A cGMP-dependent cascade enhances an L-type-like Ca2+ current in identified snail neurons.
We studied the impact of an NO-cGMP dependent signalling pathway on the high-voltage-activated (HVA) Ca(2+) current in identified neurons of the pulmonate snail, Helix pomatia, using Ba(2+) as charge carrier. The 3',5'-cyclic guanosine monophosphate (cGMP) analogues, dibutyryl-cGMP and 8-bromo-cGMP, consistently induced a biphasic response, consisting of an increase superseded by a decline of the Ba(2+) current. The NO donor, sodium nitroprusside (SNP), modulated only in a minority of neurons the Ba(2+) current. Blockade of protein kinase activity with 1-[5-isoquinolinesulfonyl]-2 methyl piperazine (H 7), a nonselective protein kinase inhibitor, or Rp-8-pCPT-cGMP, a selective protein kinase G (PKG) inhibitor, decreased, whereas Rp-cAMP, a selective protein kinase A (PKA) inhibitor, increased the Ba(2+) current upon application of cGMP analogues or SNP. Okadaic acid or calyculin, inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), augmented the Ba(2+) current. Under these conditions, cGMP analogues or SNP had an additive-enhancing effect on the Ba(2+) current. When neurons were exposed to the nonselective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), cGMP analogues induced a persistent increase of the Ba(2+) current, whereas SNP induced a biphasic response. These data suggest coexistence of cGMP-PKG and cGMP-PDE pathways as well as crosstalk between cGMP and 3',5'-cyclic adenosine monophosphate (cAMP) pathways, which converge on HVA Ca channels in Helix neurons. In this model, augmentation of the Ba(2+) current through HVA Ca channels is accomplished by PKA and PKG, whereas attenuation is mediated by PDEs, which prevent activation of protein kinases via hydrolysis of cyclic nucleotides.
Zsombok
Andrea
A
Division of Animal Physiology, Department of Cellular Biology, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria.
Schrofner
Siegfried
S
Hermann
Anton
A
Kerschbaum
Hubert H
HH
D000818:Animals; D001464:Barium / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000378:metabolism*; D006152:Cyclic GMP / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006360:Heat-Shock Proteins / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009433:Neural Inhibition / Q000187:drug effects; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D018408:Patch-Clamp Techniques / Q000379:methods; D010446:Peptide Fragments / Q000494:pharmacology; D012908:Snails
D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D006360:Heat-Shock Proteins; D010446:Peptide Fragments; C063820:SNP nonapeptide; D001464:Barium; D006152:Cyclic GMP