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

Abstract

The modulation by muscarine or LHRH of the potassium M-current (IM) in whole-cell voltage-clamped bullfrog sympathetic neurons presents an initial phase of current reduction, followed, after agonist removal, by a transient enhancement or “overrecovery.” Employing a fast solution exchange system, an inhibitory process and an enhancing process were distinguished kinetically. The extent of overrecovery increased with the extent of the preceding inhibition. The rate and degree of inhibition increased with the concentration of agonist. In contrast, the rate of recovery and the extent of overrecovery were independent. The half-lives of the inhibitory and enhancing processes were 21 and 53 sec, respectively. Several observations suggest that arachidonic acid (AA) may be involved in overrecovery: (1) AA enhanced IM in a dose- dependent and reversible manner, with an IC50 of 2.8 microM. (2) Muscarine inhibited the A-current (IA), a potassium current that is blocked by AA. (3) Phospholipase A2 inhibitors (quinacrine and bromophenacyl bromide) and a lipoxygenase inhibitor (nordihydroguaiaretic acid) prevented overrecovery, without affecting the rate or extent of IM inhibition significantly. However, kinetic analysis indicates that these drugs were preventing overrecovery by prolonging the half-life of the inhibitory process to > 80 sec (e.g., not necessarily by blocking the enhancing pathway). In addition, the extent of IA inhibition was less than expected if AA was mediating both IM enhancement and IA inhibition. The observed relation between extent and rate of overrecovery, and the action of arachidonic acid metabolism inhibitors can be accounted for by a model proposing that the agonist alters the equilibrium between three pools of M-channels.

M1Muscarinic Receptors Inhibit L-type Ca2+Current and M-Current by Divergent Signal Transduction Cascades

“…Moreover, our unanticipated findings that OPC, BSA, or the absence of cPLA 2 greatly attenuated M-current recovery from inhibition indicated a role for cPLA 2 and AA in recovery from the inhibitory process. A role for AA in regulating M-current is supported by the finding that exogenously applied AA enhances M-current in frog sympathetic neurons (Villarroel, 1994;Yu, 1995).…”

“…These differences indicate that AA uses additional, novel mechanisms to regulate M-current compared with Ca 2ϩ channels and other K v channel activity (Oliver et al, 2004). Moreover, this signaling cascade appears remarkably similar to the PLA 2 -sensitive pathway responsible for M-current over-recovery observed after washing out of agonist in other cell types (Robbins et al, 1993;Villarroel, 1994;Yu, 1995). Mechanistically, these unexpected findings suggest that, to observe the effects of AA on recovery, sufficient PIP 2 must be present in the membrane and/or associated with the channels.…”

Uncoupling PIP2-calmodulin regulation of Kv7.2 channels by an assembly destabilizing epileptogenic mutation

“…Upon returning to the holding potential, an instantaneous current jump (corresponding to leak current) followed by a slowly developing outward relaxation (corresponding to the opening of the M-channels) was recorded. The size of the outward relaxation before and after the +100 mV jump was used to estimate the effect of DrVSP activation on M-current size (Adams et al, 1982;Villarroel, 1994).…”

Arachidonic Acid Inhibits Transient Potassium Currents and Broadens Action Potentials during Electrographic Seizures in Hippocampal Pyramidal and Inhibitory Interneurons

“…The arachidonic acid-mediated inhibition of transient currents through recombinant Kv4.2 channels (Villaroel and Schwarz, 1996) and transient currents in sympathetic neurons (Villaroel, 1993(Villaroel, , 1994 was mimicked by the nonhydrolyzable arachidonic acid analog ETYA. Because ETYA is not a substrate for metabolism by the cyclooxygenase, lipoxygenase, or cytochrome P450 (epoxygenase) pathways associated with arachidonic acid, these data suggested that arachidonic acid interacted directly with Kv4.2 channels without requiring the formation of a metabolic end product.…”