The alpha-adrenergic-mediated activation of the cardiac mitochondrial Ca2+ uniporter and its role in the control of intramitochondrial Ca2+ in vivo

Adrenergic Receptor Regulation of Mitochondrial Function in Cardiomyocytes

ABSTRACT

Adrenergic receptors (ARs) are G protein-coupled receptors that are stimulated by catecholamines to induce a wide array of physiological effects across tissue types. Both α1- and β-ARs are found on cardiomyocytes and regulate cardiac contractility and hypertrophy through diverse molecular pathways. Acute activation of cardiomyocyte β-ARs increases heart rate and contractility as an adaptive stress response. However, chronic β-AR stimulation contributes to the pathobiology of heart failure. By contrast, mounting evidence suggests that α1-ARs serve protective functions that may mitigate the deleterious effects of chronic β-AR activation. Here, we will review recent studies demonstrating that α1- and β-ARs differentially regulate mitochondrial biogenesis and dynamics, mitochondrial calcium handling, and oxidative phosphorylation in cardiomyocytes. We will identify potential mechanisms of these actions and focus on the implications of these findings for the modulation of contractile function in the uninjured and failing heart. Collectively, we hope to elucidate important physiological processes through which these well-studied and clinically relevant receptors stimulate and fuel cardiac contraction to contribute to myocardial health and disease.

The Mitochondrial Ca2+ Uniporter: Structure, Function, and Pharmacology

Mitochondrial Ca²⁺ uptake is crucial for an array of cellular functions while an imbalance can elicit cell death. In this chapter, we briefly reviewed the various modes of mitochondrial Ca²⁺ uptake and our current understanding of mitochondrial Ca²⁺ homeostasis in regards to cell physiology and pathophysiology. Further, this chapter focuses on the molecular identities, intracellular regulators as well as the pharmacology of mitochondrial Ca²⁺ uniporter complex.

Reactive gamma-ketoaldehydes formed via the isoprostane pathway disrupt mitochondrial respiration and calcium homeostasis

Isoketals (IsoKs) are gamma-ketoaldehydes formed via the isoprostane pathway of arachidonic acid peroxidation and are among the most reactive by-products of lipid peroxidation. IsoKs selectively adduct to protein lysine residues and are highly cytotoxic, but the targets and molecular events involved in IsoK-induced cell death are poorly defined. Our previous work established that physiologically relevant aldehydes induce mitochondrial dysfunction (Kristal et al., J. Biol. Chem.271:6033-6038; 1996). We therefore examined whether IsoKs induced mitochondrial dysfunction. Incubation of mitochondria with synthetic IsoKs in the presence or absence of Ca(2+) was associated with alterations in mitochondrial respiration, membrane potential (DeltaPsi), and pyridine nucleotide redox state. IsoKs dose dependently (0.5-4microM) accelerated liver mitochondria swelling induced by low concentrations of Ca(2+) and Zn(2+) or by the prooxidant tert-butylhydroperoxide, and release of cytochrome c, with similar observations in heart/brain mitochondria. The mitochondrial permeability transition (mPT) inhibitor cyclosporine A delayed IsoK-induced mitochondria dysfunction. The actions of IsoKs are consistent with interactions with cytochrome c, a protein rich in lysine residues. Direct reaction of IsoKs with select lysines in cytochrome c was demonstrated using high-resolution mass spectrometry. Overall, these results suggest that IsoKs may, in part, mediate their cytotoxic effects through induction of the mPT and subsequent activation of downstream cell death cascades.

Characteristics and possible functions of mitochondrial Ca(2+) transport mechanisms

Mitochondria produce around 92% of the ATP used in the typical animal cell by oxidative phosphorylation using energy from their electrochemical proton gradient. Intramitochondrial free Ca(2+) concentration ([Ca(2+)](m)) has been found to be an important component of control of the rate of this ATP production. In addition, [Ca(2+)](m) also controls the opening of a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP), which plays a role in mitochondrial control of programmed cell death or apoptosis. Therefore, [Ca(2+)](m) can control whether the cell has sufficient ATP to fulfill its functions and survive or is condemned to death. Ca(2+) is also one of the most important second messengers within the cytosol, signaling changes in cellular response through Ca(2+) pulses or transients. Mitochondria can also sequester Ca(2+) from these transients so as to modify the shape of Ca(2+) signaling transients or control their location within the cell. All of this is controlled by the action of four or five mitochondrial Ca(2+) transport mechanisms and the PTP. The characteristics of these mechanisms of Ca(2+) transport and a discussion of how they might function are described in this paper.

Preservation of mitochondrial function may contribute to cardioprotective effects of Na+/Ca2+ exchanger inhibitors in ischaemic/reperfused rat hearts

BACKGROUND AND PURPOSE

Na+/Ca2+ exchanger (NCX) inhibitors are known to attenuate myocardial reperfusion injury. However, the exact mechanisms for the cardioprotection remain unclear. The present study was undertaken to examine the mechanism underlying the cardioprotection by NCX inhibitors against ischaemia/reperfusion injury.

EXPERIMENTAL APPROACH

Isolated rat hearts were subjected to 35-min ischaemia/60-min reperfusion or 20-min ischaemia/60-min reperfusion. NCX inhibitors (3-30 microM KB-R7943 (KBR) or 0.3-1 microM SEA0400 (SEA)) were given for 5 min prior to ischaemia (pre-ischaemic treatment) or for 10 min after the onset of reperfusion (post-ischaemic treatment).

KEY RESULTS

With 35-min ischaemia/60-min reperfusion, pre- or post-ischaemic treatment with KBR or SEA neither enhanced post-ischaemic contractile recovery nor attenuated ischaemia- or reperfusion-induced Na+ accumulation and damage to mitochondrial respiratory function. With the milder model (20-min ischaemia/reperfusion), pre- or post-ischaemic treatment with 10 microM KBR or 1 microM SEA significantly enhanced the post-ischaemic contractile recovery, associated with reductions in reperfusion-induced Ca2+ accumulation, damage to mitochondrial function, and decrease in myocardial high-energy phosphates. Furthermore, Na+ influx to mitochondria in vitro was enhanced by increased concentrations of NaCl. KBR (10 microM) and 1 microM SEA partially decreased the Na+ influx.

CONCLUSIONS AND IMPLICATIONS

The NCX inhibitors exerted cardioprotective effects during relatively mild ischaemia. The mechanism may be attributable to prevention of mitochondrial damage, possibly mediated by attenuation of Na+ overload in cardiac mitochondria during ischaemia and/or Ca2+ overload via the reverse mode of NCX during reperfusion.

Integration of rapid cytosolic Ca2+signals by mitochondria in cat ventricular myocytes

Decoding of fast cytosolic Ca2+concentration ([Ca2+]i) transients by mitochondria was studied in permeabilized cat ventricular myocytes. Mitochondrial [Ca2+] ([Ca2+]m) was measured with fluo-3 trapped inside mitochondria after removal of cytosolic indicator by plasma membrane permeabilization with digitonin. Elevation of extramitochondrial [Ca2+] ([Ca2+]em) to >0.5 μM resulted in a [Ca2+]em-dependent increase in the rate of mitochondrial Ca2+accumulation ([Ca2+]emresulting in half-maximal rate of Ca2+accumulation = 4.4 μM) via Ca2+uniporter. Ca2+uptake was sensitive to the Ca2+uniporter blocker ruthenium red and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone and depended on inorganic phosphate concentration. The rates of [Ca2+]mincrease and recovery were dependent on the extramitochondrial [Na+] ([Na+]em) due to Ca2+extrusion via mitochondrial Na+/Ca2+exchanger. The maximal rate of Ca2+extrusion was observed with [Na+]emin the range of 20–40 mM. Rapid switching (0.25–1 Hz) of [Ca2+]embetween 0 and 100 μM simulated rapid beat-to-beat changes in [Ca2+]i(with [Ca2+]itransient duration of 100–500 ms). No [Ca2+]moscillations were observed, either under conditions of maximal rate of Ca2+uptake (100 μM [Ca2+]em, 0 [Na+]em) or with maximal rate of Ca2+removal (0 [Ca2+]em, 40 mM [Na+]em). The slow frequency-dependent increase of [Ca2+]margues against a rapid transmission of Ca2+signals between cytosol and mitochondria on a beat-to-beat basis in the heart. [Ca2+]mchanges elicited by continuous or pulsatile exposure to elevated [Ca2+]emshowed no difference in mitochondrial Ca2+uptake. Thus in cardiac myocytes fast [Ca2+]itransients are integrated by mitochondrial Ca2+transport systems, resulting in a frequency-dependent net mitochondrial Ca2+accumulation.

Effects of reduced free fatty acid availability on skeletal muscle PDH activation during aerobic exercise. Pyruvate dehydrogenase

This study investigated the effect of reduced free fatty acid (FFA) availability on pyruvate dehydrogenase activation (PDHa) and carbohydrate metabolism during moderate aerobic exercise. Eight active male subjects cycled for 40 min at 55% Vo(2 peak) on two occasions. During one trial, subjects ingested 20 mg/kg body mass of the antilipolytic drug nicotinic acid (NA) during the hour before exercise to reduce FFA. Nothing was ingested in the control trial (CON). Blood and expired gas measurements were obtained throughout the trials, and muscle biopsy samples were obtained immediately before exercise and at 5, 20, and 40 min of exercise. Plasma FFA were lower in the NA trial (0.13 +/- 0.01 vs. 0.48 +/- 0.03 mM, P < 0.05), and the respiratory exchange ratio (RER) was increased with NA (0.93 +/- 0.01 vs. 0.89 +/- 0.01, P < 0.05), resulting in a 14.5 +/- 1.8% increase in carbohydrate oxidation compared with CON. PDHa increased rapidly in both trials at exercise onset but was approximately 15% higher (P < 0.05) throughout exercise in the NA trial (2.44 +/- 0.19 and 2.07 +/- 0.12 mmol x kg wet muscle(-1) x min(-1) for NA and CON at 40 min). Muscle glycogenolysis was 15.3 +/- 9.6% greater in the NA trial vs. the CON trial but did not reach statistical significance. Glucose 6-phosphate contents were elevated (P < 0.05) in the NA trial at 30 and 40 min of exercise, but pyruvate and lactate contents were unaffected. These data demonstrate that the reduction of exogenous FFA availability increased the activation of PDH and carbohydrate oxidation during moderate aerobic exercise in men. The increased activation of PDH was not explained by changes in muscle pyruvate or the ATP/ADP ratio but may be related to a decrease in the NADH/NAD(+) ratio or an epinephrine-induced increase in calcium concentration.

Localisation of intracellular calcium stores in the striated muscles of the jellyfishPolyorchis penicillatus: possible involvement in excitation–contraction coupling

When jellyfish striated muscles were stimulated directly, the amplitude of contractile tension increased as the stimulation frequency increased. Application of 10 mmol l–1 caffeine reduced the amplitude of contractile tension and abolished this facilitatory relationship, indicating that calcium stores participate in excitation–contraction coupling. Calcium stores were identified ultrastructurally using enzymatic histochemistry to localize CaATPases, and potassium dichromate to precipitate calcium. Electron energy-loss spectroscopy was used to verify the presence of calcium in precipitates. Both CaATPase and calcium were localised in membrane-bound vesicles beneath the sarcolemma. We concluded that sub-sarcolemmal vesicles could act as calcium stores and participate in excitation–contraction coupling.

No effect of mild heat stress on the regulation of carbohydrate metabolism at the onset of exercise

To investigate the influence of heat stress on the regulation of skeletal muscle carbohydrate metabolism, six active, but not specifically trained, men performed 5 min of cycling at a power output eliciting 70% maximal O2 uptake in either 20 degrees C (Con) or 40 degrees C (Heat) after 20 min of passive exposure to either environmental condition. Although muscle temperature (T(mu)) was similar at rest when comparing trials, 20 min of passive exposure and 5 min of exercise increased (P < 0.05) T(mu) in Heat compared with Con (37.5 +/- 0.1 vs. 36.9 +/- 0.1 degrees C at 5 min for Heat and Con, respectively). Rectal temperature and plasma epinephrine were not different at rest, preexercise, or 5 min of exercise between trials. Although intramuscular glycogen phosphorylase and pyruvate dehydrogenase activity increased (P < 0.05) at the onset of exercise, there were no differences in the activities of these regulatory enzymes when comparing Heat with Con. Accordingly, glycogen use in the first 5 min of exercise was not different when comparing Heat with Con. Similarly, no differences in intramuscular concentrations of glucose 6-phosphate, lactate, pyruvate, acetyl-CoA, creatine, phosphocreatine, or ATP were observed at any time point when comparing Heat with Con. These results demonstrate that, whereas mild heat stress results in a small difference in contracting T(mu), it does not alter the activities of the key regulatory enzymes for carbohydrate metabolism or glycogen use at the onset of exercise, when plasma epinephrine levels are unaltered.

Adrenaline increases skeletal muscle glycogenolysis, pyruvate dehydrogenase activation and carbohydrate oxidation during moderate exercise in humans

1. To evaluate the role of adrenaline in regulating carbohydrate metabolism during moderate exercise, 10 moderately trained men completed two 20 min exercise bouts at 58 +/- 2 % peak pulmonary oxygen uptake (V(O2,peak)). On one occasion saline was infused (CON), and on the other adrenaline was infused intravenously for 5 min prior to and throughout exercise (ADR). Glucose kinetics were measured by a primed, continuous infusion of 6,6-[(2)H]glucose and muscle samples were obtained prior to and at 1 and 20 min of exercise. 2. The infusion of adrenaline elevated (P < 0.01) plasma adrenaline concentrations at rest (pre-infusion, 0.28 +/- 0.09; post-infusion, 1.70 +/- 0.45 nmol l(-1); means +/- S.E.M.) and this effect was maintained throughout exercise. Total carbohydrate oxidation increased by 18 % and this effect was due to greater skeletal muscle glycogenolysis (P < 0.05) and pyruvate dehydrogenase (PDH) activation (P < 0.05, treatment effect). Glucose rate of appearance was not different between trials, but the infusion of adrenaline decreased (P < 0.05, treatment effect) skeletal muscle glucose uptake in ADR. 3. During exercise muscle glucose 6-phosphate (G-6-P) (P = 0.055, treatment effect) and lactate (P < 0.05) were elevated in ADR compared with CON and no changes were observed for pyruvate, creatine, phosphocreatine, ATP and the calculated free concentrations of ADP and AMP. 4. The data demonstrate that elevated plasma adrenaline levels during moderate exercise in untrained men increase skeletal muscle glycogen breakdown and PDH activation, which results in greater carbohydrate oxidation. The greater muscle glycogenolysis appears to be due to increased glycogen phosphorylase transformation whilst the increased PDH activity cannot be readily explained. Finally, the decreased glucose uptake observed during exercise in ADR is likely to be due to the increased intracellular G-6-P and a subsequent decrease in glucose phosphorylation.

Rat arterial smooth muscle devoid of ryanodine receptor function: effects on cellular Ca(2+) handling

The roles of intracellular Ca(2+) stores and ryanodine (Ry) receptors for vascular Ca(2+) homeostasis and viability were investigated in rat tail arterial segments kept in organ culture with Ry (10 - 100 microM) for up to 4 days. Acute exposure to Ry or the non-deactivating ryanodine analogue C(10)-O(eq) glycyl ryanodine (10 microM) eliminated Ca(2+) release responses to caffeine (20 mM) and noradrenaline (NA, 10 microM), whereas responses to NA, but not caffeine, gradually returned to normal within 4 days of exposure to RY: Ry receptor protein was detected on Western blots in arteries cultured either with or without RY: Brief Ca(2+) release events (sparks) were absent after culture with Ry, whereas Ca(2+) waves still occurred. The propagation velocity of waves was equal ( approximately 19 microm s(-1)) in tissue cultured either with or without RY: Inhibition of Ca(2+) accumulation into the sarcoplasmic reticulum (SR) by culture with caffeine (5 mM), cyclopiazonic acid or thapsigargin (both 10 microM) decreased contractility due to Ca(2+)-induced cell damage. In contrast, culture with Ry did not affect contractility. Removal of Ca(2+) from the cytosol following a Ca(2+) load was retarded after Ry culture. Thapsigargin reduced the rate of Ca(2+) removal in control cultured rings, but had no effect after Ry culture. It is concluded that intracellular Ca(2+) stores recover during chronic Ry treatment, while Ry receptors remain non-functional. Ry receptor activity is required for Ca(2+) sparks and for SR-dependent recovery from a Ca(2+) load, but not for Ca(2+) waves or basal Ca(2+) homeostasis.

Inositol hexakisphosphate binding sites in rat heart and brain

1. Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol hexakisphosphate (InsP6) are produced in response to stimulation of cardiac alpha 1-adrenoceptors. While the role of Ins(1,4,5)P3 and Ins(1,4,5)P3 receptors is well-defined in many tissues including brain, the functional role of the putative InsP6-InsP6 receptor system in cardiac function is less clear. Using quantitative autoradiography, this study examined the characteristics and regional localization of [3H]-InsP6 binding sites in rat heart and compared the affinity of a range of inositol polyphosphates for [3H]-InsP6 and [3H]-Ins(1,4,5)P3 binding sites in heart and brain. 2. [3H]-InsP6 bound to a single, high affinity site in sections of rat heart (KD ranging from 22 +/- 1.9 nM in right atria to 35 +/- 2.6 nM in the interventricular septum, n = 7). The maximal number of binding sites (Bmax) ranged from 5.1 +/- 0.48 to 12 +/- 1.8 pmol mg-1 protein in left atrium and left ventricle, respectively. Inositol phosphates inhibited binding of [3H]-InsP6 with the order of potency: InsP6 > Ins(1,4,5)PS3 > inositol 1,3,4,5-tetrakisphosphate > or = inositol pentakisphosphate > Ins(1,4,5)P3 > > inositol mono- and bisphosphates, consistent with the labelling of an InsP6 binding site. 3. The Ins(1,4,5)P3 analogue, Ins(1,4,5)PS3, originally investigated as a putative selective radioligand for the Ins(1,4,5)P3 receptor, was a potent inhibitor of [3H]-InsP6 binding in all heart regions (K1 = 170-260 nM). The K1 of Ins(1,4,5)PS3 for the inhibition of [3H]-Ins(1,4,5)P3 binding in rat brain (60-220 nM) was similar to that observed for the inhibition of [3H]-InsP6 binding in heart, suggesting that Ins(1,4,5)PS3 is not a specific ligand for either Ins(1,4,5)P3 or InsP6 receptor binding sites. 4. Previous studies have detected [3H]-InsP6 binding in mitochondrial and sarcoplasmic reticulum fractions of heart and links between InsP6 and cardiac mitochondrial Ca2+ regulation have been proposed, suggesting further studies are warranted to determine the functional role(s) of InsP6 and InsP6 receptor binding sites in cardiac tissue.

The hormonal regulation of pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex has a central role in the regulation of mammalian metabolism as it represents the point-of-no-return in the utilization of carbohydrate. This article summarizes our studies into how signalling systems initiated by hormones binding to cell surface receptors can reach the pyruvate dehydrogenase system which is located within the inner mitochondrial membrane. One class of hormones which activate pyruvate dehydrogenase are those that increase cytoplasmic Ca2+. A wide range of studies on isolated enzymes, separated mitochondria and intact cell preparations have shown that the activation is due to the stimulation of pyruvate dehydrogenase phosphatase. Two other intramitochondrial dehydrogenases which regulate the citrate acid cycle are activated in parallel and this is an important means of balancing the supply of ATP to increasing cell demand. Insulin is also able to activate pyruvate dehydrogenase, but this is restricted to fat and other cells capable of lipogenesis. Insulin acts by stimulating pyruvate dehydrogenase phosphatase, but the activation does not involve alterations in Ca2+. The signalling pathway involved has not been established, but it appears to be quite distinct from those involved in many other actions of insulin.

Calcium regulation of glycolysis, glucose oxidation, and fatty acid oxidation in the aerobic and ischemic heart

Although Ca2+ is an important regulator of energy metabolism, the effects of increasing extracellular [Ca2+] on energy substrate preference are not clear. We determined the relationship between [Ca2+], fatty acids, and ischemia on rates of glycolysis, glucose oxidation, and palmitate oxidation in isolated working rat hearts. Hearts were perfused with Krebs-Henseleit buffer containing 11 mM glucose, 100 microU/mL insulin, and either 1.25 or 2.5 mM Ca2+, in the presence or absence of 1.2 mM palmitate. Rates of glycolysis and glucose oxidation or palmitate oxidation were measured in the hearts using [5-3H,14C(U)]glucose or [1-14C]palmitate, respectively. In the absence of fatty acids, glycolysis and glucose oxidation rates were similar, regardless of whether [Ca2+ was 1.25 or 2.5 mM. Addition of 1.2 mM palmitate to the perfusate of hearts perfused with 1.25 mM Ca2+ significantly decreased rates of both glycolysis (from 4623 +/- 438 to 1378 +/- 238 nmol.min-1.g-1 dry weight) and glucose oxidation (from 1392 +/- 219 to 114 +/- 22 nmol.min-1.g-1 dry weight). When [Ca2+] was increased from 1.25 to 2.5 mM in hearts perfused with 1.2 mM palmitate, glycolysis and glucose oxidation increased by 164 and 271%, respectively, with no change in palmitate oxidation rates. Increasing [Ca2+] from 1.25 to 2.5 mM increased the contribution of glucose to ATP production from 9.3 to 18.7%. When hearts were subjected to low-flow ischemia (by reducing coronary flow to 0.5 mL.min-1) oxidative metabolism was essentially abolished. Under these conditions, glycolytic rates were not dependent on either [Ca2+] or the presence or absence of fatty acids. These results demonstrate that perfusate [Ca2+] is an important determinant of myocardial glucose metabolism in aerobic hearts, and that glycolysis and glucose oxidation are more responsive to changes in [Ca2+] than is fatty acid oxidation.

Physiological role of mitochondrial Ca2+ transport

A model has been proposed in which mitochondrial Ca2+ ion transport serves to regulate mitochondrial matrix free Ca2+ ([Ca2+]m), with the advantage to the animal that this allows the regulation of pyruvate dehydrogenase and the tricarboxylate cycle in response to energy demand. This article examines recent evidence for dehydrogenase activation and for increases in [Ca2+]m in response to increased tissue energy demands, especially in cardiac myocytes and in heart. It critiques recent results on beat-to-beat variation in [Ca2+]m in cardiac muscle and also briefly surveys the impact of mitochondrial Ca2+ transport on transient changes in cytosolic free Ca2+ in excitable tissues. Finally, it proposes that a failure to elevate [Ca2+]m sufficiently in response to work load may underlie some cardiomyopathies of metabolic origin.

Effects of tetrahydroberberine on isoproterenol-stimulated mitochondrial 45Ca uptake in rabbit myocardia

Cardiac mitochondria of rabbit was prepared by differential centrifugation. Tetrahydroberberine (THB) 10 mumol/L inhibited isoproterenol 1 mumol/L induced 45Ca uptake in the mitochondria by 48.58%. The inhibition is concentration-dependent and may play an important role in the protection of mitochondrial function in cardiac ischemia and reperfusion injury.

Alpha 1-adrenergic effects on intracellular pH and calcium and on myofilaments in single rat cardiac cells

1. The cellular effects of alpha 1-adrenoceptor stimulation by phenylephrine were studied in the presence of propranolol in single cells isolated from the ventricles of rat hearts. 2. Phenylephrine (10-100 microM) induced a biphasic pattern of inotropism in these cells: a transient negative followed by a sustained positive inotropic effect as usually observed in cardiac tissues. 3. In Snarf-1-loaded cells, phenylephrine induced an alkalinization. This effect was reversible on wash-out and inhibited by prazosin, an alpha 1-adrenoceptor antagonist. 4. The alpha 1-adrenoceptor-mediated increase in intracellular pH (pHi) was 0.1 pH unit in HEPES buffer containing 4.4 mM-NaHCO3 and in Krebs buffer containing 25 mM-NaHCO3. 5. The alkalinization was blocked by the Na(+)-H+ antiport blocker, ethylisopropylamiloride (EIPA). 6. The recovery from an acidosis induced by a NH4Cl pre-pulse was accelerated by phenylephrine. The phenylephrine-induced alkalinization was attributed to activation of the Na(+)-H+ antiport. 7. Despite its ability to increase pHi, phenylephrine did not alter Ca2+ current amplitude and kinetics. 8. Ca2+ transients recorded in Indo-1-loaded cells were not augmented by phenylephrine. Diastolic calcium level was decreased. 9. In single skinned cells, the Ca2+ sensitivity of the contractile proteins was increased by a pre-treatment with phenylephrine even when the alpha 1-adrenoceptor-mediated alkalinizing effect had been prevented by EIPA. 10. These results lead us to propose that the alpha 1-adrenergic-induced positive inotropic response of heart muscle could result from an increased sensitivity of the myofilaments to Ca2+ ions. This alpha 1-adrenoceptor-mediated Ca2+ sensitization could result both from an intracellular alkalinization and from a direct effect on contractile proteins.

Catalytic properties of the inorganic pyrophosphatase in rat liver mitochondria

Intact rat liver mitochondria have very low hydrolytic activity, if any, toward exogenous pyrophosphate. The activity can be unmasked by making mitochondria permeable to PPi by toluene treatment or disrupting them with detergents or ultrasound, indicating that the active site of pyrophosphatase is located in the matrix. Initial rates of PPi hydrolysis by toluene-permeabilized mitochondria and purified pyrophosphatase were found to depend in a similar manner on PPi and Mg2+ concentrations. The simplest model consistent with the data in both cases implies that the reaction proceeds through two pathways and requires MgPPi as the substrate and, at least, one Mg2+ ion as the activator. In the presence of 0.4 mM Mg2+ (physiological concentration), the inhibition constant for Ca2+ is 12 microM and the enzyme activity is, at least, 50% maximal. The results suggest that the activity of pyrophosphatase in mitochondria is high enough to keep free PPi concentration at a level close to that at equilibrium.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Evidence that angiotensin II decreases mitochondrial calcium in the glomerulosa cell

The present studies were performed using primary monolayer cultures of bovine glomerulosa cells to determine whether the elevation in cytosolic calcium concentration produced by angiotensin II was accompanied by an elevation in mitochondrial calcium. Exchangeable mitochondria calcium content was assessed indirectly by measuring the changes in cytosolic calcium concentration and calcium efflux produced by the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Total mitochondrial calcium content was also assessed directly by atomic absorption spectroscopy. CCCP had a direct effect to promote calcium release from an oligomycin/antimycin-sensitive (mitochondrial) calcium pool in permeabilized cells. In intact cells, CCCP caused rapid reductions in cellular ATP content and the ratio of ATP to ADP. Still, its effects on calcium dynamics were exerted primarily at the mitochondrial level as evidenced by inhibition with ruthenium red, but not dantrolene. As expected, angiotensin II produced a rapid increase in calcium efflux and an equally rapid and sustained increase in cytosolic calcium concentration. Nonetheless, CCCP-stimulated elevations in cytosolic calcium concentration and calcium efflux were reduced by angiotensin II in a concentration-dependent manner. Total mitochondrial calcium content was also lower in angiotensin-treated than in control cells. These results indicate that angiotensin II causes a net decrease in mitochondrial calcium stores. On the basis of these data, it is proposed that alterations in calcium metabolism initiated by angiotensin II are exerted not only at the membrane and cytosolic levels but also at the level of the mitochondria. Changes in mitochondrial calcium dynamics may directly contribute to the regulation of mitochondrial steroidogenic enzymes by angiotensin II.

Bay K 8644, modifier of calcium transport and energy metabolism in rat heart mitochondria: a new intracellular site of action

1. The dihydropyridine Ca2+ channel agonist Bay K 8644 (10-200 microM) produced a concentration-dependent increase in State 4 respiration in the rat heart mitochondria with the highest concentration (200 microM) increasing the rate from 33.1 +/- 0.7 to 187.0 +/- 13.3 ng atoms O2 consumed min-1 mg-1 protein. 2. Bay K 8644 (200 microM) reduced State 3 respiration from 247.2 +/- 11.7 to 174.4 +/- 0.06 ng atoms O2 min-1 mg-1 protein, reduced the respiratory control index (RCI) from 5.3 +/- 0.45 to 1.1 +/- 0.03 and reduced the ADP:O ratio from 2.75 +/- 0.03 to 1.3 +/- 0.15. 3. A similar, but smaller, stimulation of State 4 respiration was seen with nitrendipine (25-200 microM), the rate increasing from 22.6 +/- 1.0 to 33.1 +/- 1.8 ng atoms O2 consumed min-1 mg-1 protein in the presence of 200 microM nitrendipine. 4. Bay K 8644 (10-60 microM) increased the total Ca2+ uptake into rat heart mitochondria, the total increasing from 248.8 +/- 8.4 to 406.9 +/- 17.6 ng Ca2+ mg-1 protein at 60 microM Bay K 8644 (EC50 = 18.9 +/- 1.4 microM). 5. Bay K 8644 (10-60 microM) produced a concentration-dependent reduction in the Ca2+ influx rate (IC50 = 52.5 +/- 2.8 microM). Similar effects were seen with (+)-Bay K 8644 and (-)-Bay K 8644. 6. Nitrendipine (40-120 microM) stimulated Ca2+ efflux from mitochondria preloaded with the ion; the efflux rate increasing from 2.9 +/- 0.05 to 114.2 +/- 6.2 nmol Ca2+ min-1 mg-1 protein (EC50 = 57.3 +/- 1.3 microM). 7. These data indicate dihydropyridine-induced changes in the activity of the mitochondrial Na+/Ca2 . antiporter pathway; nitrendipine causing stimulation and Bay K 8644 causing inhibition.

The role of mitochondrial Ca2+ transport and matrix Ca2+ in signal transduction in mammalian tissues

The pyruvate, NAD(+)-isocitrate and 2-oxoglutarate dehydrogenases are key regulatory enzymes in intramitochondrial oxidative metabolism in mammalian tissues, and can all be activated by increases in Ca2+ in the micromolar range. There is now mounting evidence that hormones and other stimuli which act by increasing cytosolic Ca2+ also, as a result, cause increases in mitochondrial matrix Ca2+ and hence activation of these enzymes, suggesting that the primary physiological function of mitochondrial Ca2(+)-transport is to be involved in this relay mechanism. This may also explain how in such circumstances rates of ATP production may be increased to meet the greater demand, but without any decreases in ATP/ADP occurring.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

The role of Ca2+ ions in the regulation of intramitochondrial metabolism and energy production in rat heart

In the heart and other mammalian tissues, there are three exclusively intramitochondrial dehydrogenases that occupy key regulatory sites in oxidative metabolism which can be activated by increases in Ca2+ in the approximate range 0.05-5 microM; they are the pyruvate, NAD+-isocitrate and 2-oxoglutarate dehydrogenases. Activation of these enzymes can be demonstrated within intact mitochondria, incubated under expected physiological conditions, when the extramitochondrial concentration of Ca+ is raised within the expected physiological range. Recent studies with fura-2-loaded mitochondria have established that matrix Ca2+ is indeed in the 0.02-2 microM range as the enzymes are activated. There is now good evidence that in the rat heart, increases in cytoplasmic [Ca2+] caused by various inotropic agents result in increases in intramitochondrial Ca2+ and activation of these dehydrogenases. It is argued therefore that matrix Ca2+ may thus be a key regulator of oxidative phosphorylation under such circumstances. The major advantage of such a mechanism of dehydrogenase-based control of this process would be to the energy homeostasis of the cell by allowing stimulated ATP production without the need to decrease the ATP/ADP ratio. Therefore it is also proposed that the major function of the mitochondrial Ca2+-transport system is to regulate matrix Ca2+, and that the ability of mitochondria to buffer the extramitochondrial concentration of Ca2+ may thus only be reserved for pathophysiological conditions of abnormal sarcolemmal Ca2+ influx as perhaps may occur in ischaemia-reperfusion.

Inhibition of inorganic pyrophosphatase of animal mitochondria by calcium

Calcium ion is an uncompetitive inhibitor of the inorganic pyrophosphatases of bovine heart and rat liver mitochondria with respect to substrate MgPPi at pH 8.5 and a non-competitive inhibitor of the former enzyme at pH 7.2. The concentration of Ca2+ required to decrease the maximal velocities for both enzymes at pH 8.5, 0.4 mM Mg2+ was about 10 microM. The inhibition results from the binding of two Ca2+ ions to both free enzymes and their complexes with the substrate. The results suggest that Ca2+ regulates pyrophosphatase activity and hence PPi level in mammalian mitochondria.

Direct evidence for a role of intramitochondrial Ca2+ in the regulation of oxidative phosphorylation in the stimulated rat heart. Studies using 31P n.m.r. and ruthenium red

1. The concentrations of free ATP, phosphocreatine (PCr), Pi, H+ and ADP (calculated) were monitored in perfused rat hearts by 31P n.m.r. before and during positive inotropic stimulation. Data were accumulated in 20 s blocks. 2. Administration of 0.1 microM-(-)-isoprenaline resulted in no significant changes in ATP, transient decreases in PCr, and transient increases in ADP and Pi. However, the concentrations of all of these metabolites returned to pre-stimulated values within 1 min, whereas cardiac work and O2 uptake remained elevated. 3. In contrast, in hearts perfused continuously with Ruthenium Red (2.5 micrograms/ml), a potent inhibitor of mitochondrial Ca2+ uptake, administration of isoprenaline caused significant decreases in ATP, and also much larger and more prolonged changes in the concentrations of ADP, PCr and Pi. In this instance values did not fully return to pre-stimulated concentrations. Administration of Ruthenium Red alone to unstimulated hearts had minor effects. 4. It is proposed that, in the absence of Ruthenium Red, the transmission of changes in cytoplasmic Ca2+ across the mitochondrial inner membrane is able to maintain the phosphorylation potential of the heart during positive inotropic stimulation, through activation of the Ca2+-sensitive intramitochondrial dehydrogenases (pyruvate, NAD+-isocitrate and 2-oxoglutarate dehydrogenases) leading to enhanced NADH production. 5. This mechanism is unavailable in the presence of Ruthenium Red, and oxidative phosphorylation must be stimulated primarily by a fall in phosphorylation potential, in accordance with the classical concept of respiratory control. However, the full oxidative response of the heart to stimulation may not be achievable under such circumstances.

Inhibition by cyclosporin A of a Ca2+-dependent pore in heart mitochondria activated by inorganic phosphate and oxidative stress

The capacity of cyclosporin A to inhibit opening of a Ca2+-dependent pore in the inner membrane of heart mitochondria was investigated. Whereas in the presence of 25 nmol of Ca2+/mg of mitochondrial protein and 5 mM-Pi mitochondria were unable to maintain accumulated Ca2+, inner-membrane potential and sucrose impermeability, all three parameters were preserved when cyclosporin was included. Pore opening was assayed directly by [14C]sucrose entry and entrapment in the matrix space. [14C]Sucrose entry induced by both Ca2+ plus Pi and Ca2+ plus t-butyl hydroperoxide was almost completely inhibited by 60 pmol of cyclosporin/mg of mitochondrial protein. It is concluded that cyclosporin A is a potent inhibitor of the pore.

Characterization of the effects of Ca2+ on the intramitochondrial Ca2+-sensitive dehydrogenases within intact rat-kidney mitochondria

The regulatory properties of the Ca2+-sensitive intramitochondrial enzymes (pyruvate dehydrogenase phosphate phosphatase, NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase) in extracts of rat kidney mitochondria were found to be essentially similar to those described previously for other mammalian tissues; in particular each enzyme could be activated severalfold by Ca2+ with half-maximal effects (K0.5 values) of about 1 microM and effective ranges of approx. 0.1-10 microM Ca2+. In intact mitochondria prepared from whole rat kidneys incubated in a KCl-based medium containing respiratory substrates, the amount of active, nonphosphorylated pyruvate dehydrogenase could be increased severalfold by increases in extramitochondrial [Ca2+]; these effects could be blocked by ruthenium red. Similarly, Ca2+-dependent activations of NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase could be demonstrated in intact, fully coupled, rat kidney mitochondria by either following O2 uptake (in the presence of ADP) and NAD(P)H reduction (in the absence of ADP) on presentation of non-saturating concentrations of either threo-Ds-isocitrate or 2-oxoglutarate, respectively, under appropriate conditions, or for the latter enzyme only, also by following 14CO2 production from 2-oxo[1-14C]glutarate (in the absence or presence of ADP). Effects of Na+ (as a promoter of egress) and Mg2+ (as an inhibitor of uptake) on Ca2+-transport by rat kidney mitochondria could be readily demonstrated by assaying for the Ca2+-sensitive properties of the intramitochondrial Ca2+-sensitive dehydrogenases within intact rat kidney mitochondria. In the presence of physiological concentrations of Na+ (10 mM) and Mg2+ (2 mM), activation of the enzymes was achieved by increases in extramitochondrial [Ca2+] within the expected physiological range (0.05-5 microM) and with apparent K0.5 values in the approximate range of 300-500 nM. The implications of these results on the role of the Ca2+-transport system of kidney mitochondria are discussed.

Regulation of NAD+-linked isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase by Ca2+ ions within toluene-permeabilized rat heart mitochondria. Interactions with regulation by adenine nucleotides and NADH/NAD+ ratios

1. Toluene-permeabilized rat heart mitochondria have been used to study the regulation of NAD+-linked isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase by Ca2+, adenine and nicotinamide nucleotides, and to compare the properties of the enzymes in situ, with those in mitochondrial extracts. 2. Although K0.5 values (concn. giving half-maximal effect) for Ca2+ of 2-oxoglutarate dehydrogenase were around 1 microM under all conditions, corresponding values for NAD+-linked isocitrate dehydrogenase were in the range 5-43 microM. 3. For both enzymes, K0.5 values for Ca2+ observed in the presence of ATP were 3-10-fold higher than those in the presence of ADP, with values increasing over the ADP/ATP range 0.0-1.0. 4. 2-Oxoglutarate dehydrogenase was less sensitive to inhibition by NADH when assayed in permeabilized mitochondria than in mitochondrial extracts. Similarly, the Km of NAD+-linked isocitrate dehydrogenase for threo-Ds-isocitrate was lower in permeabilized mitochondria than in extracts under all the conditions investigated. 5. It is concluded that in the intact heart Ca2+ activation of NAD+-linked isocitrate dehydrogenase may not necessarily occur in parallel with that of the other mitochondrial Ca2+-sensitive enzymes, 2-oxoglutarate dehydrogenase and the pyruvate dehydrogenase system.

Parallel measurement of oxoglutarate dehydrogenase activity and matrix free Ca2+ in fura-2-loaded heart mitochondria

The entrapment of the Ca2+-sensitive fluorescence indicators fura-2 or quin2 in the matrix space of isolated heart mitochondria renders possible the direct monitoring of the matrix free Ca2+ [( Ca2+]m) [(1987) Biochem J. 248, 609-613]. In this paper the correlation between the [Ca2+]m and the in situ activity of oxoglutarate dehydrogenase (OGDH) in fura-2-loaded mitochondria is shown. At the initial value of [Ca2+]m, 64 nM, which corresponded to 0.36 nmol/mg mitochondrial Ca content, the OGDH activity was 12% of the maximal. Half-maximal and maximal activation were attained at 0.8 and 1.6 microM [Ca2+]m, respectively. The results indicate that an increase of the mitochondrial Ca content in the physiological range enhances the OGDH activity by means of elevation of [Ca2+]m.

Mechanisms of mitochondrial calcium transport

Mitochondria are known to possess a rapid calcium uptake mechanism or uniport and both sodium-dependent and sodium-independent efflux mechanisms. Whether sodium-independent calcium efflux is mediated and whether sodium-dependent calcium efflux can be found in liver mitochondria have been questioned. Kinetics results relevant to the answers of these questions are discussed below. A slow, mediated, sodium-independent calcium efflux mechanism is identified which shows second order kinetics. This mechanism, which shows "nonessential activation" kinetics, has a Vmax around 1.2 nmol calcium per mg protein per min and a half maximal velocity around 8.4 nmol calcium per mg protein. A slow, sodium-dependent calcium efflux mechanism is identified, which is first order in calcium and second order in sodium. This mechanism has a Vmax around 2.6 nmol of calcium per mg protein per min. The sodium dependence is half saturated at an external sodium concentration of 9.4 mM, and the calcium dependence is half saturated at an internal calcium concentration of 8.1 nmol calcium per mg protein. The cooperativity of the sodium dependence effectively permits a terreactant system to be fit by a bireactant model in which [Na] only appears as the square of [Na]. This liver system shows simultaneous, as opposed to ping-pong, kinetics. It is also found to be sensitive to inhibition by tetraphenyl phosphonium, magnesium, and ruthenium red. A model is proposed in which mitochondrial calcium transport could function to "shape the pulses" of cytosolic calcium. Simultaneously, mitochondria may mediate a "calcium memory" coupled perhaps to activation of cytosolic events through calmodulin or perhaps to activation of electron transport through the activation of specific dehydrogenases by intramitochondrial calcium.

Measurement of the matrix free Ca2+ concentration in heart mitochondria by entrapped fura-2 and quin2

A method was developed to monitor continuously the matrix free Ca2+ concentration ([Ca2+]m) of heart mitochondria by use of the fluorescent Ca2+ indicators, fura-2 and quin2. The acetoxymethyl esters of fura-2 and quin2 were accumulated in and hydrolysed by isolated mitochondria. An increase of the mitochondrial Ca content from 0.3 nmol/mg of protein to 6 nmol/mg corresponded to a rise of [Ca2+]m from 30 to 1000 nM. The results indicate that physiological fluctuations of the mitochondrial Ca content elicit changes of [Ca2+]m in that range which regulates the matrix dehydrogenases.

Evidence for the presence of a reversible Ca2+-dependent pore activated by oxidative stress in heart mitochondria

Rat heart mitochondria became permeabilized to sucrose when incubated with 100 nmol of Ca2+/mg of protein in the presence of Pi. Ca2+ chelation with EGTA restored impermeability to sucrose, which became entrapped in the matrix space. t-Butylhydroperoxide markedly promoted permeabilization in the presence of Ca2+ but not in its absence, and Ca2+-plus-t-butylhydroperoxide-induced permeabilization was reversed by EGTA. The data suggest that Ca2+ and oxidative stress synergistically promote the reversible opening of an inner membrane pore.

The effects of Mg2+ and adenine nucleotides on the sensitivity of the heart mitochondrial Na+-Ca2+ carrier to extramitochondrial Ca2+. A study using arsenazo III-loaded mitochondria

The technique of reversible Ca2+-induced permeabilization [Al Nasser & Crompton (1986) Biochem. J. 239, 19-29, 31-40] has been applied to the preparation of heart mitochondria loaded with the Ca2+ indicator arsenazo III (2 nmol of arsenazo III/mg of mitochondrial protein). The loaded mitochondria ('mitosomes') were used to study the control of the Na+-Ca2+ carrier by extramitochondrial Ca2+ mediated by putative regulatory sites. The Vmax. of the Na+-Ca2+ carrier and the degree of regulatory-site-mediated inhibition were similar to normal heart mitochondria. Ca2+ occupation of the sites in mitosomes yields partial inhibition, which is half-maximal with 0.8 microM external free Ca2+. The inhibition consists of a small decrease in Vmax. and a relatively large increase in apparent Km for internal Ca2+. Mg2+ also appears to interact with the sites, but this is largely abolished by ATP and ADP (but not AMP) under conditions in which the free [Mg2+] is maintained constant. The results indicate that the regulatory sites are effective in controlling the Na+-Ca2+ carrier at physiological concentrations of adenine nucleotides, Mg2+, intra- and extra-mitochondrial free Ca2+.

The calcium sensitive dehydrogenases of vertebrate mitochondria

Three important dehydrogenases in vertebrate mitochondria are activated by Ca2+ ions with half-maximal effects at about 1 microM. These are pyruvate dehydrogenase, NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase. Activation of these enzymes can also be demonstrated within intact mitochondria when extramitochondrial Ca2+ is increased within the range of concentrations generally considered to occur in the cytoplasm of vertebrate cells. It is argued that the main role of the calcium transport system in the inner membrane of vertebrate mitochondria is to relay changes in the cytoplasmic concentration of Ca2+ into the mitochondrial matrix. In this way, hormones and other extracellular stimuli which stimulate ATP-requiring processes such as contraction and secretion through increases in the cytoplasmic concentration of Ca2+ may also increase intramitochondrial oxidative metabolism and hence the replenishment of ATP.

Ca2+ transport by mammalian mitochondria and its role in hormone action

Three key dehydrogenases in mammalian mitochondria have been found to be activated by Ca2+ with a half-maximal effect at approximately 1 microM. These are pyruvate dehydrogenase, NAD+-isocitrate dehydrogenase, and oxoglutarate dehydrogenase. Activation of these enzymes can also be demonstrated in intact coupled mitochondria when extra mitochondrial Ca2+ is increased in the range of concentrations (0.1 to 2 microM) generally considered to occur in the cytoplasm of normal cells. It is argued that the main role of the calcium transport system in mammalian mitochondria is to relay changes in cytoplasmic Ca2+ into the mitochondrial matrix. Hormones and other extracellular messengers which stimulate ATP-requiring processes such as secretion or muscle contraction through increasing the cytoplasmic concentration of Ca2+ could in this way also increase intramitochondrial oxidative metabolism and hence promote the replenishment of ATP. Recent evidence obtained with heart and liver preparations in support of this view is reviewed.