Comparison of norepinephrine- and veratrine-induced phosphoinositide hydrolysis in rat brain

Stimulation of phosphoinositide hydrolysis by depolarization with veratrine was compared to that produced by stimulation of alpha-1 adrenoceptors by norepinephrine. The phosphoinositides in rat cerebral cortex were labeled with [myo-3H]inositol and the effects of the drugs on the formation of the following inositol phosphates were determined: inositol 1-phosphate (IP); inositol 1,4-bisphosphate (IP2); mixture of inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate (IP3). Termination of the hydrolysis by trichloroacetic acid resulted in lower basal levels and more reproducible results than termination by water lysis or a chloroform-methanol mixture (CHCl3-MeOH). The amounts of IP and IP2 formed by a maximal concentration of veratrine were about one half of that formed by a maximal concentration of norepinephrine although the amount of IP3 formed after stimulation by veratrine was only about 10% of that produced by norepinephrine. The increase in IP was linear with time (30 min) for both norepinephrine and veratrine. Stimulation of IP2 and IP3 formation by veratrine reached a maximum at 5 min whereas that produced by norepinephrine continued to increase for 30 min. Blockade of voltage-dependent calcium channels with manganese produced nearly complete antagonism of the veratrine response while only partially antagonizing the norepinephrine response. Norepinephrine-induced IP2 formation was less sensitive to manganese than was formation of IP or IP3. These data suggest that either veratrine and norepinephrine cause hydrolysis of different pools of phosphoinositide or that the hydrolysis occurs by different mechanisms. The data also suggest that IP and IP2 may be produced directly from phosphatidylinositol and phosphatidylinositol 4-phosphate rather than solely as a metabolite of IP3.