Regulation of atrial muscarinic K+ channel activity by a cytosolic protein via G protein-independent pathway

Properties and modulation of the G protein-coupled K+ channel in rat cerebellar granule neurons: ATP versus phosphatidylinositol 4,5-bisphosphate

Cerebellar granule (CG) neurons express a G protein-gated K+ current (GIRK) that is involved in the neurotransmitter regulation of the excitatory input to the Purkinje fibres of the cerebellum. Here, we characterized the single-channel behaviour of GIRK in CG neurons, and examined the effects of several known modulators of GIRK and their putative physiological roles. Whole-cell GIRKs were activated by baclofen, a GABAB receptor agonist. In cell-attached patches, baclofen activated GIRK with a single-channel conductance of 34 pS and a mean open time of 0.5 ms. In inside-out patches, application of GTPgammaS to the cytoplasmic side activated GIRK with similar kinetic properties. Addition of 2 mM ATP resulted in a marked increase in GIRK activity and induced longer-lived openings with a mean open time of 2.3 ms (ATP-dependent gating). Brain cytosolic fraction or free fatty acids inhibited this effect of ATP, and this was reversed by addition of purified recombinant brain fatty acid binding protein. Applying phosphatidylinositol 4,5-bisphosphate (PIP2) to inside-out patches in place of ATP also increased GIRK activity; however, only an increase in the frequency of opening was observed. The stimulatory effect of PIP2 on GIRK activity was not inhibited by the cytosolic fraction. Following maximal activation by PIP2, ATP caused an additional 2.2-fold increase in GIRK activity. These results show that GIRKs in CG neurons are regulated by positive and negative modulators that affect frequency as well as open time duration. The net effect is that the ligand-activated GIRK is in the 'low activity' state associated with short-lived openings, mainly due to strong action of the cytosolic inhibitor of ATP-dependent gating. Our results also show that intracellular ATP modulates GIRK via pathways different from that of PIP2 in CG neurons.

Role of receptor kinase in long-term desensitization of the cardiac muscarinic receptor-K+ channel system

Desensitization of the cardiac muscarinic K+ channel was studied in cultured neonatal rat atrial cells and in Chinese hamster ovary (CHO) cells transfected with muscarinic receptor (HM(2)), G protein-coupled inward rectifying K+ channels 1 and 4, and G protein-coupled receptor kinase 2. In atrial cells incubated in 10 microM carbachol for 24 h, channel activity in cell-attached patches was substantially reduced as a result of long-term desensitization. The long-term desensitization was also observed in CHO cells transfected with the wild-type receptor and receptor kinase (as well as the channel). However, long-term desensitization was greatly reduced or abolished if the cells were 1) not transfected with the receptor kinase, 2) transfected with a mutant receptor lacking phosphorylation sites (rather than the wild-type receptor), or 3) transfected with a mutant receptor kinase lacking kinase activity (rather than the wild-type receptor kinase). We suggest that long-term desensitization of the cardiac muscarinic receptor-K+ channel system to muscarinic agonist may involve phosphorylation of the receptor by receptor kinase.

Evidence of involvement of GIRK1/GIRK4 in long-term desensitization of cardiac muscarinic K+ channels

The cardiac M2 muscarinic receptor/G protein/K+ channel system was studied in neonatal rat atrial cells cultured with and without 10 microM carbachol (CCh) for 24 h. Channel activity in CCh-pretreated cells was substantially reduced as a result of long-term desensitization regardless of whether the channel was activated by ACh in cell-attached patches or GTP in inside-out patches. Channel activity in CCh-pretreated cells was also low when the receptor was bypassed and the G protein and channel were directly activated by [gamma-S]GTP or both the receptor and G protein were bypassed and the channel was directly activated by trypsin. Finally, in CCh-pretreated cells, the whole cell K+ current was low when the channel was activated via the independent adenosine receptor. This suggests that the channel is involved in long-term desensitization. However, in CCh-pretreated cells, although the receptor was internalized, there was no internalization of the channel. We suggest that the function of the muscarinic K+ channel declines in long-term desensitization of the cardiac M2 muscarinic receptor/G protein/K+ channel system.

Cytoplasmic unsaturated free fatty acids inhibit ATP-dependent gating of the G protein-gated K(+) channel

This study reports the identification of an endogenous inhibitor of the G protein-gated (K(ACh)) channel and its effect on the K(ACh) channel kinetics. In the presence of acetylcholine in the pipette, K(ACh) channels in inside-out atrial patches were activated by applying GTP to the cytoplasmic side of the membrane. In these patches, addition of physiological concentration of intracellular ATP (4 mM) upregulated K(ACh) channel activity approximately fivefold and induced long-lived openings. However, such ATP-dependent gating is normally not observed in cell-attached patches, indicating that an endogenous substance that inhibits the ATP effect is present in the cell. We searched for such an inhibitor in the cell. ATP-dependent gating of the K(ACh) channel was inhibited by the addition of the cytosolic fraction of rat atrial or brain tissues. The lipid component of the cytosolic fraction was found to contain the inhibitory activity. To identify the lipid inhibitor, we tested the effect of approximately 40 different lipid molecules. Among the lipids tested, only unsaturated free fatty acids such as oleic, linoleic, and arachidonic acids (0.2-2 microM) reversibly inhibited the ATP-dependent gating of native K(ACh) channels in atrial cells and hippocampal neurons, and of recombinant K(ACh) channels (GIRK1/4 and GIRK1/2) expressed in oocytes. Unsaturated free fatty acids also inhibited phosphatidylinositol-4, 5-bisphosphate (PIP(2))-induced changes in K(ACh) channel kinetics but were ineffective against ATP-activated background K(1) channels and PIP(2)-activated K(ATP) channels. These results show that during agonist-induced activation, unsaturated free fatty acids in the cytoplasm help to keep the cardiac and neuronal K(ACh) channels downregulated by antagonizing their ATP-dependent gating. The opposing effects of ATP and free fatty acids represent a novel regulatory mechanism for the G protein-gated K(+) channel.

Modulation of rat atrial G protein-coupled K+ channel function by phospholipids

1. G protein-gated K+ channels (KACh channels) in the heart and brain are activated by the betagamma subunit of inhibitory G protein. Phosphatidylinositol-4,5-bisphosphate (PIP2) has recently been reported to directly activate KACh channels (GIRK) expressed in oocytes, as well as to support activation by the betagamma subunit in the presence of Na+. We examined the effect of Na+, PIP2 and other phospholipids on the KACh channel to understand better their role in KACh channel activation and modulation. 2. In atrial membrane patches, none of the phospholipids tested including PIP2 caused activation of the KACh channel in either the presence or the absence of 30 mM Na+. PIP2 (3 microM) and other phospholipids (30 microM) blocked acetylcholine-induced activation of the KACh channel. 3. When KACh channels were first activated with GTPgammaS, however, all phospholipids (100 microM) tested augmented the KACh channel activity 1.5- to 2-fold. Phosphatidylinositol-4-phosphate (PIP) and PIP2 were an order of magnitude more potent than other phospholipids. The increase in KACh channel activity was the result of a shift in the gating mode of the channel from a short-lived to a longer-lived open state. Such a modulatory effect was qualitatively similar to that produced by intracellular ATP. Trypsin blocked the ATP effect but not the phospholipid effect on the KACh channel kinetics. 4. The phosphate group linked to the glycerol backbone was important for KACh channel modulation by phospholipids. The higher potency of PIP and PIP2 was due to the presence of inositol phosphates. 5. Intracellular Na+ (30 mM) increased the frequency of KACh channel opening approximately 2-fold if the channels were already active, but did not affect modulation by phospholipids. The effects of Na+ and phospholipids on KACh channel activity were additive. 6. A low concentration of ATP (20 microM), which had no effect on the KACh channel by itself, potentiated the stimulatory action of phospholipids, indicating that ATP and phospholipids interacted to modulate KACh channel function. 7. We conclude that exogenously applied PIP2 and other phospholipids block agonist-mediated KACh channel activation. However, if the KACh channel is already activated with GTPgammaS, phospholipids augment the existing activity by increasing the number of longer-lived channel openings. The evidence for and against the role of PIP and PIP2 in the stimulatory effect of ATP on the KACh channel is presented and discussed.

Heterologous expression and coupling of G protein-gated inwardly rectifying K+ channels in adult rat sympathetic neurons

1. G protein-gated inwardly rectifying K+ (GIRK) channels were heterologously expressed in rat superior cervical ganglion (SCG) neurons by intranuclear microinjection. The properties of GIRK channels and their coupling to native receptors were characterized using the whole-cell patch-clamp technique. 2. Following coinjection of either GIRK1-2 or GIRK1-4 cDNA, application of noradrenaline (NA) produced large inwardly rectifying K+ currents. Injection of cDNA encoding individual GIRK subunits produced only small and inconsistent NA-activated inward currents. Current arising from the native expression of GIRK channels in SCG neurons was not observed. 3. NA-mediated activation of GIRK channels was abolished by pertussis toxin (PTX) pretreatment, indicating coupling via G proteins of the Gi/Go subfamily. Conversely, vasoactive intestinal peptide (VIP) activated GIRK channel currents via a cholera toxin-sensitive pathway suggesting coupling through Galphas. Pretreatment of neurons with PTX caused a significant increase in amplitude of the VIP-mediated GIRK channel currents when compared with untreated cells. 4. Application of adenosine, prostaglandin E2 and somatostatin resulted in activation of GIRK channel currents. Activation of m1 muscarinic acetylcholine receptors (i.e. application of oxotremorine M to PTX-treated neurons) failed to elicit overt GIRK channel currents. 5. GIRK channel overexpression decreased basal Ca2+ channel facilitation significantly when compared with uninjected neurons. Furthermore, the NA-mediated inhibition of Ca2+ channels was significantly attenuated. 6. In summary, the ability to heterologously express GIRK channels in adult sympathetic neurons allows the experimental alteration of receptor-G protein-effector stoichiometry. Such studies may increase our understanding of the mechanisms underlying ion channel modulation by G proteins in a neuronal environment.

Role of receptor kinase in short-term desensitization of cardiac muscarinic K+ channels expressed in Chinese hamster ovary cells

1. The cardiac muscarinic receptor-K+ channel system was reconstructed in Chinese hamster ovary (CHO) cells by transfecting the cells with the various components of the system. The activity of the muscarinic K+ channel was measured with the cell-attached configuration of the patch clamp technique. 2. In CHO cells transfected with the channel (Kir3.1/Kir3.4), receptor (hm2) and receptor kinase (GRK2), on exposure to agonist, there was a decline in channel activity as a result of desensitization, similar to that in atrial cells. 3. Whereas the desensitization was almost abolished by not transfecting with the receptor kinase or by transfecting with a mutant receptor lacking phosphorylation sites, it was only reduced (by approximately 39%) by transfecting with a mutant receptor kinase with little/kinase activity. 4. These results suggest that the receptor kinase is responsible for desensitization of the muscarinic K+ channel and that this involves phosphorylation-dependent and -independent mechanisms.

ATP-dependent desensitization of the muscarinic K+ channel in rat atrial cells

1. Fast desensitization of the muscarinic K+ channel has been studied in excised patches from rat atrial cells. 2. In inside-out patches, ACh was present in the pipette and GTP was applied via the bath to activate the channel. In outside-out patches, GTP was present in the pipette and ACh was applied via the bath to activate the channel. In both cases, during a 30 s exposure to GTP or ACh there was a decline in channel activity as a result of fast desensitization if ATP was present. 3. In inside-out patches, fast desensitization was still observed if the muscarinic ACh receptor was bypassed and the channel was activated by GTP gamma S. This suggests that fast desensitization is a result of a modification of the channel (or the connecting G protein) and not the receptor. 4. In both inside-out and outside-out patches, channel activity was depressed and fast desensitization was reduced or absent, if ATP was not present. 5. The non-hydrolysable analogue of ATP, AMP-PNP, did not substitute for ATP in its effects on the channel. 6. The results are consistent with the hypothesis that fast desensitization of the muscarinic K+ channel is the result of a dephosphorylation of the channel.