ACTIVATION OF INWARDLY RECTIFYING POTASSIUM CHANNELS BY MUSCARINIC RECEPTOR-LINKED G-PROTEIN IN ISOLATED HUMAN VENTRICULAR MYOCYTES

Muscarinic receptor-linked G protein, G(i), can directely activate the specific K+ channel (IK(ACh)) in the atrium and in pacemaker tissues in the heart. Coupling of G(i) to the K+ channel in the ventricle has not been well defined. G protein regulation of K+ channels in isolated human ventricular myocytes was examined using the patch-clamp techniq ue. Bath application of 1 mu M acetylcholine (ACh) reversibly shortene d the action potential duration to 74.4 +/- 12.1% of control (at 90% r epolarization, mean +/- SD, n = 8) and increased the whole-cell membra ne current conductance without prior beta-adrenergic stimulation in hu man ventricular myocytes, The ACh effect was reversed by atropine (1 m u M). In excised inside-out patch configurations, application of GTP g amma S (100 mu M) to the bath solution (internal surface) caused activ ation of I-K(ACH) and/or the background inwardly-rectifying K+ channel (IK(ACh) in ventricular cell membranes. IK(ACh) exhibited rapid gatin g behavior with a slope conductance of 44 +/- 2 pS (n = 25) and a mean open lifetime of 1.8 +/- 0.3 msec (n = 21). Single channel activity o f GTP gamma S-activated I-K1 demonstrated longlasting bursts with a sl ope conductance of 30 +/- 2 pS (n = 16) and a mean open lifetime of 36 .4 +/- 4.1 msec (n = 12). Unlike I-K(ACh), G protein-activated I-K1 di d not require GTP to maintain channel activity, suggesting that these two channels may be controlled by G proteins with different underlying mechanisms. The concentration of GTP at half-maximal channel activati on was 0.22 mu M in I-K(ACh) and 1.2 mu M in I-KI. Myocytes pretreated with pertussis toxin (PTX) prevented GTP from activating these channe ls, indicating that muscarinic receptor-linked PTX-sensitive G protein , G(i), is essential for activation of both channels, G protein-activa ted channel characteristics from patients with terminal heart failure did not differ from those without heart failure or guinea pig. These r esults suggest that ACh can shorten the action potential by activating I-K(ACh) and I-KI via musca rinic receptor-linked G(i) proteins in hu man ventricular myocytes.