29 July 2013
Adenosine receptors (AR; A 1, A 2A, A 2B, and A 3) contract and relax smooth muscle through different signaling mechanisms. Deciphering these complex responses remains difficult because relationships between AR subtypes and various end-effectors (e.g., enzymes and ion channels) remain to be identified. A 1AR stimulation is associated with the production of 20–hydroxyeicosatetraenoic acid (20–HETE) and activation of protein kinase C (PKC). 20–HETE and PKC can inhibit large conductance Ca 2+/voltage-sensitive K + (BK) channels that regulate smooth muscle contraction. We tested the hypothesis that activation of A 1AR inhibits BK channels via a PKC-dependent mechanism. Patch clamp recordings and Western blots were performed using aortae of wild type (WT) and A 1AR knockout (A 1KO) mice. There were no differences in whole-cell K + current or α and β1 subunits expression between WT and A 1KO. 20–HETE (100 nmol/L) inhibited BK current similarly in WT and A 1KO mice. NECA (5′–N–ethylcarboxamidoadenosine; 10 μmol/L), a nonselective AR agonist, increased BK current in myocytes from both WT and A 1KO mice, but the increase was greater in A 1KO (52 ± 15 vs. 17 ± 3%; P < 0.05). This suggests that A 1AR signaling negatively regulates BK channel activity. Accordingly, CCPA (2–chloro–N(6)-cyclopentyladenosine; 100 nmol/L), an A 1AR-selective agonist, inhibited BK current in myocytes from WT but not A 1KO mice (81 ± 4 vs. 100 ± 7% of control; P < 0.05). Gö6976 (100 nmol/L), a PKCα inhibitor, abolished the effect of CCPA to inhibit BK current (99 ± 3% of control). These data lead us to conclude that, in aortic smooth muscle, A 1AR inhibits BK channel activity and that this occurs via a mechanism involving PKCα.