Permeability to Ca2+ of the acetylcholine receptor channel of denervated mouse muscles in the presence of Na+ and of some other cations.

Calcium uptake caused by exposure to azelainylcholine and the additional membrane slope conductance caused by the same agonist were compared in partially depolarized mouse soleus muscles denervated for 3-5 days. Ca uptake was estimated from the amount of 45Ca retained after a 2 min exposure to the tracer (1 min in the presence of azelainylcholine) and a subsequent 17 min period of tracer washout. The amount taken up in the presence of Na+ was 0.152 m mole/kg fresh muscle. The uptake was by about 60% higher when Na+ was replaced by N-methyl-D-glucamine. For 10 other monovalent cations Ca uptake was less than with Na. Ca uptake was not related to the molecular weight, size or structure of the cation. The slope conductance in the presence of 10 microM azelainylcholine was 4 microseconds and it was 21% of that value when Na+ was replaced by N-methyl-D-glucamine, i.e. conductance was decreased when Ca uptake was increased. This discrepancy points to a major difference in the way cations such as Ca2+ and K+ pass the receptor channel.