The M3 Receptor-mediated K+ Current (IKM3), a Gq Protein-coupled K+ Channel

Abstract

Stimulation of muscarinic acetylcholine receptors (mAChRs) can activate an inward rectifier K؉ current (I KACh ), which is mediated by the M 2 subtype of mAChR in cardiac myocytes. Recently, a novel delayed rectifierlike K ؉ current mediated by activation of the cardiac M 3 receptors (designated I KM3 ) was identified, which is distinct from I KACh and other known K ؉ currents. While I KACh is known to be a G i protein-gated K ؉ channel, the signal transduction mechanisms for I KM3 activation remained unexplored. We studied I KM3 with whole-cell patch clamp and macropatch clamp techniques. Whole cell I KM3 activated by choline persisted with minimal rundown over 2 h in presence of internal GTP. When GTP was replaced by guanyl-5-yl thiophosphate, I KM3 demonstrated rapid and extensive rundown. While I KACh (induced by ACh) was markedly reduced in cells pretreated with pertussis toxin, I KM3 was unaltered. Intracellular application of antibodies targeting ␣-subunit of G i/o protein suppressed I KACh without affecting I KM3 . Antibodies targeting the N and the C terminus, respectively, of G q protein ␣-subunit substantially depressed I KM3 but failed to alter I KACh . The antibody against ␤-subunits of G proteins inhibited both I KACh and I KM3 . I KM3 activated by choline in the cell-attached mode of macropatches persisted in the cell-free configuration. Application of purified G q protein ␣-subunit or ␤␥-subunit of G proteins or guanosine 5-O-(thiotriphosphate) to the internal solution activated I KM3 -like currents in inside-out patches. Our findings revealed a novel aspect of receptor-channel signal transduction mechanisms, and I KM3 represents the first G q protein-coupled K ؉ channel. We propose that the G protein-coupled K ؉ channel family could be divided into two subfamilies: G i protein-coupled K ؉ channel subfamily and G q proteincoupled K ؉ channel subfamily.While M 2 receptors are commonly believed to be the only functional mAChRs 1 in cardiac tissues, this concept has been challenged by recent findings revealing the presence of M 3 receptors in the hearts of various species including guinea pig (1, 2), rat (3), dog (4 -6), and human (7-11). We discovered that the cardiac M 3 receptors mediate the activation of a novel delayed rectifier K ϩ current (we have named it I KM3 ) distinct from I KACh and other known K ϩ currents. I KACh is characterized by strong inward rectification, whereas I KM3 conducts a delayed rectifier-like K ϩ current. We also found that M 3 receptors and I KM3 play a significant role in regulating heart rates, cardiac resting membrane potential, and membrane repolarization (1, 2). The findings suggest that we are no longer able to consider parasympathetic control of the heart as due to a simple ACh-M 2 interaction; we have to understand cholinergic effects in terms of the consequence of activating multiple subtypes of mAChRs, with potentially varying signal transduction and effector systems (such as different K ϩ channels) (6). The M 1 and M 3 receptors are characterized biochemica...

Functional M3 cholinoreceptors are present in pacemaker and working myocardium of murine heart

“…These effects in many ways resemble well-known negative action of selective activation of M2 receptors, although they are mediated by another intracellular signaling pathway activated via G q proteins. Although molecular mechanisms of discovered effects were not investigated in the present study, it is known that shortening of APs during M3 stimulation may be produced via direct activation of potassium current I KM3 by subunits of G q protein [10] or activation of phosphoinositide pathway leading to various consequences including suppression of I CaL by PKC [1]. The mechanisms of negative chronotropy may be quite similar as long as activation of potassium current obviously leads to hyperpolarization and slowing of SDD in the pacemaker myocytes.…”

“…M3 cholinoreceptors are coupled to G q proteins; therefore, M3 stimulation leads to enhancing of intracellular phosphoinositide hydrolysis due to the α q -mediated activation of phospholipase C. At the same time, the outward potassium current I KM3 , which is carried by G q proteincoupled K + channels, is activated by G q proteins [9,10]. Moreover, stimulation of M3 receptors alters intercellular communication via modulation of connexin 43 (Cx43) gap junctional channels, particularly inhibiting ischemiainduced dephosphorylation of Cx43 [17], and activates antiapoptotic and other cytoprotective mechanisms [8,16].…”

Chemical activation of a food deprivation signal extends lifespan

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