cAMP-dependent protein kinase is in an active state in rat small arteries possessing a myogenic tone

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 microM Rp-CPT-cAMPS and 9 microM 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-beta-phenyl-1,N(2)-etheno-guanosine 3', 5'-cyclic monophosphothioate (Rp-8-Br-PET-cGMPS) did not alter vessel diameter up to a concentration of 10 microM. Neither endothelium removal nor inhibition of neural transmission affected the action of Rp-CPT-cAMPS. The effect of 300 microM Rp-CPT-cAMPS was reduced by 82% after pretreatment of the vessel with 100 nM iberiotoxin, a blocker of calcium-activated potassium (K(Ca)) channels. However, the effect of 300 microM Rp-CPT-cAMPS was not altered after pretreatment with 1 mM 4-aminopyridine, a blocker of delayed rectifier potassium channels, or 10 microM 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 microM MgATP increased K(Ca) 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) K(Ca) channels mediate, at least partly, this effect of PKA.