Multiple second messenger routes enhance two high-voltage-activated calcium currents in molluscan neuroendocrine cells.
Two types of high-voltage-activated calcium currents were identified in whole-cell voltage-clamp recordings of the neuroendocrine caudodorsal cells, which control egg-laying in the freshwater snail Lymnaea stagnalis. The currents were: (i) a rapidly inactivating high-voltage-activated current, with an activation threshold of -40 mV and maximal amplitude at +10 mV; and (ii) a slowly inactivating high-voltage-activated current, with a threshold of -10 mV and a peak at +30 mV. Both currents were reduced by nifedipine and verapamil, but not by omega-conotoxin GVIA, suggesting that they belong to the L-type family of calcium currents. The voltage-dependence of inactivation of the rapidly inactivating high-voltage-activated current was bell-shaped. Time-constants of inactivation ranged from 10 to 25 ms. Steady-state inactivation was characterized by a potential of half maximal inactivation of -21.7 +/- 3.4 mV and a slope factor of 8.1 +/- 1.7 mV. The voltage-dependence of inactivation of the slowly inactivating high-voltage-activated current was S-shaped. Time-constants of inactivation increased with depolarization up to a maximum of 300 ms. The steady-state inactivation parameters were a potential of half maximal inactivation of +6.8 +/- 2.2 mV and a slope factor of 6.0 +/- 1.1 mV. The membrane-permeable analog of cAMP, 8-chlorophenylthio-cyclic AMP, predominantly increased the slowly inactivating high-voltage-activated current, and shifted its voltage-dependence of activation and inactivation 10 mV to the left. The rapidly inactivating high-voltage-activated current was slightly increased by 8-chlorophenylthio-cyclic AMP. 8-Bromo-cyclic GMP and the phorbol ester, 12-O-tetradecanoyl-13-phorbol acetate, had qualitatively similar effects. Both agents enhanced the rapidly inactivating current and, to a lesser degree, the slowly inactivating current, without affecting their voltage-dependence. The cyclic AMP-dependent protein kinase inhibitor, Walsh inhibitor peptide, antagonized the stimulating effect of 8-chlorophenylthio-cyclic AMP. The broad-spectrum protein kinase inhibitor 1-(5-isoquino-linylsulfonyl)-2-methyl-piperazine (H-7) strongly attenuated the effects of 8-chlorophenylthio-cyclic AMP, 8-bromo-cyclic GMP and 12-O-tetradecanoyl-13-phorbol acetate, suggesting that all treatments increase both types of high-voltage-activated calcium currents through phosphorylation of the channel-complex.
Dreijer
A M
AM
Graduate School of Neurosciences of Amsterdam, Neuroscience Research Institute, Vrije Universiteit, Faculty of Biology, The Netherlands.
Kits
K S
KS
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000818:Animals; D015220:Calcium Channels / Q000502:physiology*; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology; D004594:Electrophysiology; D007546:Isoquinolines / Q000494:pharmacology; D008195:Lymnaea; D009490:Neurosecretory Systems / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D010879:Piperazines / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology; D015290:Second Messenger Systems / Q000502:physiology*; D013873:Thionucleotides / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology; D020866:omega-Conotoxin GVIA
D015220:Calcium Channels; D007546:Isoquinolines; D010455:Peptides; D010879:Piperazines; D013873:Thionucleotides; C016276:8-bromocyclic GMP; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D020866:omega-Conotoxin GVIA; D014700:Verapamil; D000242:Cyclic AMP; D011493:Protein Kinase C; D006152:Cyclic GMP; D009543:Nifedipine