Potassium currents in freshly dissociated uterine myocytes from nonpregnant and late-pregnant rats.
In freshly dissociated uterine myocytes, the outward current is carried by K+ through channels highly selective for K+. Typically, nonpregnant myocytes have rather noisy K+ currents; half of them also have a fast-inactivating transient outward current (ITO). In contrast, the current records are not noisy in late pregnant myocytes, and ITO densities are low. The whole-cell IK of nonpregnant myocytes respond strongly to changes in [Ca2+]o or changes 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 to dissociating enzymes. By holding at -80, -40, or 0 mV and digital subtractions, the whole-cell IK 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+ currents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in late-pregnant myocytes. In detached membrane patches from the latter, identified 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 coli myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium, charybdotoxin, and iberiotoxin contribute 30-35% of the total IK in nonpregnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptible, small-conductance delayed rectifier currents are not seen in nonpregnant myocytes, but contribute approximately 20% of total IK in late-pregnant myocytes. Thus, in late-pregnancy, myometrial excitability is increased by changes in K+ currents that include a suppression of the ITO, a redistribution of IK expression from large-conductance Ca2+-activated channels to smaller-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a positive shift of the activation of some large-conductance Ca2+-activated channels.
Wang
S Y
SY
Department of Pharmacology, State University of New York Health Science Center, Brooklyn, New York 11203, USA.
Yoshino
M
M
Sui
J L
JL
Wakui
M
M
Kao
P N
PN
Kao
C Y
CY
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D018999:Charybdotoxin / Q000494:pharmacology; D004546:Elapid Venoms / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D009215:Myometrium / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D010782:Photolysis; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000378:metabolism*; D051381:Rats; D019789:Tetraethylammonium / Q000494:pharmacology
D004546:Elapid Venoms; D010455:Peptides; D015221:Potassium Channels; D018999:Charybdotoxin; D001030:Apamin; C008331:mast cell degranulating peptide; D019789:Tetraethylammonium; C020273:dendrotoxin; C064719:iberiotoxin; D015761:4-Aminopyridine; D002118:Calcium