Neurotensin modulates the amplitude and frequency of voltage-activated Ca2+ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway.
Many excitatory neurotransmitters and neuropeptides regulate the activity of neuronal and endocrine cells by modulating voltage-operated Ca2+ channels. Paradoxically, however, excitatory neuromediators that provoke mobilization of intracellular calcium from inositol trisphosphate (IP3)-sensitive stores usually inhibit voltage-gated Ca2+ currents. We have recently demonstrated that neurotensin (NT) stimulates the electrical and secretory activities of frog pituitary melanotrophs, and increases intracellular calcium concentration in these cells. In the present study, we have investigated the effects of NT on Ca2+ currents in cultured frog melanotrophs by using the perforated patch-clamp technique. Frog neurotensin (f NT) reduced the amplitude and facilitated the inactivation of both L- and N-type Ca2+ currents. Application of the membrane-permeant Ca2+ chelator BAPTA-AM, the sarcoendoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, or the IP3 receptor antagonist 2-APB suppressed the reduction of Ca2+ currents induced by f NT. Incubation of melanotrophs with the diacylglycerol analogue PMA, which causes desensitization of protein kinase C (PKC), or with the PKC inhibitors chelerythrine and calphostin C, reduced the inhibitory effect of f NT. The NT-induced action potential waveforms, applied as voltage-clamp commands, decreased the amplitude of Ca2+ currents, and enhanced Ca2+ influx by increasing the Ca2+ spike frequency. Altogether, these data indicate that the inhibitory effect of f NT on Ca2+ currents results from activation of the IP3/PKC pathway. The observation that NT controls Ca2+ signalling through both amplitude and frequency modulations of Ca2+ currents suggests that NT might induce spacial and temporal changes of intracellular Ca2+ concentration leading to stimulation of exocytosis.
Belmeguenai
Amor
A
European Institute for Peptide Research (IFRMP 23), Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U-413, UA CNRS, University of Rouen, 76821 Mont-Saint-Aignan, France.
Leprince
Jérôme
J
Tonon
Marie-Christine
MC
Vaudry
Hubert
H
Louiset
Estelle
E
D000470:Alkaloids; D000818:Animals; D053119:Benzophenanthridines; D001896:Boron Compounds / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000502:physiology*; D020864:Calcium Channels, N-Type / Q000187:drug effects / Q000502:physiology*; D018929:Cell Culture Techniques; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008297:Male; D009281:Naphthalenes / Q000494:pharmacology; D009496:Neurotensin / Q000494:pharmacology / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010617:Phenanthridines / Q000494:pharmacology; D010902:Pituitary Gland / Q000187:drug effects / Q000502:physiology*; D011493:Protein Kinase C / Q000502:physiology*; D011895:Rana ridibunda; D018160:Receptors, Cytoplasmic and Nuclear / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology*; D015398:Signal Transduction; D019284:Thapsigargin / Q000494:pharmacology
D000470:Alkaloids; D053119:Benzophenanthridines; D001896:Boron Compounds; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D020864:Calcium Channels, N-Type; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009281:Naphthalenes; D010617:Phenanthridines; D018160:Receptors, Cytoplasmic and Nuclear; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D009496:Neurotensin; D004533:Egtazic Acid; D019284:Thapsigargin; C016299:chelerythrine; C109986:2-aminoethoxydiphenyl borate; D011493:Protein Kinase C; C058819:calphostin C