The depolarizing effect of suxamethonium on the membrane potential of striated muscle fibres at endplate-free regions

Influence of chloride and sodium pump activity on carbachol- and acetylaholine-induced depolarizations in denervated rat diphragm

The effect was measured of low chloride medium and ouabain on carbachol- and acetylcholine-induced depolarizations in denervated rat diaphragm. Membrane potentials were measured in rat diaphragm muscle fibers 10-14 days after denervation. Depolarizations induced by carbachol and acetycholine were increased when the extracellular chloride concentration was diminished from 110 to 40 or 10 mM. Sodium pump inhibition by ouabain (10(-4) M) dramatically enhanced the carbachol depolarizations. In 40 mM Cl- solutions, dose-response relations were determineed with ACh in the absence of presence of dTc. For ACh a pD2 value of 5.2 was found; the pA2 value for dTC appeared to be about 6.4. Therefore, the binding properties of the post-denervational receptors do not appear to be influenced by the chloride gradient. It was concluded that drug-induced depolarizations of the denervated rat diaphragm are short-circuited by chloride ions and counteracted by sodium pumping.

The masking role of the sodium pump and chloride ions on the effect of carbachol in- and outside the endplate region of rat diaphragm muscle

The effect of carbachol (10(-3)M) on the membrane potential of rat diaphragm muscle fibres near and remote from the endplate has been investigated by means of a microelectrode technique. In the endplate region a rather slow depolarization was observed followed by spontaneous repolarization. Outside the endplate region similar, but smaller, responses could be measured. In a muscle part separated from the endplate zone, carbachol had no depolarizing effect. Inhibition of the sodium pump by ouabain (10(-4)M) or potassium-free medium not changing the membrane potential itself, allowed carbachol to exert a depolarizing effect on this endplate-free membrane, while in endplate-free regions continuous with the endplate-zone the depolarizations already observed were enhanced. In low chloride solutions carbachol also had a depolarizing effect on endplate-free muscle membrane. It is concluded from these experiments that a direct effect of cholinergic drugs on endplate-free muscle membrane of rat diaphragm is counteracted by the sodium pump in combination with passive chloride movements. The results are discussed in the context of known receptor densities and chloride effects.