Characterizing voltage-dependent Ca2+ channels coupled to VIP release and NO synthesis in enteric synaptosomes

In enteric synaptosomes of the rat, the role of voltage-dependent Ca2+ channels in K+ induced VIP release and nitric oxide (NO) synthesis was investigated. Basal VIP release was 39 +/- 4 pg/mg, and cofactor-substituted NO synthase activity was 7.0 +/- 0.8 fmol.mg(-1).min(-1).K+ depolarization (65 mM) stimulated VIP release Ca2+ dependently (basal, 100%; K+, 172.2 +/- 16.2%; P < 0.05, n = 5). K+-stimulated VIP release was reduced by blockers of the P-type (ω-agatoxin-IVA, 3 x 10(-8) M) and N-type (ω-conotoxin-GVIA, 10(-6) M) Ca2+ channels by ∼50 and 25%, respectively, but not by blockers of the L-type (isradipine, 10(-8) M), Q-type (ω-conotoxin-MVIIC, 10(-6) M), or T-type (Ni2+,10(-6) M) Ca2+ channels. In contrast, NO synthesis was suppressed by ω-agatoxin-IVA, ω-conotoxin-GVIA, and isradipine by ∼79, 70, and 70%, respectively, whereas Ni2+ and ω-conotoxin-MVIIC had no effect. These findings are suggestive of a coupling of depolarization-induced VIP release primarily to the P- and N-type Ca2+ channels, whereas NO synthesis is presumably dependent on Ca2+ influx not only via the P- and N- but also via the L-type Ca2+ channel. In contrast, none of the Ca2+ channel blockers affected VIP release evoked by exogenous NO, suggesting that NO induces VIP secretion by a different mechanism, presumably involving intracellular Ca2+ stores.