INHIBITION OF [CA2-Y - ROLE FOR A CGMP-DEPENDENT PROTEIN KINASE-ACTIVATED K+ CONDUCTANCE(](I) TRANSIENTS IN RAT ADRENAL CHROMAFFIN CELLS BYNEUROPEPTIDE)

The effects of neuropeptide Y on the intracellular level of Ca2+ ([Ca2 +](i)) were studied in cultured rat adrenal chromaffin cells loaded wi th fura-2. A proportion (16%) of cells exhibited spontaneous rhythmic [Ca2+](i) oscillations. In silent cells, oscillations could be induced by forskolin and 1,9-dideoxyforskolin. This action of forskolin was n ot modified by H-89, an inhibitor of protein kinase A. Spontaneous [Ca 2+](i) fluctuations and [Ca2+](i) fluctuations induced by forskolin- a nd 1,9-dideoxyforskolin were inhibited by neuropeptide Y, Increases in [Ca2+](i) induced by 10 and 20 mM KCl but not by 50 mM KCl were dimin ished by neuropeptide Y. However, neuropeptide Y had no effect on [Ca2 +](i) increases evoked by (-)BAY K8644 and the inhibitory effect of ne uropeptide Y on responses induced by 20 mM KCl was not modified by ome ga-conotoxin GVIA, consistent with neither L- nor N-type voltage-sensi tive Ca2+ channels being affected by neuropeptide Y, Rises in [Ca2+](i ) provoked by 10 mM tetraethylammonium were not decreased by neuropept ide Y, suggesting that K+ channel blockade reduces the effect of neuro peptide V. However, [Ca2+](i) transients induced by 1 mM tetraethylamm onium and charybdotoxin were still inhibited by neuropeptide Y, as wer e those to 20 mM KCl in the presence of apamin, The actions of neurope ptide Y on [Ca2+](i) transients provoked by 20 and 50 mM KCl, 1 mM tet raethylammonium, (-)BAY K8644 and charybdotoxin were mimicked by 8-bro mo-cGMP. In contrast, 8-bromo-cAMP did not modify responses to 20 mM K Cl or 1 mM tetraethylammonium. The inhibitory effects of neuropeptide Y and 8-bromo-cGMP on increases in [Ca2+](i) induced by 1 mM tetraethy lammonium were abolished by the Rp-8-pCPT-cGMPS, an inhibitor of prote in kinase G, but not by H-89. A rapid, transient increase in cGMP leve l was found in rat adrenal medullary tissues stimulated with 1 mu M ne uropeptide Y. Rises in [Ca2+](i) produced by DMPP, a nicotinic agonist , but not by muscarine, were decreased by neuropeptide V. Our data sug gest that neuropeptide Y activates a K+ conductance via a protein kina se G-dependent pathway, thereby opposing the depolarizing action of K channel blocking agents and the associated rise in [Ca2+](i).