The modulation of action potential generation by calcium-induced calcium release is enhanced by mitochondrial inhibitors in mudpuppy parasympathetic neurons

Mitochondrial ATP production provides long-range control of endothelial inositol trisphosphate-evoked calcium signaling

Endothelial cells are reported to be glycolytic and to minimally rely on mitochondria for ATP generation. Rather than providing energy, mitochondria in endothelial cells may act as signaling organelles that control cytosolic Ca2+ signaling or modify reactive oxygen species (ROS). To control Ca2+ signaling, these organelles are often observed close to influx and release sites and may be tethered near Ca2+ transporters. In this study, we used high-resolution, wide-field fluorescence imaging to investigate the regulation of Ca2+ signaling by mitochondria in large numbers of endothelial cells (∼50 per field) in intact arteries from rats. We observed that mitochondria were mostly spherical or short-rod structures and were distributed widely throughout the cytoplasm. The density of these organelles did not increase near contact sites with smooth muscle cells. However, local inositol trisphosphate (IP3)-mediated Ca2+ signaling predominated near these contact sites and required polarized mitochondria. Of note, mitochondrial control of Ca2+ signals occurred even when mitochondria were far from Ca2+ release sites. Indeed, the endothelial mitochondria were mobile and moved throughout the cytoplasm. Mitochondrial control of Ca2+ signaling was mediated by ATP production, which, when reduced by mitochondrial depolarization or ATP synthase inhibition, eliminated local IP3-mediated Ca2+ release events. ROS buffering did not significantly alter local Ca2+ release events. These results highlight the importance of mitochondrial ATP production in providing long-range control of endothelial signaling via IP3-evoked local Ca2+ release in intact endothelium.

The mitochondrial membrane potential and Ca2+ oscillations in smooth muscle

Ca2+ uptake by mitochondria might both modulate the cytosolic Ca2+ concentration ([Ca2+]c) and depolarize the mitochondrial membrane potential (delta Psi m) to limit ATP production. To investigate how physiological Ca2+ signaling might affect energy production, delta Psi m was examined during Ca2+ oscillations in smooth muscle cells. In single, voltage-clamped smooth muscle cells, inhibition of mitochondrial Ca2+ accumulation inhibited inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3]-evoked Ca2+ release and prolonged the time required for restoration of [Ca2+]c following activation of plasmalemmal Ca2+ currents (ICa). Ca2+ could be released from mitochondria immediately (within 15 seconds) after a [Ca2+]c rise evoked by Ins(1,4,5)P3 or ICa. Despite this evidence of mitochondrial Ca2+ accumulation, no change in delta Psi m was observed during single or repetitive [Ca2+]c oscillations evoked by these conditions. Occasionally, spontaneous, repetitive, persistent Ca 2+ oscillations were observed. In these cases, mitochondria displayed stochastic delta Psi m depolarizations, which were independent both of events in neighboring mitochondria and of the timing of the [Ca 2+]c oscillations themselves. Such delta Psi m depolarizations could be mimicked by increased exposure to either fluorescence excitation light or the delta Psi m-sensitive dye tetramethylrhodamine ethyl ester (TMRE) and were inhibited by antioxidants (ascorbic acid, catalase, Trolox and TEMPO) or the mitochondrial permeability transition pore (mPTP)-inhibitor cyclosporin A (CsA). Individual mitochondria within smooth muscle cells might depolarize during repetitive Ca2+ oscillations or during oxidative stress but not during the course of single [Ca2+]c transients evoked by Ca2+ influx or store release.

Bursts of potential elicited by d-amphetamine in central snail neuron: effect of sodium azide

Effects of sodium azide (NaN(3)) on spontaneously generated action potential and bursts of potential elicited by d-amphetamine (d-amphetamine-elicited BoP) were studied on the right parietal 4 (RP4) neuron of the snail Achatina fulica Ferussac in vitro. Sodium azide altered the spontaneous action potential of RP4 neuron in a concentration-dependent manner. In lower concentrations, neither NaN(3) (30, 100, 300 microM; 1 and 3 mM) nor d-amphetamine (135 microM) affect the resting membrane potential, amplitude and frequency of RP4 neurons, while in the higher concentrations NaN(3) (30 mM) did abolish the spontaneous action potential on RP4 neurons and depolarized the RP4 neurons reversibly. At lower concentration, NaN(3) (30 microM) facilitated the d-amphetamine-elicited BoP. The BoP elicited by NaN(3) (30 microM) and d-amphetamine (135 microM) were decreased following treatment with KT5720 (protein kinase A inhibitor), or intracellular injection of EGTA [ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid]. However, the BoP was not affected by applying U73122 (1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione) or neomycin (phospholipase inhibitors). Voltage clamp studies revealed that NaN(3) (30 microM) did not alter the total fast inwards currents (70 msec.) and the steady-state outwards currents (5 sec.). It appeared that the BoP elicited by NaN(3) (30 microM) and d-amphetamine (135 microM) was mainly due to protein kinase A-related messenger system and intracellular calcium. It is concluded that d-amphetamine-elicited BoP was not mainly due to inhibition of the function of mitochondria in the neuron while the function of mitochondria did alter the BoP elicited by amphetamine.

Impaired development of mitochondria plays a role in the central nervous system defects of fetal alcohol syndrome

BACKGROUND

Alcohol consumption during pregnancy can induce a wide spectrum of adverse effects in offspring. Microcephaly and mental retardation are two major defects of central nervous system (CNS). Most mechanism studies of alcohol-related CNS defects have been focused on the morphologically abnormal tissues, and more attention has been paid to nuclear alteration as opposed to organelle development.

METHODS

A mouse model of fetal alcohol syndrome (FAS) was used to investigate the effect of alcohol on fetal cerebral mitochondria development. Pregnant mice were given different doses of ethanol intragastrically from GD6 to GD15. Fetal cerebral mitochondria were isolated and analyzed on GD18.

RESULTS

Excessive cell apoptosis was found in the cerebra of prenatal alcohol exposure fetuses. Proliferation and differentiation of fetal cerebral mitochondria were inhibited by alcohol. Affected mitochondrial volume constriction and adenosine triphosphate (ATP) accumulation, reduced activities of respiratory chain complex I and IV and ATP synthase were detected in the cerebral tissue without obvious malformed appearance.

CONCLUSIONS

Impaired mitochondria development plays a role in the CNS defects induced by prenatal alcohol exposure.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Calcium-induced calcium release regulates action potential generation in guinea-pig sympathetic neurones

Experiments were done using guinea-pig sympathetic neurones dissociated from the stellate ganglia to establish whether calcium-induced calcium release (CICR) modulated action potential (AP) generation in mammalian neurones. Using measurements of intracellular calcium ([Ca(2+)](i)) with the Ca(2+)-sensitive dye fluo-3, we demonstrated that 10 mM caffeine activated ryanodine receptors and caused a rise in [Ca(2+)](i) in both Ca(2+)-containing and Ca(2+)-deficient solutions. We also demonstrated that combined treatment with caffeine and 1 microm thapsigargin or caffeine and 20 microm ryanodine blocked subsequent caffeine-induced elevations of [Ca(2+)](i). Treatment with thapsigargin, ryanodine or 200 microM Cd(2+) to disrupt CICR decreased the latency to AP generation during 400 ms depolarizing current ramps using the perforated patch whole cell patch clamp in current clamp mode. Treatment with 500 microM tetraethylammonium also decreased the latency to AP generation during depolarizing current ramps in control cells, but not in cells pretreated with thapsigargin to deplete internal Ca(2+) stores. In summary, we propose that an outward current, carried at least in part through BK channels, is activated by CICR at membrane voltages approaching the threshold for AP initiation and that this current opposed depolarizing current ramps applied to guinea-pig sympathetic stellate neurones.