Sy. Wang et al., Potassium currents in freshly dissociated uterine myocytes from nonpregnant and late-pregnant rats, J GEN PHYSL, 112(6), 1998, pp. 737-756
In freshly dissociated uterine myocytes, the outward current is carried by
K+ through channels highly selective for K+. Typically, nonpregnant myocyte
s have rather noisy K+ currents; half of them also have a fast-inactivating
transient outward current (I-TO). In contrast, the current records are not
noisy in late pregnant myocytes, and I-TO densities are low. The whole-cel
l I-K of nonpregnant myocytes respond strongly to changes in [Ca2+](o) or c
hanges in [Ca2+](i) caused by photolysis of caged Ca2+ compounds, nitr 5 or
DM-nitrophene, but that of late-pregnant myocytes respond weakly or not at
all. The Ca2+ insensitivity of the latter is present before any exposure t
o dissociating enzymes. By holding at -80, -40, or 0 mV and digital subtrac
tions, the whole-cell I-K of each type of myocyte can be separated into one
noninactivating and two inactivating components with half-inactivation at
approximately -61 and -22 mV. The noninactivating components, which consist
mainly of iberiotoxin-susceptible large-conductance Ca2+-activated K+ curr
ents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in
late-pregnant myocytes. In detached membrane patches from the latter, ident
ified 139 pS, Ca2+-sensitive K+ channels also have a half-open probability
at 68 mV, and are less sensitive to Ca2+ than similar channels in taenia co
li myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium,
charybdotoxin, and iberiotoxin contribute 30-35% of the total I-K in nonpr
egnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptibl
e, small-conductance delayed rectifier currents are not seen in nonpregnant
myocytes, but contribute similar to 20% of total I-K in late-pregnant myoc
ytes. Thus, in late-pregnancy, myometrial excitability is increased by chan
ges in K+ currents that include a suppression of the I-TO, a redistribution
of I-K expression from large-conductance Ca2+-activated channels to smalle
r-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a
positive shift of the activation of some large-conductance Ca2+-activated
channels.