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Omega-conotoxin differentially blocks acetylcholine and adenosine triphosphate releases from Torpedo synaptosomes
I Fariñas1, C Solsona, J Marsal
1Departament de Biologia Cellular i Anatomia Patològica, Facultat de Medicina, Hospital de Bellvitge, Universitat de Bärcelona, Spain.
Abstract:
We have examined the effect of several blockers of voltage-sensitive calcium channels on the release of acetylcholine and ATP from synaptosomes isolated from Torpedo marmorata electric organ. Depolarization of these nerve terminals with high K(+)-containing solutions resulted in a calcium-dependent release of both molecules. Cadmium ions (10(-6) to 10(-3) M) inhibited similarly both releases whereas nickel ions (10(-4) M) in the external medium did not affect either neurotransmitter or nucleotide release. Both releases were completely resistant to the effect of 1,4-dihydropyridines (antagonists nimodipine, nifedipine and agonist Bay K 8644) and of a related compound (diltiazem) at concentrations up to 10(-5) M. These drugs failed to cause any effect even when synaptosomes were submaximally depolarized during incubation. Omega-conotoxin (10(-8) to 5 x 10(-5) M) showed a differential effect on acetylcholine and ATP releases. Nucleotide release was inhibited 90% at the highest concentration tested (50 microns) while acetylcholine release was only moderately decreased (30%). EC50 values for acetylcholine and ATP were of 167 and 2 microM respectively. The results suggest the implication of different types of calcium channels in the release of these molecules.
Insights
Voltage-sensitive calcium channels control acetylcholine and ATP release from Torpedo synaptosomes. Different channel types mediate the release of these molecules, as shown by blocker effects.
Area of Science:
- Neuroscience
- Biochemistry
- Pharmacology
Background:
- Synaptosomes from Torpedo electric organ release acetylcholine and ATP.
- Calcium influx triggers neurotransmitter and nucleotide release.
Purpose of the Study:
- Investigate the role of voltage-sensitive calcium channels (VSCCs) in acetylcholine and ATP release.
- Differentiate the types of VSCCs involved in releasing these molecules.
Main Methods:
- Isolated Torpedo marmorata synaptosomes were depolarized using high K+ solutions.
- The effects of various calcium channel blockers (cadmium, nickel, dihydropyridines, diltiazem, omega-conotoxin) on release were measured.
Main Results:
- Cadmium ions inhibited both acetylcholine and ATP release similarly.
- Nickel ions and 1,4-dihydropyridines/diltiazem had no significant effect on release.
- Omega-conotoxin differentially affected release, strongly inhibiting ATP release (90%) but only moderately inhibiting acetylcholine release (30%).
Conclusions:
- The findings suggest distinct types of VSCCs mediate acetylcholine and ATP release.
- Omega-conotoxin's differential effect highlights specific channel subtypes involved in nucleotide versus neurotransmitter release.