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 s ubunit 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 ope n time was 1.60+/-0.57 ms, mean time constant ( tau(o)) was 1.59+/-0.5 3 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) throug h a defined sequence near its carboxyl terminus. A 28-amino-acid pepti de derived from the carboxyl terminus of beta ARK1 (peptide G) increas ed the closed time to 10.04 ms (P<.001) and decreased opening probabil ity (NPo) by 71% (P<.001). Fusion proteins containing the entire carbo xyl terminus of beta ARK1, glutathione S-transferase beta ARK1ct and h exahistidine beta ARK1ct, decreased NPo by 67% (P=.03) and 48% (P=.009 ), respectively. They also both significantly increased the closed tim e. 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 furt her evidence for the role of G(beta gamma) in the activation of I-K.AC h.