R. W. Hadley

Elevated Postsynaptic [Ca2+]iand L-Type Calcium Channel Activity in Aged Hippocampal Neurons: Relationship to Impaired Synaptic Plasticity

Considerable evidence supports a Ca2+dysregulation hypothesis of brain aging and Alzheimers disease. However, it is still not known whether (1) intracellular [Ca2+]i is altered in aged brain neurons during synaptically activated neuronal activity; (2) altered [Ca2+]i is directly correlated with impaired neuronal plasticity; or (3) the previously observed age-related increase in L-type voltage-sensitive Ca2+ channel (L-VSCC) density in hippocampal neurons is sufficient to impair synaptic plasticity. Here, we used confocal microscopy to image [Ca2+]i in single CA1 neurons in hippocampal slices of young-adult and aged rats during repetitive synaptic activation. Simultaneously, we recorded intracellular EPSP frequency facilitation (FF), a form of short-term synaptic plasticity that is impaired with aging and inversely correlated with cognitive function. Resting [Ca2+]i did not differ clearly with age. Greater elevation of somatic [Ca2+]i and greater depression of FF developed in aged neurons during 20 sec trains of 7 Hz synaptic activation, but only if the activation triggered repetitive action potentials for several seconds. Elevated [Ca2+]i and FF also were negatively correlated in individual aged neurons. In addition, the selective L-VSCC agonist Bay K8644 increased the afterhyperpolarization and mimicked the depressive effects of aging on FF in young-adult neurons. Thus, during physiologically relevant firing patterns in aging neurons, postsynaptic Ca2+ elevation is closely associated with altered neuronal plasticity. Moreover, selectively increasing postsynaptic L-VSCC activity, as occurs in aging, negatively regulated a form of short-term plasticity that enhances synaptic throughput. Together, the results elucidate novel processes that may contribute to impaired cognitive function in aging.

Ca2+ and voltage inactivate Ca2+ channels in guinea-pig ventricular myocytes through independent mechanisms.

1. L‐type Ca2+ currents and Ca2+ channel gating currents were studied in isolated guinea‐pig ventricular heart cells using the whole‐cell patch‐clamp technique, in order to investigate the mechanism of Ca(2+)‐dependent inactivation. The effect of altering the intracellular Ca2+ concentration ([Ca2+]i) on these currents was studied through photorelease of intracellular Ca2+ ions using the photolabile Ca2+ chelators DM‐nitrophen and nitr‐5. 2. We found that step increases in [Ca2+]i produced by photorelease could either increase or decrease the L‐type Ca2+ current. Specifically, Ca2+ photorelease from DM‐nitrophen almost exclusively caused inactivation of the Ca2+ current. In contrast, Ca2+ photorelease from nitr‐5 had a biphasic effect: a small, rapid inactivation of the Ca2+ current was followed by a slow potentiation. These two Ca(2+)‐dependent processes seemed to differ in their Ca2+ dependence, as small Ca2+ photoreleases elicited potentiation without a preceding inactivation, whereas larger photoreleases elicited both inactivation and potentiation. 3. The mechanism of the Ca(2+)‐dependent inactivation of Ca2+ channels was explored by comparing the effects of voltage and photoreleased Ca2+ on the Ca2+ current and the Ca2+ channel gating current. Voltage was found to reduce both the Ca2+ current and the gating current proportionally. However, Ca2+ photorelease from intracellular DM‐nitrophen inactivated the Ca2+ current without having any effect on the gating current. 4. The dephosphorylation hypothesis for Ca(2+)‐dependent inactivation was tested by applying isoprenaline to the cells before eliciting a maximal rise of [Ca2+]i (maximal flash intensity, zero external [Na+]i). Isoprenaline could completely prevent Ca(2+)‐dependent inactivation under these conditions, even when [Ca2+]i rose so high as to cause an irreversible contracture of the cell. 5. We concluded from these experiments that voltage and Ca2+ ions inactivate the L‐type Ca2+ channel through separate, independent mechanisms. In addition, we found that Ca(2+)‐dependent inactivation does not result in the immobilization of gating charge, and apparently closes the Ca2+ permeation pathway through a mechanism that does not involve the voltage‐sensing region of the channel. Furthermore, we found that Ca(2+)‐dependent inactivation is entirely sensitive to beta‐adrenergic stimulation. These facts suggest that either Ca(2+)‐dependent inactivation results from Ca(2+)‐dependent dephosphorylation of the Ca2+ channel, or that Ca(2+)‐dependent inactivation is modulated by protein kinase A.

An intrinsic potential‐dependent inactivation mechanism associated with calcium channels in guinea‐pig myocytes.

1. Currents through Ca2+ channels of single guinea‐pig ventricular myocytes were studied using patch electrodes for whole‐cell recording. Currents through Na+ and K+ channels were suppressed by the application of drugs or the substitution of impermeant ions. 2. Inactivation of the Ca2+ current (ICa) was investigated using a two‐pulse protocol. The amount of inactivation left behind by a pre‐pulse appeared to be related to current magnitude, as others have reported. The dependence of inactivation on the pre‐pulse potential was partially U‐shaped, as the amount of inactivation peaked at 0 mV and then declined with more positive pre‐pulses. 3. Non‐specific current carried by monovalent ions through Ca2+ channels (Ins) was induced by lowering the extracellular Ca2+ concentration with EGTA. Ins peaked in an inward direction at ‐20 mV, reversed direction at +22 mV, and became a large outward current at more positive potentials. 4. Ins inactivated with a slow time course. The inactivation was not due to accumulation or depletion phenomena. Studies using two‐pulse protocols showed that the amount of inactivation left by a pre‐pulse was directly related to the pre‐pulse potential. 5. The addition of micromolar amounts of free Ca2+ to the external solution induced outward rectification of Ins. Inward currents were small or absent, while larger outward currents could still be seen at very positive potentials. Ca2+‐channel inactivation still occurred under these conditions, even in the absence of any significant ionic movement. 6. The time courses of Ins inactivation and recovery were studied. The half‐time of Ins inactivation decreased with larger depolarizations. Recovery of Ins was very slow, but could be accounted for by changes in the surface charge of the membrane. 7. It is concluded that Ins inactivation is due solely to a voltage‐dependent inactivation process which is intrinsic to myocardial Ca2+ channels. Voltage‐dependent inactivation appears to account for a significant proportion of total Ca2+‐channel inactivation at negative potentials, and appears to account for almost all of the inactivation at very positive potentials, even in the presence of millimolar concentration of external Ca2+.

Influence of cytosolic and mitochondrial Ca2+, ATP, mitochondrial membrane potential, and calpain activity on the mechanism of neuron death induced by 3-nitropropionic acid.

3-Nitropropionic acid (3NP), an irreversible inhibitor of succinate dehydrogenase, induces both rapid necrotic and slow apoptotic death in rat hippocampal neurons. Low levels of extracellular glutamate (10 microM) shift the 3NP-induced cell death mechanism to necrosis, while NMDA receptor blockade results in predominantly apoptotic death. In this study, we examined the 3NP-induced alterations in free cytosolic and mitochondrial calcium levels, ATP levels, mitochondrial membrane potential, and calpain and caspase activity, under conditions resulting in the activation of apoptotic and necrotic pathways. In the presence of 10 microM glutamate, 3NP administration resulted in a massive elevation in [Ca(2+)](c) and [Ca(2+)](m), decreased ATP, rapid mitochondrial membrane depolarization, and a rapid activation of calpain but not caspase activity. In the presence of the NMDA receptor antagonist MK-801, 3NP did not induce a significant elevation of [Ca(2+)](c) within the 24h time period examined, nor increase [Ca(2+)](m) within 1h. ATP was maintained at control levels during the first hour of treatment, but declined 64% by 16h. Calpain and caspase activity were first evident at 24h following 3NP administration. 3NP treatment alone resulted in a more rapid decline in ATP, more rapid calpain activation (within 8h), and elevated [Ca(2+)](m) as compared to the results obtained with added MK-801. Together, the results demonstrate that 3NP-induced necrotic neuron death is associated with a massive calcium influx through NMDA receptors, resulting in mitochondrial depolarization and calpain activation; while 3NP-induced apoptotic neuron death is not associated with significant elevations in [Ca(2+)](c), nor with early changes in [Ca(2+)](m), mitochondrial membrane potential, ATP levels, or calpain activity.

Membrane electrical properties of vesicular Na-Ca exchange inhibitors in single atrial myocytes.

Na-loading single frog atrial cells produce changes in membrane currents that are similar to the creep currents originally observed in Na-loaded cardiac Purkinje fibers. Exposure to the Na ionophore, monensin, was used to induce creep currents in isolated atrial cells. The sensitivity of myocardial creep currents to three compounds that have been shown to be inhibitors of Na-Ca exchange flux activity in isolated sarcolemmal vesicles was assessed. Dodecylamine, quinacrine, and the amiloride analog, 3′,4′-dichlorobenzamil block creep currents at concentrations well below those required to block Na-dependent Ca uptake in sarcolemmal vesicles. The estimated Kis for inhibition of myocardial creep currents were 3μM for dodecylamin, 10μm for quinacrine, and 4μM for 3,4-dichlorobenzamil. The sensitivity of creep currents to these compounds is consistent with the hypothesis that creep currents may represent the electrogenic activity of a Na-Ca exchange carrier. In an additional series of experiments, the relative specificity of these compounds was tested by examining their effects on myocardial membrane channels. Both dodecylamine and 3′,4′-dichlorobenzamil were found to inhibit myocardial Ca and K currents over the same range of concentrations in which block of exchange activity occurs. These results seriously question the use of these exchange carrier inhibitors as selective experimental probes for defining the role of Na-Ca exchange in various physiological processes.

Contribution of the Na+channel and Na+/H+exchanger to the anoxic rise of [Na+] in ventricular myocytes

The aim of this study was to quantify the contribution of the Na+/H+exchanger (NHE) and the Na+channel to the rise in cytosolic Na+ concentration ([Na+]) that is seen in anoxic guinea pig ventricular myocytes. [Na+] was measured with the use of microfluorometry and was found to rise to 44 mM after prolonged anoxia. This rise was partially sensitive to either TTX or HOE-642, selective inhibitors of the Na+ channel and NHE1, respectively. [Na+] did not significantly rise when both drugs were present, suggesting that other routes of Na+ entry were insignificant. However, the relative contributions of the NHE and the Na+ channel were found to be remarkably sensitive to ionic conditions expected to occur during ischemia. The Na+ channel was the dominant Na+ source during acidic anoxia. However, the NHE was the dominant Na+ source during both hyperkalemic anoxia and simulated ischemia (hyperkalemia, low pH, and anoxia). The data suggest that the NHE may prove to be the best pharmacological target to reduce Na+ entry during true ischemia and that inhibition of Na+ influx could contribute strongly to the cardioprotective effects of NHE inhibitors.

Intramitochondrial [Ca2+] and membrane potential in ventricular myocytes exposed to anoxia-reoxygenation

The aim of this study was to investigate the role of mitochondrial ionic homeostasis in promoting reoxygenation-induced hypercontracture in cardiac muscle. Mitochondrial membrane potential and intramitochondrial Ca2+ concentration ([Ca2+]) were measured using confocal imaging in guinea pig ventricular myocytes exposed to anoxia and reoxygenation. Anoxia produced a variable, but often profound, mitochondrial depolarization. Some cells mounted a recovery of their mitochondrial membrane potential during reoxygenation; the depolarization was sustained in other cells. Recovery of the mitochondrial membrane potential seemed essential to avoid reoxygenation-induced hypercontracture. Reoxygenation also caused a sizable elevation in intramitochondrial [Ca2+], the amplitude of which was correlated with the likelihood of a cell undergoing hypercontracture. A sustained Ca2+load analogous to that seen during reoxygenation was imposed on cardiac mitochondria through permeabilization of the plasma membrane. Elevation of intracellular [Ca2+] to 800 nM caused a substantial mitochondrial depolarization. We propose that the conditions seen in guinea pig ventricular myocytes during reoxygenation are well suited to produce Ca2+-dependent mitochondrial depolarization, which may play a significant role in promoting irreversible cell injury.

Intramembrane charge movement in guinea-pig and rat ventricular myocytes

1. Non‐linear capacitative current (charge movement) was studied in isolated guinea‐pig and rat ventricular myocytes. Linear capacitance was subtracted using standard procedures. Most of the experiments were done with guinea‐pig myocytes, while rat myocytes were used for comparison. 2. When a myocyte was held at ‐100 mV, depolarizing clamp steps produced a rapid outward current transient, which was followed by an inward current transient upon repolarization. This current was identified as the movement of charged particles in the cell membrane, rather than ionic movement across the membrane, for the following reasons: (1) the current saturated at membrane potentials positive to +20 mV; (2) the current was capacitative in nature, having no reversal potential; (3) in general, the charge moved during depolarization (Qon) approximated the charge moved during repolarization (Qoff). 3. Qoff was significantly less than Qon for a depolarization from ‐100 mV to 0 mV. However, the Qoff/Qon ratio approached unity if the cell was instead repolarized to ‐140 mV. This was interpreted as being due to the immobilization of a fraction of the charge during the depolarization, which recovered rapidly enough to be measured at ‐140 mV, but recovered too slowly at ‐100 mV. 4. Charge movement in these cells had a sigmoidal dependence on the membrane potential, which could be empirically described by the two‐state Boltzmann equation Q = Qmax/(1 + exp[‐(V‐V*)/kappa]), where Q is the charge movement at potential V, Qmax is the maximum charge, V* is the membrane potential at Q = Qmax/2, and kappa is a slope factor. Qmax was 11.7 nC/microF, V* was ‐18 mV and kappa was 16 mV in guinea‐pig myocytes held at ‐100 mV, while the values in rat myocytes were 10.9 nC/microF, ‐32 mV and 13 mV. 5. The charge movement could be partially immobilized by a prior depolarization. This effect developed over a broad voltage range, from ‐120 to +20 mV. The fraction of charge that could be immobilized by a 10 s pre‐pulse to +20 mV was 59%. 6. The time course of decay of both Qon and Qoff could basically be described as a single‐exponential process. The time constant was largest at ‐40 mV and decreased at both more positive and negative test potentials. A second, slower Qoff time constant, possibly representing remobilization of immobilized charge, could be seen under some conditions. 7. The temperature dependence of charge movement was studied between 11 and 35 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

High affinity and low affinity ouabain binding sites in the rat heart

Ventricular muscle of rat heart has two classes of receptors which are responsible for the positive inotropic effect of ouabain. Low affinity receptors are apparently related to Na+, K+-ATPase. To determine if high affinity receptors are also sarcolemmal Na+, K+-ATPase of muscle cells, their characteristics were examined. Binding of [3H]ouabain to the high affinity binding site required ATP in the presence of Mg2+ and Na+, was stimulated by Na+ in the presence of Mg2+ and ATP, and was inhibited by K+. Digoxin, digitoxin and cassaine all inhibited [3H]ouabain binding to the high affinity site. Cassaine was about an order of magnitude less potent than the glycosides. These results indicate similarities in high affinity ouabain binding sites in ventricular muscle of rat heart and Na+, K+-ATPase obtained from other sources. Destruction of sympathetic nerve terminals with 6-hydroxydopamine failed to affect the high affinity ouabain binding sites indicating that high affinity sites do not represent the Na+, K+-ATPase in sympathetic nerve terminals. Labeling of Na+, K+-ATPase from [gamma-32P]ATP indicates that high affinity ouabain binding sites account for 25% of the total enzyme molecules present in ventricular muscle of rat heart.

Excitation-contraction coupling in heart cells. Roles of the sodium-calcium exchange, the calcium current, and the sarcoplasmic reticulum

Experimental data do not support the idea that excitation-contraction coupling in heart muscle can be explained by a simple calcium-induced calcium release mechanism alone or a simple voltage-dependent calcium release mechanism alone. Our data on excitation-contraction coupling combined with the results of others suggest the need to either develop more complex and more sophisticated single-mechanism models, or to establish dual-control models.