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Department of Biological Sciences and Center for Environmental Signal Transduction, College of Arts and Sciences, Western Michigan University, Kalamazoo, Michigan 49008
We examined the functional role of
large-conductance Ca2+-activated
K+
(KCa) channels in the hamster
cremasteric microcirculation by intravital videomicroscopy and
characterized the single-channel properties of these channels in
inside-out patches of membrane from enzymatically isolated cremasteric
arteriolar muscle cells. In second-order (39 ± 1 µm,
n = 8) and third-order (19 ± 2 µm, n = 8) cremasteric arterioles
with substantial resting tone, superfusion with the
KCa channel antagonists
tetraethylammonium (TEA, 1 mM) or iberiotoxin (IBTX, 100 nM) had no
significant effect on resting diameters
(P > 0.05). However, TEA potentiated
O2-induced arteriolar constriction
in vivo, and IBTX enhanced norepinephrine-induced contraction of
cremasteric arteriolar muscle cells in vitro. Patch-clamp studies
revealed unitary K+-selective and
IBTX-sensitive currents with a single-channel conductance of 240 ± 2 pS between 
60 and 60 mV (n = 7 patches) in a symmetrical 140 mM
K+ gradient. The free
Ca2+ concentration
([Ca2+]) for
half-maximal channel activation was 44 ± 3, 20 ± 1, 6 ± 0.4, and 3 ± 0.5 µM at membrane potentials of 60,
30, +30, and +60 mV, respectively
(n = 5), with a Hill coefficient of
1.9 ± 0.2. Channel activity increased
e-fold for a 16 ± 1 mV
(n = 6) depolarization. The
plot of log[Ca2+] vs.
voltage for half-maximal activation
(V1/2) was
linear (r2 = 0.9843, n = 6); the change in
V1/2 for a
10-fold change in
[Ca2+] was 84 ± 5 mV, and the [Ca2+] for
half-maximal activation at 0 mV
(Ca0; the
Ca2+ set point) was 9 µM. Thus, in vivo,
KCa channels are silent in cremasteric arterioles at rest but can be recruited during
vasoconstriction. We propose that the high
Ca0 is responsible for the
apparent lack of activity of these channels in resting cremasteric
arterioles, and we suggest that this may result from expression of
unique KCa channels in the
microcirculation.
microcirculation; vasoconstriction; iberiotoxin; tetraethylammonium; skeletal muscle; vascular smooth muscle; calcium ions; oxygen; patch clamp; norepinephrine; hamster; cremaster muscle
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