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1 University of Rostock, Institute of Physiology, D-18055 Rostock, Germany; and 2 Institute of Experimental Cardiology, Cardiology Research Center, 121552 Moscow, Russia
The hypothesis that cAMP-dependent protein
kinase (protein kinase A; PKA) is in an active state in small arteries
possessing a myogenic tone was investigated in pressurized rat tail
small arteries. At a pressure of 80 mmHg, these vessels constricted to
71.6 ± 1.0% (n = 32) of the
diameter of the fully relaxed state. The PKA inhibitors
Rp-8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphothioate (Rp-CPT-cAMPS) and
N-(2-{[3-(4-bromophenyl)-2-propenyl]amino}-ethyl)-5-isoquinolinesulfonamide HCl (H-89) constricted these vessels dose dependently. For
example, 300 µM Rp-CPT-cAMPS and 9 µM H-89 reduced vessel diameter
by 11.0 ± 1.2% (n = 8) and 14.3 ± 3.6% (n = 5), respectively. The
cGMP-dependent protein kinase (protein kinase G; PKG) inhibitor
Rp-8-bromo-
-phenyl-1,N2-etheno-guanosine
3',5'-cyclic monophosphothioate (Rp-8-Br-PET-cGMPS) did not
alter vessel diameter up to a concentration of 10 µM. Neither
endothelium removal nor inhibition of neural transmission affected the
action of Rp-CPT-cAMPS. The effect of 300 µM Rp-CPT-cAMPS was reduced
by 82% after pretreatment of the vessel with 100 nM iberiotoxin, a
blocker of calcium-activated potassium
(KCa) channels. However, the
effect of 300 µM Rp-CPT-cAMPS was not altered after pretreatment with
1 mM 4-aminopyridine, a blocker of delayed rectifier potassium
channels, or 10 µM ryanodine, a blocker of ryanodine receptor-generated calcium sparks. In inside-out patch-clamp
experiments on cells isolated from rat tail small arteries, 10 U/ml of
the catalytic subunit of PKA together with 100 µM MgATP increased KCa channel activity 30.1 ± 9.8-fold (n = 9). Additionally,
neither inhibition of PKA or PKG nor moderate activation of PKA or PKG altered the vessel response to a pressure step from 80 to 120 mmHg.
These results suggest that in rat tail small arteries possessing a
myogenic tone 1) PKA is in an active
state modulating the level of the myogenic tone, and
2)
KCa channels mediate, at least
partly, this effect of PKA.
vascular smooth muscle; calcium-activated potassium channel; protein kinase G; myogenic response
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