Muscarinic regulation of two ionic currents in the bullfrog sympathetic neurone

The mechanism of slow muscarinic excitation in bullfrog sympathetic ganglia was studied using a single-electrode voltage clamp technique. The membrane current induced by muscarine (0.01-30 microM: Imus) consisted of a voltage-dependent component (dIM) induced by the inhibition of K+-current (known as IM), a voltage-independent cation-selective current (ID), or both. In the last case, the magnitude of either component varied in different cells. Generation of both dIM and ID were equally suppressed by pirenzepine with a dissociation constant (Ki) of 30 nM, while they were inhibited less by AF-DX116 with a Ki of 600 nM. Kd values for muscarinic induction of dIM and ID were 0.35 and 5 microM respectively. This difference was also seen even after reducing the receptor population by pretreatment with propylbenzilylcholine mustard. ID was enhanced after blockade of dIM by Ba2+ (4 mM), with a little change in Kd. This effect was larger when the control ID was smaller. These results suggest that a single subtype (M1) of muscarinic receptor generates both the dIM and ID with different efficacies and that Ba2+, known as a blocker of IM (accordingly of a high-efficacy current, dIM), potentiates a low-efficacy current (ID).