Muscarinic receptor-mediated inhibition of voltage-activated Ca current in neuroblastoma x glioma hybrid (NG 108-15) cells--reduction of muscarinic agonist and antagonist potency by tetraethylammonium (TEA).
Patch clamp techniques were used to record voltage-activated Ca currents from NG 108-15 cells. The muscarinic agonists acetylcholine (ACh) and oxotremorine-M partially inhibited the Ca current (mean inhibitor 37%) with EC50 values of 0.32 microM and 0.14 microM, respectively. The EC50 for ACh in the presence of neostigmine (1 microM) was 0.11 microM. Tetraethylammonium (TEA) (10 mM) (10 mM) shifted ACh (+ neostigmine) and oxotremorine dose response curves to the right by 36- and 20-fold, respectively. TEA also inhibited muscarinic receptor binding (Ki values in brackets) of (i) N-methyl scopolamine ([3H]NMS) to homogenates of rat heart (86 microM) and rat submandibular gland (178 microM) and to intact NG 108-15 cells (358 microM) and (ii) [3H]pirenzepine binding to rat cerebral cortex homogenates (111 microM). Atropine antagonized muscarinic agonist responses and was found to have a pKB of 9.8 +/- 0.39, but 10 mM TEA reduced the potency of atropine by 69-fold, probably because of the weak antagonist action of TEA.