The action of cobalt ions on neuromuscular transmission in the frog
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Abstract:
1. Cobalt ions, in concentrations of 0.05-2 mM, block neuromuscular transmission in the frog sartorius muscle.2. The reduction in the e.p.p. amplitude produced by Co(2+) is due to a decrease in the amount of transmitter released by a nerve impulse (mean quantum content). This reduction is associated with little change in the resting membrane potential of the muscle fibre or in the mean amplitude of spontaneous m.e.p.p.s.3. The reduction in evoked transmitter release produced by Co(2+) may be antagonized by elevation of the external Ca(2+) concentration. It is suggested that the antagonism between Co(2+) and Ca(2+) is competitive in nature.4. The mean dissociation constant for Co(2+) and its hypothetical membrane complex was found to be 0.18 mM. On this basis, it is concluded that Co(2+) is about 20 times more potent than Mg(2+) in suppressing evoked transmitter release.5. In contrast to the inhibitory action on evoked release, Co(2+) increases spontaneous transmitter release. However, concentrations of Co(2+) 20-60 times greater than those which decrease the e.p.p. amplitude are needed to produce a significant increase in m.e.p.p. frequency.
Insights
Cobalt ions block neuromuscular transmission by reducing neurotransmitter release. However, cobalt ions also increase spontaneous release at higher concentrations, suggesting a complex role in synaptic function.
Area of Science:
- Neuroscience
- Pharmacology
- Muscle Physiology
Background:
- Neuromuscular transmission relies on precise regulation of neurotransmitter release.
- Cations like calcium and magnesium play critical roles in synaptic function.
- The specific effects of cobalt ions on neurotransmission are not fully elucidated.
Purpose of the Study:
- To investigate the effects of cobalt ions (Co(2+)) on neuromuscular transmission in the frog sartorius muscle.
- To determine the mechanism by which cobalt ions affect neurotransmitter release.
- To compare the potency of cobalt ions with magnesium ions in modulating synaptic transmission.
Main Methods:
- Electrophysiological recordings of neuromuscular junctions in frog sartorius muscle.
- Measurement of end-plate potential (e.p.p.) amplitude and spontaneous miniature e.p.p.s (m.e.p.p.s).
- Varying concentrations of cobalt ions and calcium ions to assess effects and interactions.
Main Results:
- Cobalt ions (0.05-2 mM) significantly reduced evoked neurotransmitter release (e.p.p. amplitude) by decreasing mean quantum content.
- Cobalt ions had minimal impact on resting membrane potential and spontaneous m.e.p.p. amplitude.
- Elevated external calcium concentration antagonized the inhibitory effect of cobalt ions on evoked release, suggesting competitive interaction.
- Cobalt ions were found to be approximately 20 times more potent than magnesium ions in suppressing evoked release.
- Higher concentrations of cobalt ions (20-60 times greater) increased the frequency of spontaneous m.e.p.p.s.
Conclusions:
- Cobalt ions inhibit evoked neurotransmitter release at the neuromuscular junction through a mechanism likely involving competitive antagonism with calcium.
- Cobalt ions exhibit a dual effect, suppressing evoked release while enhancing spontaneous release at higher concentrations.
- The findings provide insights into the role of divalent cations in regulating synaptic transmission.