Bk. Fleischmann et al., MUSCARINIC ACTIVATION AND CALCIUM PERMEATION OF NONSELECTIVE CATION CURRENTS IN AIRWAY MYOCYTES, American journal of physiology. Cell physiology, 41(1), 1997, pp. 341-349
We examined the activation and Ca2+ permeation of nonselective cation
channels in voltage-damped (nystatin), fura 2-loaded equine tracheal m
yocytes at 35 degrees C. Methacholine (50 mu M) induced a biphasic inc
rease in intracellular Ca2+ concentration ([Ca2+](i)) and a biphasic i
nward current consisting of a large, rapidly inactivating Ca2+-activat
ed Cl- current [I-Cl(Ca)] and a smaller, sustained nonselective cation
current (I-cat). I-Cl(Ca) but not I-cat was activated by caffeine. Ne
ither I-cat nor the sustained rise in [Ca2+](i) was blocked by nisoldi
pine, whereas both were rapidly blocked by Ni2+. I-cat was determined
to be Ca2+ permeant, since 1) a sustained elevation of [Ca2+](i) occur
red when I-cat was activated, and blockade of I-cat produced a rapid d
ecline in [Ca2+](i); 2) increasing extracellular Ca2+ during I-cat inc
reased [Ca2+](i); 3) 110 mM extracellular Ca2+ shifted the reversal po
tential of I-cat to 12 mV (Ca2+-to-Cs+ permeability ratio = 3.6); and
4) instantaneous voltage-clamp steps to negative potentials during I-c
at increased the current and [Ca2+](i), whereas depolarizing steps dec
reased the current and [Ca2+](i). The fraction of I-cat carried by Ca2
+ under physiological conditions was estimated to be 14% at -60 mV.