Fatty acid-activated K+ channels in autonomic neurons: activation by an endogenous source of free fatty acids

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Fatty Acid Regulation of Voltage- and Ligand-Gated Ion Channel Function

Free fatty acids (FFA) are essential components of the cell, where they play a key role in lipid and carbohydrate metabolism, and most particularly in cell membranes, where they are central actors in shaping the physicochemical properties of the lipid bilayer and the cellular adaptation to the environment. FFA are continuously being produced and degraded, and a feedback regulatory function has been attributed to their turnover. The massive increase observed under some pathological conditions, especially in brain, has been interpreted as a protective mechanism possibly operative on ion channels, which in some cases is of stimulatory nature and in other cases inhibitory. Here we discuss the correlation between the structure of FFA and their ability to modulate protein function, evaluating the influence of saturation/unsaturation, number of double bonds, and cis vs. trans isomerism. We further focus on the mechanisms of FFA modulation operating on voltage-gated and ligand-gated ion channel function, contrasting the still conflicting evidence on direct vs. indirect mechanisms of action.

Insulin downregulates M(2)-muscarinic receptors in adult rat atrial cardiomyocytes: a link between obesity and cardiovascular complications

OBJECTIVE

To determine whether decreased cardiac parasympathetic activity observed in obesity is due to insulin-induced alterations in cardiac M(2)-muscarinic receptors and/or adenylyl cyclase activity.

DESIGN AND METHODS

After incubation with increasing concentrations of insulin, adult rat atrial cardiomyocytes were assayed for M(2)-muscarinic receptor binding density and affinity, and for M(2)R mRNA expression using RT-PCR analysis. Forskolin-stimulated adenylyl cyclase activity and its inhibition by carbachol were also assayed, as was endothelial nitric oxide synthase mRNA expression. The effects of insulin on M(2)-muscarinic receptor density and mRNA expression levels were analyzed using the insulin signaling inhibitors rapamycin, wortmanin and PD 098059.

RESULTS

Insulin induces a concentration- and time-dependent decrease in expression of the M(2)R mRNA, and in [(3)H]N-methylscopolamine binding by the receptor. These effects on the M(2)R mRNA levels and on [(3)H]N-methylscopolamine binding were prevented by PD 98059, but not by wortmanin or rapamycin. Basal and forskolin-induced cAMP production did not differ, but the inhibition of forskolin-simulated enzyme activity by carbachol was blunted by insulin. No change in the mRNA levels for endothelial nitric oxide synthase was observed.

CONCLUSION

In rat atrial cardiomyocytes, insulin markedly alters both the M(2)-muscarinic receptor density, and its mRNA expression through transcriptional regulation and adenylyl cyclase activity. These data suggest that the obesity-associated decrease in cardiac parasympathetic tone may be related to hyperinsulinemia, which could directly contribute to cardiovascular morbidity in obese patients.

Histamine activates a background, arachidonic acid-sensitive K channel in embryonic chick dorsal root ganglion neurons

Histamine has been proposed to be an important modulator of developing neurons, but its mechanism of action remains unclear. In embryonic chick dorsal root ganglion neurons we found that histamine activates, through the pyrilamine-sensitive H1 receptor, a K-selective, background channel. The K channel activated by histamine was also activated by arachidonic acid in a dose-dependent way, with a KD of 4 microM and a slope of 2.5, had a unitary conductance of about 150 pS (symmetrical 140 KCl) and a moderate voltage dependence. The channel was insensitive to the classical K channel blockers tetraethylammonium, charybdotoxin, 4-aminopyridine, but inhibited by millimolar Ba2+. Channel activity could also be increased by lowering the intracellular pH from 7.2 to 5.5, or by applying negative pressure pulses through the patch pipette. Experiments aimed at delineating the metabotropic pathway leading to K channel activation by histamine indicated the involvement of a pertussis toxin-insensitive G protein, and a quinacrine-sensitive cytosolic phospholipase A2. The histamine-induced K channel activation was observed only with elevated internal Ca2+ (achieved using 0.5 microM ionomycin or elevated external KCl). An increase in the histamine-induced phosphoinositide hydrolysis was also observed upon internal Ca2+ elevation, showing the presence of a Ca2+ dependent step upstream to inositol 1,4,5-triphosphate production. In view of the functional importance of K conductances during cell differentiation, we propose that histamine activation of this K channel may have a significant role during normal development of embryonic chick neurons.

Active and passive membrane properties of rat sympathetic preganglionic neurones innervating the adrenal medulla

The intravascular release of adrenal catecholamines is a fundamental homeostatic process mediated via thoracolumbar spinal sympathetic preganglionic neurones (AD-SPN). To understand mechanisms regulating their excitability, whole-cell patch-clamp recordings were obtained from 54 retrogradely labelled neonatal rat AD-SPN. Passive membrane properties included a mean resting membrane potential, input resistance and time constant of -62 +/- 6 mV, 410 +/- 241 MOmega and 104 +/- 53 ms, respectively. AD-SPN were homogeneous with respect to their active membrane properties. These active conductances included transient outward rectification, observed as a delayed return to rest at the offset of the membrane response to hyperpolarising current pulses, with two components: a fast 4-AP-sensitive component (A-type conductance), contributing to the after-hyperpolarisation (AHP) and spike repolarisation; a slower prolonged Ba(2+)-sensitive component (D-like conductance). All AD-SPN expressed a Ba(2+)-sensitive instantaneous inwardly rectifying conductance activated at membrane potentials more negative than around -80 mV. A potassium-mediated, voltage-dependent sustained outward rectification activated at membrane potentials between -35 and -15 mV featured an atypical pharmacology with a component blocked by quinine, reduced by low extracellular pH and arachidonic acid, but lacking sensitivity to Ba(2+), TEA and intracellular Cs(+). This quinine-sensitive outward rectification contributes to spike repolarisation. Following block of potassium conductances by Cs(+) loading, AD-SPN revealed the capability for autorhythmicity and burst firing, mediated by a T-type Ca(2+) conductance. These data suggest the output capability is dynamic and diverse, and that the range of intrinsic membrane conductances expressed endow AD-SPN with the ability to generate differential and complex patterns of activity. The diversity of intrinsic membrane properties expressed by AD-SPN may be key determinants of neurotransmitter release from SPN innervating the adrenal medulla. However, factors other than active membrane conductances of AD-SPN must ultimately regulate the differential ratio of noradrenaline (NA) versus adrenaline (A) release secreted in response to various physiological and environmental demands.