CARDIAC MUSCARINIC POTASSIUM CHANNEL ACTIVITY IS ATTENUATED BY INHIBITORS OF G(BETA-GAMMA)

The cardiac muscarinic potassium channel (I-K.ACh) is activated by a G protein upon receptor stimulation with acetylcholine. The G protein subunit responsible for activation (G(alpha) versus G(beta gamma)) has been disputed. We used G(beta gamma) inhibitors derived from the beta-adrenergic kinase 1 (beta ARK1) to assess the relative importance of G(beta gamma) in I-K.ACh activation. In rabbit atrial myocytes, I-K.ACh had a conductance of 49+/-6.2 pS. In inside-out patches, the mean open time was 1.60+/-0.57 ms, mean time constant ( tau(o)) was 1.59+/-0.53 ms, and mean closed time was 3.02+/-1.35 ms (n=38). beta ARK1 is a G(beta gamma)-sensitive enzyme that interacts with G(beta gamma) through a defined sequence near its carboxyl terminus. A 28-amino-acid peptide derived from the carboxyl terminus of beta ARK1 (peptide G) increased the closed time to 10.04 ms (P<.001) and decreased opening probability (NPo) by 71% (P<.001). Fusion proteins containing the entire carboxyl terminus of beta ARK1, glutathione S-transferase beta ARK1ct and hexahistidine beta ARK1ct, decreased NPo by 67% (P=.03) and 48% (P=.009), respectively. They also both significantly increased the closed time. None of the inhibitors affected mean open time or channel amplitude. A control peptide derived from a neighboring region of beta ARK1 had no significant effect on I-K.ACh activity. These results provide further evidence for the role of G(beta gamma) in the activation of I-K.ACh.