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Intracellular Mechanisms of Calcium Channel Modulation By Serotonin in Identified Helix Pomatia Neurones

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

Effects of application of serotonin (5-HT) on voltage-gated calcium currents (ICa) were studied on isolated intracellularly perfused Helix pomatia neurones. Cells were identified in the pedal ganglion in which 5-HT induced reversible potentiation (up to 50%) of the current amplitude. ICa in these cells could also be increased by intracellular introduction of cAMP (100 μM) but not cGMP. The effects were not additive and could be potentiated by theophylline (5 mM) and IBMX (100-500 μM). They could be mimicked by forskolin (10-50 μM) and abolised by tolbutamide (1-5 mM) or protein PK-inhibitor (500 μg/ml). Methiothepin (10-50 μM), a 5-HT1,2 receptor antagonist, irreversibly inhibited this effect, while antagonists of 5-HT2 receptors (cyproheptadine or ketanserine) as well as 5-HT3 receptor antagonists (ISC 205-930 and cocaine) were without effect. Elevation of intracellular free calcium level ([Ca]i) by perfusing the cell with Cabuffered solutions depressed the up-regulatory effect in a complicated concentration-dependent way, indicating the involvement of different Ca-binding processes. Addition of calmodulin antagonists (50 μM trifluoperazine or 50 μM chlorpromazine), phosphodiesterase inhibitor (IBMX 100-500 μM) and protein phosphatase antagonist (2 μM okadaic acid) modified the complex Ca-binding isotherm in a specific way, revealing two tetramolecular binding processes with Kd's of 0.04 and 0.69 μM respectively. They represent activation of two calmodulin-dependent enzymes: phosphodiesterase which prevents channel phosphorylation by reducing the intracellular cAMP level and calcineurin which increases its dephosphorylation. Due to this difference in Kd values, changes of [Ca]i in the physiological range may serve as a negative feedback mechanism determining the direction, precision and rate of calcium channel modulation by serotonin.

Affiliations: 1: Bogomoletz Institute of Physiology, National Academy of Sciences of Ukraine, Bogomoletz Str. 4, Kiev-24, 252601 GSP, Ukraine


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