Protein Kinase FA/Glycogen Synthase Kinase-3α After Heparin Potentiation Phosphorylates τ on Sites Abnormally Phosphorylated in Alzheimer's Disease Brain

In this work, we tested the effect of ion channel blockers and of phorbol ester treatments on ³H]dopamine (³H]DA) release and neurotensin (NT)-induced facilitation of ³H]DA release from cultures of rat fetal mesencephalic cells. The potassium channel blockers tetraethylammonium and 4-aminopyridine increased basal ³H]DA release and decreased K⁺-evoked ³H]DA release, whereas apamin was without effect. K⁺-evoked ³H]DA release was decreased by ω-conotoxin and nifedipine, totally suppressed by cadmium, and unaffected by amiloride. These results show the differential sensitivity of ³H]DA release to blockade of various ion channels and suggest the involvement of N-type, L-type, and non-L-non-N-type, but not T-type, voltage-sensitive calcium channels in K⁺-evoked release. Phorbol 12-myristate 13-acetate increased both spontaneous and K⁺-evoked ³H]DA release, suggesting a modulatory action of protein kinase C on DA release in this system. Unexpectedly, however, the effects of the phorbol ester were not counteracted by the protein kinase C inhibitors H7, staurosporine, or polymyxin B. NT-induced facilitation of K⁺-evoked ³H]DA release was insensitive to most of the ion channel blockers, except cadmium (64% decrease in NT effect), suggesting that the corresponding potassium' and calcium channels were not involved in the effect of NT on ³H]DA release in this system. The NT effect was totally suppressed by phorbol ester treatments, indicating a possible desensitization of the corresponding transduction mechanisms after protein kinase C activation.