Arachidonic acid metabolites alter G protein-mediated signal transduction in heart. Effects on muscarinic K+ channels.

Abstract

The muscarinic acetylcholine receptor (mAChR)-stimulated, inwardly rectifying K + current (Ir4^Cal) was examined in single bullfrog atrial cells using the whole-cell patch clamp technique. Iir was activated either by bath addition of 1 /~M ACh or via activation of the G protein, Gk, with guanosine-'r-thiotriphosphate (GTP3~S). Arachidonic acid (AA) modulated IKtAC~] under both conditions. AA decreased mAChR-stimulated Ir4Aoq and increased the rate of decay from the peak current (desensitization). In addition, AA affected GTPTS-activated IK[AC~] by modulation of C~. The effects of AA and its metabolites on Gk were assessed by examining their effects on both the basal rate of Gk activation by GTPTS, and the mAChR-mediated increase in activation rate produced by nanomolar ACh. AA increased the basal rate of GTP"/S-mediated IKtxchl activation, but reduced the ACh-induced augmentation of this rate. All of the effects of AA on GTP3,Smediated IgtACa] activation were produced by metabolites. A lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), decreased the basal and ACh-enhanced rate of IKIAChl activation in both the presence and absence of exogenous AA. In contrast, indomethacin (INDO), a cyclooxygenase inhibitor, increased the basal rate of IKtAChj activation by GTP'rS in both the presence and absence of exogenous AA, and reversed the effects of AA on the ACh-augmented basal rate. AA metabolites produced via lipoxygenase and cyclooxygenase pathways thus have opposing effects on the signal transduction pathway from mAChR to IV4ACh]. We directly tested a lipoxygenase pathway metabolite, LTC4, on GTP3,S-mediated IK[AChl activation and found that it not only overcame the inhibitory effects of NDGA, but also increased both the basal and ACh-augmented rate of Ir4Aoq activation. From these data, we propose that AA metabolites modulate the function of G k by altering its kinetic properties.Address reprint requests to Dr.

Modulation of calcium channel currents by arachidonic acid in single smooth muscle cells from vas deferens of the guinea‐pig

“…This type of direct action has also been suggested for the mechanism of the AA-induced increase in large conductance Ca-dependent K channel (BK channel) activity in pulmonary smooth muscle cells (Kirber et al, 1992). Since internal application of 0.3 mM GTPyS or 1 mM GDPfiS did not affect significantly the AA-induced decrease in IBa (Figure 6), activation of a GTP binding protein is unlikely to be involved in this AA effect, unlike the AA-induced regulation of the cardiac K+ channel (Kim et al, 1989;Scherer & Breitwieser, 1990). One mechanism by which AA acts directly on Ca channel proteins or phospholipids, is by being incorporated into the membrane bilayer.…”

On the role of arachidonic acid in M-current modulation by muscarine in bullfrog sympathetic neurons

“…AA increased Z, after treatment with 4-BPB, indicating that 4-BPB was probably preventing AA release. At low concentrations, NDGA is a relatively specific lipoxygenase inhibitor (but see Vacher et al, 1989;Kom and Horn, 1990), suggesting that a lipoxygenase metabolite participates in the termination of inhibition, possibly by altering the function of the G-protein (Scherer and Breitwieser, 1990) involved in inhibition (Pfaffinger, 1988;Lopez and Adams, 1989;Tokimasa and Akasu, 1990). Taken together, these results suggest that, as found in Aplysia neurons (Carlson and Levitan, 1990), there is a rapid turnover of AA, creating steady state conditions with relatively low concentrations of AA (so IA can be observed) and relatively high concentrations of a lipoxygenase metabolite, such that under resting conditions the rate of termination of inhibition is fast.…”

Heterologously expressed serotonin 1A receptors couple to muscarinic K+ channels in heart.

“…3A). Other mechanisms for the effects of hydrolysis-resistant GTP analogs-e.g., involving GTPase-activating proteins (35), arachidonate metabolites (36,37), and 8y subunits-are possible.…”