On the mechanism of G protein beta gamma subunit activation of the muscarinic K+ channel in guinea pig atrial cell membrane. Comparison with the ATP-sensitive K+ channel.

Abstract

A B ST R AC T The mechanism of G protein ~3,/subunit (Gl3v)-induced activation of the muscarinic K + channel (KAch) in the guinea pig atrial cell membrane was examined using the inside-out patch clamp technique. G0v and GTP-~,S-bound ct subunits (G*ds) of pertussis toxin (PT)-sensitive G proteins were purified from bovine brain. Either in the presence or absence of Mg 2+, GI~ ~ activated the KACh channel in a concentration-dependent fashion. 10 nM G~ almost fully activated the channel in 132 of 134 patches (98.5%). The Gl~-induced maximal channel activity was equivalent to or sometimes larger than the GTP-TS-induced one. Half-maximal activation occurred at ~ 6 nM Gt3 ~. Detergent (CHAPS) and boiled G~ preparation could not activate the KACh channel. G~v suspended by Lubrol PX instead of CHAPS also activated the channel. Even when G~ was pretreated in Mg2+-free EDTA internal solution containing GDP analogues (24---48 h) to inactivate possibly contam0t' inating G~ s, the G0v activated the channel. Furthermore, GI~ ~ preincubated with excessive GDP-bound Go,, did not activate the channel. These results indicate that GI~ v itself, but neither the detergent CHAPS nor contaminating G~, activates the KACh channel. Three different kinds of G,*, at 10 pM-10 nM could weakly activate the KACh channel. However, they were effective only in 40 of 124 patches (32.2%) and their maximal channel activation was ~ 20% of that induced by GTP-~/S or Gay.

A comparison of adenosine-induced cardioprotection and ischemic preconditioning in dogs. Efficacy, time course, and role of KATP channels.

“…Preconditioning with either 10 or 60 minutes of reperfusion produced a marked reduction (P<.05) in infarct size (6.7±2.5% and 8.7±2.6%, respectively, versus 26.9±4.3% in controls). Administration of adenosine with a 10-minute drug-free period before 60 minutes of occlusion resulted in a marked decrease in infarct size similar to that seen with preconditioning (9.6±1.7% versus 26.9±4.3% in controls); however, the protection disappeared when a 60-minute drugfree period was allowed after adenosine administration (23 Adenosine is a metabolite of adenine nucleotides; it accumulates quickly in large amounts in the ischemic myocardium.4 A large body of evidence suggests that adenosine can protect the heart against ischemia/reperfusion injury.5 Numerous recent studies have shown that activation of adenosine A, receptors mimicked and blockade of these receptors abolished the effect of ischemic preconditioning to reduce infarct size. 5 Liu and coworkers6 have further demonstrated that in isolated blood-perfused rabbit hearts, intracoronary infusion of adenosine for 5 minutes followed by 10 beats per minute, the heart was paced at that rate with rectangular pulses of 4-millisecond duration and a voltage twice threshold via bipolar electrodes sutured to the left atrial appendage.…”

Adenosine-induced activation of ATP-sensitive K+channels in excised membrane patches is mediated by PKC

“…Kirsch et al (16) showed that adenosine activates K ATP channels at reduced [ATP] i via the ␣-subunit of inhibitory G protein (16), in contrast to the G i ␤␥-mediated regulation of acetylcholine-sensitive K ϩ current (13). G protein regulates K ATP by removing [ATP] i -induced inhibition (14,15,36).…”

“…adenosine A 1 receptor activation stimulates K ATP channels via an inhibitory G protein (G* i␣ )-mediated pathway (13,14,16,17,20,36). Recently, the stimulation of PKC has been shown to increase K ATP channel activity in cardiac myocytes (11,23,26).…”

Role of heterotrimeric G proteins in membrane traffic.

“…Logothetis et al (1987) showed that fly activated a K+ channel. Although this result was originally controversial, it recently has been confirmed and extended (Ito et al, 1992). Perhaps the most revealing case is control of adenylyl cyclase by Gs.…”