CATION CHANNEL ACTIVATED BY MUSCARINIC AGONISTS ON PORCINE ADRENAL CHROMAFFIN CELLS

A large portion (70%) of the secretory response to muscarinic agonists in porcine adrenal chromaffin cells has previously been shown to be d ependent on extracellular Ca2+ (Xu et al., J. Neurochem. 56: 1899-1896 , 1991). Results presented here show that muscarinic agonists activate a cation-selective channel which is permeable to divalent cations. Th e muscarinic agonist, methacholine, was found to activate the uptake o f Mn2+, which paralleled the ability of methacholine to activate Ca-45 (2+) uptake as shown previously. Secretion induced by methacholine was not affected by nifedipine, a compound that inhibits dihydropyridine- sensitive voltage-gated Ca2+ channels. In voltage-clamped cells, metha choline activated whole cell currents, which reversed at approximately -20 mV in standard salt solutions. However, with the standard whole c ell configuration, the currents were slow to activate and were often e rratic. In contrast, when the perforated-patch (nystatin) technique wa s used to measure whole cell currents, methacholine rapidly activated sustained inward currents. Ion-substitution experiments indicated that the inward currents were carried by Na+, Ba2+, or Ca2+ but not by Cl- . Single-channel currents activated by methacholine were observed in o utside-out vesicles, which were electrically accessed using the perfor ated-patch technique. These channels reversed at -15 mV, had a slope c onductance of 20 pS, and were 14-fold more likely to be open in the pr esence of methacholine. These channels are probably responsible for th e extracellular Ca2+-dependent secretory response to muscarinic recept or stimulation in porcine adrenal chromaffin cells.