Ca2+ channel inhibition induced by nitric oxide in rat insulinoma RINm5F cells

The effect of nitric oxide (NO) donors on high-voltage-activated Ca2+ chann els in insulin-secreting RINm5F cells was investigated using the patch-clam p technique in the whole-cell configuration. Sodium nitroprusside (SNP, 2-4 00 mu) induced a dose-dependent reduction in Ba2+ currents with maximal inh ibition of 58%. The IC50 for SNP was 45 mu M. A different NO donor, (+/-)S- nitroso-N-acetylpenicillamine (SNAP, 500 mu M), also produced a 50% decreas e in current amplitude. When 200 mu M SNP was administered together with th e NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidozoline-1- oxyl-3 -oxide (carboxy-PTIO, 300 mu M), the Ba2+ current inhibition was lowered to 7%. Administration of 500 mu M 8-bromoguanosine 3':5'-cyclic monophosphate sodium salt (8-Br-cGMP) mimicked the effects of SNP, causing a comparable decrease (56%) in peak-current amplitude. When soluble guanylyl cyclase was blocked by 10 mu M 1H- [1,2,4] oxadiazole [4,3-a] quinoxalin-1-one (ODQ), the inhibitory effect of 200 mu M SNP was reduced from 39% to 15%. The SNP- induced current decrease was 36% of controls after the blockade of L-type C a2+ channels and 30% in the presence of 2.5 mu M omega-conotoxin-MVIIC. The se data indicate that NO inhibits both L-type and P/Q-type Ca2+ channels in RINm5F cells, probably by an increase in the intracellular levels of cGMP. NO may then significantly influence the Ca2+-dependent release of hormones from secretory cells as well as that of neurotransmitters from nerve termi nals.