The role of calcium in M-current inhibition by muscarinic agonists in rat sympathetic neurons.

I have investigated the role of Ca2+ on M-current (IK(M)) inhibition by the muscarinic agonist oxo-M using the perforated patch voltage clamp technique. Oxo-M inhibited IK(M) in cultured SCG cells with an IC50 of 1.2 microM in 2 mM [Ca2+]o, and 13.1 microM in nominally Ca(2+)-free external solution. BAPTA-AM, ryanodine and thapsigargin (substances which modulate [Ca2+]i) did not affect IK(M) or the inhibitory action of oxo-M in either 2 or 0 mM extracellular Ca2+. Caffeine (10 mM) inhibited M current by approximately 30% in both 2 and 0 mM [Ca2+]o; this inhibition was not affected by [Ca2+]i modulators. Unexpectedly, the effect of oxo-M (10 microM) was enhanced after application of caffeine (10 mM) in either 2 or 0 mM [Ca2+]o. Thus, the effect of muscarinic agonists on IK(M) was blunted in Ca(2+)-free extracellular solutions, but neither oxo-M nor caffeine appeared to inhibit IK(M) through an elevation of [Ca2+]i. I suggest that resting levels of [Ca2+]i are necessary for a normal inhibition, with lower levels inducing an impairment of the inhibition of IK(M) by muscarinic agonists.