Cation channel activated by muscarinic agonists on porcine adrenal
chromaffin cells.
Forsberg, Erik J., Qiumin Li, and Yanping Xu.
Department of Physiology, University of Wisconsin Medical School,
Madison, WI 53706
APStracts 2:0037E, 1995.
A large portion (70%) of the secretory response to muscarinic agonists
in porcine adrenal chromaffin cells has previously been shown to be
dependent on extracellular Ca2+ (Xu et al. (1991) J. Neurochem. 56,
1899-1896). Results presented here show that muscarinic agonists
activate a cation-selective channel which is permeable to divalent
cations. The muscarinic agonist, methacholine, was found to activate
the uptake of Mn2+ which paralleled the ability of methacholine to
activate 45Ca2+ uptake as shown previously. Secretion induced by
methacholine was not affected by nifedipine, a compound which
inhibits dihydropyridine-sensitive, voltage-gated, Ca2+ channels. In
voltage-clamped cells, methacholine activated whole cell currents
which reversed at approximately -20 mV in standard salt solutions.
However, using the standard whole cell configuration, the currents
were slow to activate and were often erratic. In contrast, when using
the perforated patch (nystatin) technique 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 outside-out vesicles which
were electrically accessed using the perforated patch technique.
These channels reversed at -15 mV, had a slope conductance of 20 pS,
and were 14-fold more likely to be open in the presence of
methacholine. These channels are probably responsible for the
extracellular Ca2+-dependent secretory response to muscarinic
receptor stimulation in porcine adrenal chromaffin cells.
Received 17 November 1994; accepted in final form 15 February
1995.
APS Manuscript Number E477-4.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 1 March 1995.