Effects of acetylcholine on the Na(+)-K+ pump current in guinea-pig ventricular myocytes

Serotonin, norepinephrine, and acetylcholine differentially affect astrocytic potassium clearance to modulate somatosensory signaling in male mice

Changes in extracellular potassium ([K⁺ ]_e ) modulate neuronal networks via changes in membrane potential, voltage-gated channel activity, and alteration to transmission at the synapse. Given the limited extracellular space in the central nervous system, potassium clearance is crucial. As activity-induced potassium transients are rapidly managed by astrocytic Kir4.1 and astrocyte-specific Na⁺ /K⁺ -ATPase, any neurotransmitter/neuromodulator that can regulate their function may have indirect influence on network activity. Neuromodulators differentially affect cortical/thalamic networks to align sensory processing with differing behavioral states. Given serotonin (5HT), norepinephrine (NE), and acetylcholine (ACh) differentially affect spike frequency adaptation and signal fidelity ("signal-to-noise") in somatosensory cortex, we hypothesize that [K⁺ ]_e may be differentially regulated by the different neuromodulators to exert their individual effects on network function. This study aimed to compare effects of individually applied 5HT, NE, and ACh on regulating [K⁺ ]_e in connection to effects on cortical-evoked response amplitude and adaptation in male mice. Using extracellular field and K⁺ ion-selective recordings of somatosensory stimulation, we found that differential effects of 5HT, NE, and ACh on [K⁺ ]_e regulation mirrored differential effects on amplitude and adaptation. 5HT effects on transient K⁺ recovery, adaptation, and field post-synaptic potential amplitude were disrupted by barium (200 µM), whereas NE and ACh effects were disrupted by ouabain (1 µM) or iodoacetate (100 µM). Considering the impact [K⁺ ]_e can have on many network functions; it seems highly efficient that neuromodulators regulate [K⁺ ]_e to exert their many effects. This study provides functional significance for astrocyte-mediated buffering of [K⁺ ]_e in neuromodulator-mediated shaping of cortical network activity.

Carnitine modulates crucial myocardial adenosine triphosphatases and acetylcholinesterase enzyme activities in choline-deprived rats

Choline is an essential nutrient, and choline deficiency has been associated with cardiovascular morbidity. Choline is also the precursor of acetylcholine (cholinergic component of the heart's autonomic nervous system), whose levels are regulated by acetylcholinesterase (AChE). Cardiac contraction-relaxation cycles depend on ion gradients established by pumps like the adenosine triphosphatases (ATPases) Na(+)/K(+)-ATPase and Mg(2+)-ATPase. This study aimed to investigate the impact of dietary choline deprivation on the activity of rat myocardial AChE (cholinergic marker), Na(+)/K(+)-ATPase, and Mg(2+)-ATPase, and the possible effects of carnitine supplementation (carnitine, structurally relevant to choline, is used as an adjunct in treating cardiac diseases). Adult male albino Wistar rats were distributed among 4 groups, and were fed a standard or choline-deficient diet for one month with or without carnitine in their drinking water (0.15% w/v). The enzyme activities were determined spectrophotometrically in the myocardium homogenate. Choline deficiency seems to affect the activity of the aforementioned parameters, but only the combination of choline deprivation and carnitine supplementation increased myocardial Na(+)/K(+)-ATPase activity along with a concomitant decrease in the activities of Mg(2+)-ATPase and AChE. The results suggest that carnitine, in the setting of choline deficiency, modulates cholinergic myocardial neurotransmission and the ATPase activity in favour of cardiac work efficiency.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Isoform-Specific Regulation of the Na(+)-K(+) Pump in Heart

Guinea pig ventricular myocytes coexpress two isoforms of the Na(+)-K(+) pump. These two isoforms respond differently to the physical environment and are coupled to autonomic input through different signal transduction cascades. The expression of different isoforms provides each cell type with a mechanism of programming specific responses to environmental changes.

Pyridostigmine blunts the increases in myocardial oxygen demand elicited by the stimulation of the central nervous system in anesthetized rats

The purpose of the present work was to verify the effect of pyridostigmine bromide, a reversible cholinesterase inhibitor, on the increases in cardiac work and myocardial oxygen demand produced by central sympathetic stimulation in pentobarbital-anesthetized Wistar rats. The pharmacological stimulation of the central nervous system with L-glutamate (1 mg/kg, intracerebroventricular) elicited marked increases in arterial pressure, dP/dt(max), rate-pressure product, and triple product, reproducing the cardiovascular alterations observed during physical effort and stressful situations. The oral administration of pyridostigmine bromide (5, 10 and 20 mg/kg) 2 hours before central stimulation blunted the increases in mean arterial pressure, dP/dt(max), and triple product elicited by glutamate (29, 28 and 57% for 5 mg/kg; 26, 23 and 46% for 10 mg/kg and 19, 17 and 37% for 20 mg/kg, respectively) when compared to the control group (41, 49 and 106%, respectively; p < 0.05). Our results also showed that the activity of plasmatic cholinesterase was effectively inhibited by pyridostigmine bromide. In conclusion, the increases in endogenous acetylcholine induced by cholinesterase inhibition blunted the centrally-evoked increases in myocardial oxygen demand in anesthetized rats. This effect could represent a cardioprotective action in a situation of ischemic heart disease.

alpha-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp was employed to study Na+-K+ pump current (Ip) in acutely isolated myocytes. alpha-Adrenergic receptors were activated with noradrenaline (NA) after blocking beta-adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). 2. Activation of alpha-receptors by NA increased Ip in a concentration-dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nM [Ca2+]i the K0.5 was 219 nM NA whereas at 1.4 microM [Ca2+]i it was 3 nM. 3. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14-15 %. 4. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the alpha-receptor-mediated stimulation of Ip at either high or low[Ca2+]i. 5. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. 6. Phenylephrine, methoxamine and metaraminol, three selective alpha1-adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an alpha1-antagonist, but was unaffected by yohimbine, an alpha2-antagonist. 7. We conclude noradrenaline activates ventricular alpha1-receptors, which are specifically coupled via PKC to increase Na+-K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.