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1 Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6085; and 2 Department of Physiology, University of Melbourne, Parkville, Australia 3052
These experiments were performed to determine the effects of
reducing Ca2+ influx
(Cain) on
K+ currents
(IK) in
myocytes from rat small mesenteric arteries by
1) adding external
Cd2+ or
2) lowering external
Ca2+ to 0.2 mM. When measured from
a holding potential (HP) of 
20 mV
(IK20),
decreasing Cain decreased
IK at voltages
where it was active (>0 mV). When measured from a HP of 60 mV
(IK60),
decreasing Cain increased
IK at voltages
between 30 and +20 mV but decreased IK at voltages
above +40 mV. Difference currents
(
IK) were
determined by digital subtraction of currents recorded under control
conditions from those obtained when
Cain was decreased. At test
voltages up to 0 mV,
IK60 exhibited
kinetics similar to control
IK60, with rapid
activation to a peak followed by slow inactivation. At 0 mV, peak
IK60 averaged
75 ± 13 pA (n = 8) with
Cd2+ and 120 ± 20 pA
(n = 9) with low
Ca2+ concentration. At test
voltages from 0 to +60 mV,
IK60 always had an early positive peak phase, but its apparent "inactivation" increased with voltage and its steady value became negative above +20
mV. At +60 mV, the initial peak
IK60 averaged
115 ± 18 pA with Cd2+ and 187 ± 34 pA with low Ca2+. With 10 mM pipette BAPTA, Cd2+ produced a
small inhibition of
IK20 but still
increased IK60 between 30 and +10 mV. In
Ca2+-free external solution,
Cd2+ only decreased both
IK20 and
IK60. In the
presence of iberiotoxin (100 nM) to inhibit
Ca2+-activated
K+ channels
(KCa),
Cd2+ increased
IK60 at all
voltages positive to 30 mV while BAY K 8644 (1 µM) decreased
IK60. These
results suggest that Cain, through L-type Ca2+ channels and perhaps
other pathways, increases KCa
(i.e., IK20) and decreases voltage-dependent K+
currents in this tissue. This effect could contribute to membrane depolarization and force maintenance.
L-type calcium channels; vascular smooth muscle; mesenteric arteries; electrophysiology; patch clamp
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