed mainly increased values which were totally abolished by alpha-trinositol. It is concluded that alpha-trinositol antagonizes NPY-induced contraction via mechanisms that regulate the intracellular IP formation including IP3 and IP2. This may occur via modulation of PIP2 breakdown by antagonizing phospholipase C activation. Interestingly, the effect seems to be directed selectively towards NPY mediated effects.