Modulation of cardiac Ca2+ channels by isoproterenol studied in transgenic mice with altered SR Ca2+ content

Phospholamban(PLB) ablation is associated with enhanced sarcoplasmic reticulum (SR)Ca²⁺ uptake and attenuation of thecardiac contractile responses to -adrenergic agonists. In thepresent study, we compared the effects of isoproterenol (Iso) on theCa²⁺ currents(I_Ca) ofventricular myocytes isolated from wild-type (WT) and PLB knockout(PLB-KO) mice. Current density and voltage dependence ofI_Ca were similarbetween WT and PLB-KO cells. However, I_Ca recorded fromPLB-KO myocytes had significantly faster decay kinetics. Iso increasedI_Ca amplitude inboth groups in a dose-dependent manner (50% effective concentration,57.1 nM). Iso did not alter the rate ofI_Ca inactivationin WT cells but significantly prolonged the rate of inactivation inPLB-KO cells. When Ba²⁺ was usedas the charge carrier, Iso slowed the decay of the current in both WTand PLB-KO cells. Depletion of SRCa²⁺ by ryanodine also slowed therate of inactivation ofI_Ca, and subsequent application of Iso further reduced the inactivation rate ofboth groups. These results suggest that enhancedCa²⁺ release from the SR offsetsthe slowing effects of -adrenergic receptor stimulation on the rateof inactivation ofI_Ca.

     

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⁺]_em resulting 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⁺]_m increase 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⁺]_em in the range of 20–40 mM. Rapid switching (0.25–1 Hz) of [Ca²⁺]_em between 0 and 100 µM simulated rapid beat-to-beat changes in [Ca²⁺]_i (with [Ca²⁺]_i transient duration of 100–500 ms). No [Ca²⁺]_m oscillations 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⁺]_m argues against a rapid transmission of Ca²⁺ signals between cytosol and mitochondria on a beat-to-beat basis in the heart. [Ca²⁺]_m changes elicited by continuous or pulsatile exposure to elevated [Ca²⁺]_em showed no difference in mitochondrial Ca²⁺ uptake. Thus in cardiac myocytes fast [Ca²⁺]_i transients are integrated by mitochondrial Ca²⁺ transport systems, resulting in a frequency-dependent net mitochondrial Ca²⁺ accumulation. mitochondrial Ca²⁺; excitation-contraction coupling; cardiomyocytes     

Oxidant-impaired intracellular Ca2+ signaling in pancreatic acinar cells: role of the plasma membrane Ca2+-ATPase

Pancreatitis is an inflammatory disease of pancreatic acinar cells whereby intracellular calcium concentration ([Ca²⁺]_i) signaling and enzyme secretion are impaired. Increased oxidative stress has been suggested to mediate the associated cell injury. The present study tested the effects of the oxidant, hydrogen peroxide, on [Ca²⁺]_i signaling in rat pancreatic acinar cells by simultaneously imaging fura-2, to measure [Ca²⁺]_i, and dichlorofluorescein, to measure oxidative stress. Millimolar concentrations of hydrogen peroxide increased cellular oxidative stress and irreversibly increased [Ca²⁺]_i, which was sensitive to antioxidants and removal of external Ca²⁺, and ultimately led to cell lysis. Responses were also abolished by pretreatment with (sarco)endoplasmic reticulum Ca²⁺-ATPase inhibitors, unless cells were prestimulated with cholecystokinin to promote mitochondrial Ca²⁺ uptake. This suggests that hydrogen peroxide promotes Ca²⁺ release from the endoplasmic reticulum and the mitochondria and that it promotes Ca²⁺ influx. Lower concentrations of hydrogen peroxide (10–100 µM) increased [Ca²⁺]_i and altered cholecystokinin-evoked [Ca²⁺]_i oscillations with marked heterogeneity, the severity of which was directly related to oxidative stress, suggesting differences in cellular antioxidant capacity. These changes in [Ca²⁺]_i also upregulated the activity of the plasma membrane Ca²⁺-ATPase in a Ca²⁺-dependent manner, whereas higher concentrations (0.1–1 mM) inactivated the plasma membrane Ca²⁺-ATPase. This may be important in facilitating "Ca²⁺ overload," resulting in cell injury associated with pancreatitis. oxidant stress; pancreatitis; calcium pump     

Distinct roles of PMCA isoforms in Ca2+ homeostasis of bladder smooth muscle: evidence from PMCA gene-ablated mice

We previously showed that plasma membrane Ca²⁺-ATPase (PMCA) activity accounted for 25–30% of relaxation in bladder smooth muscle (8). Among the four PMCA isoforms only PMCA1 and PMCA4 are expressed in smooth muscle. To address the role of these isoforms, we measured cytosolic Ca²⁺ ([Ca²⁺]_i) using fura-PE3 and simultaneously measured contractility in bladder smooth muscle from wild-type (WT), Pmca1^+/–, Pmca4^+/–, Pmca4^–/–, and Pmca1^+/–Pmca4^–/– mice. There were no differences in basal [Ca²⁺]_i values between bladder preparations. KCl (80 mM) elicited both larger forces (150–190%) and increases in [Ca²⁺]_i (130–180%) in smooth muscle from Pmca1^+/– and Pmca1^+/–Pmca4^–/– bladders than those in WT or Pmca4^–/–. The responses to carbachol (CCh: 10 µM) were also greater in Pmca1^+/– (120–150%) than in WT bladders. In contrast, the responses in Pmca4^–/– and Pmca1^+/–Pmca4^–/– bladders to CCh were significantly smaller (40–50%) than WT. The rise in half-times of force and [Ca²⁺]_i increases in response to KCl and CCh, and the concomitant half-times of their decrease upon washout of agonist were prolonged in Pmca4^–/– (130–190%) and Pmca1^+/–Pmca4^–/– (120–250%) bladders, but not in Pmca1^+/– bladders with respect to WT. Our evidence indicates distinct isoform functions with the PMCA1 isoform involved in overall Ca²⁺ clearance, while PMCA4 is essential for the [Ca²⁺]_i increase and contractile response to the CCh receptor-mediated signal transduction pathway. PMCA; bladder smooth muscle; gene-altered mice     

Nitric oxide induces [Ca2+]i oscillations in pituitary GH3 cells: involvement of IDR and ERG K+ currents

The role of nitric oxide (NO) in the occurrence of intracellular Ca²⁺ concentration ([Ca²⁺]_i) oscillations in pituitary GH₃ cells was evaluated by studying the effect of increasing or decreasing endogenous NO synthesis with L-arginine and nitro-L-arginine methyl ester (L-NAME), respectively. When NO synthesis was blocked with L-NAME (1 mM) [Ca²⁺]_i, oscillations disappeared in 68% of spontaneously active cells, whereas 41% of the quiescent cells showed [Ca²⁺]_i oscillations in response to the NO synthase (NOS) substrate L-arginine (10 mM). This effect was reproduced by the NO donors NOC-18 and S-nitroso-N-acetylpenicillamine (SNAP). NOC-18 was ineffective in the presence of the L-type voltage-dependent Ca²⁺ channels (VDCC) blocker nimodipine (1 µM) or in Ca²⁺-free medium. Conversely, its effect was preserved when Ca²⁺ release from intracellular Ca²⁺ stores was inhibited either with the ryanodine-receptor blocker ryanodine (500 µM) or with the inositol 1,4,5-trisphosphate receptor blocker xestospongin C (3 µM). These results suggest that NO induces the appearance of [Ca²⁺]_i oscillations by determining Ca²⁺ influx. Patch-clamp experiments excluded that NO acted directly on VDCC but suggested that NO determined membrane depolarization because of the inhibition of voltage-gated K⁺ channels. NOC-18 and SNAP caused a decrease in the amplitude of slow-inactivating (I_DR) and ether-à-go-go-related gene (ERG) hyperpolarization-evoked, deactivating K⁺ currents. Similar results were obtained when GH₃ cells were treated with L-arginine. The present study suggests that in GH₃ cells, endogenous NO plays a permissive role for the occurrence of spontaneous [Ca²⁺]_i oscillations through an inhibitory effect on I_DR and on I_ERG. voltage-gated potassium channels; ether-à-go-go-related gene potassium channels; slow-inactivating outward currents; fast-inactivating outward currents     

L-type voltage-dependent Ca2+ channels in cerebral microvascular endothelial cells and ET-1 biosynthesis

We investigatedthe role of intracellular calcium concentration([Ca²⁺]_i) in endothelin-1 (ET-1) production,the effects of potential vasospastic agents on[Ca²⁺]_i, and the presence of L-typevoltage-dependent Ca²⁺ channels in cerebral microvascularendothelial cells. Primary cultures of endothelial cells isolated frompiglet cerebral microvessels were used. Confluent cells were exposed toeither the thromboxane receptor agonist U-46619 (1 µM),5-hydroxytryptamine (5-HT; 0.1 mM), or lysophosphatidic acid (LPA; 1 µM) alone or after pretreatment with the Ca²⁺-chelatingagent EDTA (100 mM), the L-type Ca²⁺ channel blockerverapamil (10 µM), or the antagonist of receptor-operated Ca²⁺ channel SKF-96365 HCl (10 µM) for 15 min. ET-1production increased from 1.2 (control) to 8.2 (U-46619), 4.9 (5-HT),or 3.9 (LPA) fmol/µg protein, respectively. Such elevated ET-1biosynthesis was attenuated by verapamil, EDTA, or SKF-96365 HCl. Toinvestigate the presence of L-type voltage-dependent Ca²⁺channels in endothelial cells, the [Ca²⁺]_isignal was determined fluorometrically by using fura 2-AM. Superfusionof confluent endothelial cells with U-46619, 5-HT, or LPA significantlyincreased [Ca²⁺]_i. Pretreatment ofendothelial cells with high K⁺ (60 mM) or nifedipine (4 µM) diminished increases in [Ca²⁺]_i inducedby the vasoactive agents. These results indicate that 1)elevated [Ca²⁺]_i signals are involved in ET-1biosynthesis induced by specific spasmogenic agents, 2) theincreases in [Ca²⁺]_i induced by thevasoactive agents tested involve receptor as well as L-typevoltage-dependent Ca²⁺ channels, and 3) primarycultures of cerebral microvascular endothelial cells express L-typevoltage-dependent Ca²⁺ channels.

     

Hypotonic swelling-induced Ca2+ release by an IP3-insensitive Ca2+ store

Hypotonicswelling increases the intracellular Ca²⁺ concentration([Ca²⁺]_i) in vascular smooth muscle cells(VSMC). The source of this Ca²⁺ is not clear. To study thesource of increase in [Ca²⁺]_i in response tohypotonic swelling, we measured [Ca²⁺]_i infura 2-loaded cultured VSMC (A7r5 cells). Hypotonic swelling produced a40.7-nM increase in [Ca²⁺]_i that was notinhibited by EGTA but was inhibited by 1 µM thapsigargin. Priordepletion of inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ stores with vasopressin did not inhibit the increasein [Ca²⁺]_i in response to hypotonic swelling.Exposure of ⁴⁵Ca²⁺-loaded intracellular storesto hypotonic swelling in permeabilized VSMC produced an increase in⁴⁵Ca²⁺ efflux, which was inhibited by 1 µMthapsigargin but not by 50 µg/ml heparin, 50 µM ruthenium red, or25 µM thio-NADP. Thus hypotonic swelling of VSMC causes a release ofCa²⁺ from the intracellular stores from a novel sitedistinct from the IP₃-, ryanodine-, and nicotinic acidadenine dinucleotide phosphate-sensitive stores.

     

Elementary events of agonist-induced Ca2+ release in vascular endothelial cells

The subcellular spatial and temporal organization ofagonist-induced Ca²⁺ signals wasinvestigated in single cultured vascular endothelial cells.Extracellular application of ATP initiated a rapid increase ofintracellular Ca²⁺ concentration([Ca²⁺]_i)in peripheral cytoplasmic processes from where activation propagated asa[Ca²⁺]_iwave toward the central regions of the cell. The average propagation velocity of the[Ca²⁺]_iwave in the peripheral processes was 20-60 µm/s, whereas in thecentral region the wave propagated at <10 µm/s. The time course ofthe recovery of[Ca²⁺]_idepended on the cell geometry. In the peripheral processes (i.e.,regions with a high surface-to-volume ratio)[Ca²⁺]_ideclined monotonically, whereas in the central region[Ca²⁺]_idecreased in an oscillatory fashion. Propagating[Ca²⁺]_iwaves were preceded by small, highly localized[Ca²⁺]_itransients originating from 1- to 3-µm-wide regions. The average amplitude of these elementary events ofCa²⁺ release was 23 nM, and theunderlying flux of Ca²⁺ amountedto ~1-2 × 10¹⁸mol/s or ~0.3 pA, consistent with aCa²⁺ flux through a single orsmall number of endoplasmic reticulum Ca²⁺-release channels.

     

Mitochondrial transport in processes of cortical neurons is independent of intracellular calcium

Mitochondria show extensive movement along neuronal processes, but the mechanisms and function of this movement are not clearly understood. We have used high-resolution confocal microscopy to simultaneously monitor movement of mitochondria and changes in intracellular [Ca²⁺] ([Ca²⁺]_i) in rat cortical neurons. A significant percentage (27%) of the total mitochondria in cortical neuronal processes showed movement over distances of >2 µM. The average velocity was 0.52 µm/s. The velocity, direction, and pattern of mitochondrial movement were not affected by transient increases in [Ca²⁺]_i associated with spontaneous firing of action potentials. Stimulation of Ca²⁺ transients with forskolin (10 µM) or bicuculline (10 µM), or sustained elevations of [Ca²⁺]_i evoked by glutamate (10 µM) also had no effect on mitochondrial transit. Neither removal of extracellular Ca²⁺, depletion of intracellular Ca²⁺ stores with thapsigargin, or inhibition of synaptic activity with TTX (1 µM) or a cocktail of CNQX (10 µM) and MK801 (10 µM) affected mitochondrial movement. These results indicate that movement of mitochondria along processes is a fundamental activity in neurons that occurs independently of physiological changes in [Ca²⁺]_i associated with action potential firing, synaptic activity, or release of Ca²⁺ from intracellular stores. calcium transient; dendrites     

Metabolic inhibition with cyanide induces calcium release in pulmonary artery myocytes and Xenopus oocytes

We examined the effectsof metabolic inhibition on intracellular Ca²⁺ release insingle pulmonary arterial smooth muscle cells (PASMCs). Severemetabolic inhibition with cyanide (CN, 10 mM) increased intracellularcalcium concentration ([Ca²⁺]_i) and activatedCa²⁺-activated Cl currents[I_Cl(Ca)] in PASMCs, responses that were greatlyinhibited by BAPTA-AM or caffeine. Mild metabolic inhibition with CN (1 mM) increased spontaneous transient inward currents andCa²⁺ sparks in PASMCs. In Xenopus oocytes, CNalso induced Ca²⁺ release and activatedI_Cl(Ca), and these responses were inhibited by thapsigarginand cyclopiazonic acid to deplete sarcoplasmic reticulum (SR)Ca²⁺, whereas neither heparin nor anti-inositol1,4,5-trisphosphate receptor (IP₃R) antibodies affected CNresponses. In both PASMCs and oocytes, CN-evoked Ca²⁺release was inhibited by carbonyl cyanidem-chlorophenylhydrazone (CCCP) and oligomycin or CCCP andthapsigargin. Whereas hypoxic stimuli resulted in Ca²⁺release in pulmonary but not mesenteric artery myocytes, CN induced release in both cell types. We conclude that metabolic inhibition withCN increases [Ca²⁺]_i in both pulmonary andsystemic artery myocytes by stimulating Ca²⁺ release fromthe SR and mitochondria.

     

Regulation of intracellular calcium in N1E-115 neuroblastoma cells: the role of Na(+)/Ca(2+) exchange

In fura 2-loaded N1E-115 cells, regulationof intracellular Ca²⁺ concentration([Ca²⁺]_i) following a Ca²⁺ loadinduced by 1 µM thapsigargin and 10 µM carbonylcyanidep-trifluoromethyoxyphenylhydrazone (FCCP) wasNa⁺ dependent and inhibited by 5 mM Ni²⁺. Incells with normal intracellular Na⁺ concentration([Na⁺]_i), removal of bath Na⁺,which should result in reversal of Na⁺/Ca²⁺exchange, did not increase [Ca²⁺]_i unlesscell Ca²⁺ buffer capacity was reduced. When N1E-115 cellswere Na⁺ loaded using 100 µM veratridine and 4 µg/mlscorpion venom, the rate of the reverse mode of theNa⁺/Ca²⁺ exchanger was apparently enhanced,since an ~4- to 6-fold increase in [Ca²⁺]_ioccurred despite normal cell Ca²⁺ buffering. In SBFI-loadedcells, we were able to demonstrate forward operation of theNa⁺/Ca²⁺ exchanger (net efflux ofCa²⁺) by observing increases (~ 6 mM) in[Na⁺]_i. These Ni²⁺ (5 mM)-inhibited increases in [Na⁺]_i could onlybe observed when a continuous ionomycin-induced influx ofCa²⁺ occurred. The voltage-sensitive dyebis-(1,3-diethylthiobarbituric acid) trimethine oxonol was used tomeasure changes in membrane potential. Ionomycin (1 µM) depolarizedN1E-115 cells (~25 mV). This depolarization was Na⁺dependent and blocked by 5 mM Ni²⁺ and 250-500 µMbenzamil. These data provide evidence for the presence of anelectrogenic Na⁺/Ca²⁺ exchanger that is capableof regulating [Ca²⁺]_i after release ofCa²⁺ from cell stores.

     

Acidosis antagonizes intracellular calcium response to kappa -opioid receptor stimulation in the rat heart

To study the effects of -opioid receptor stimulation onintracellular Ca²⁺ concentration([Ca²⁺]_i)homeostasis during extracellular acidosis, we determined the effects of-opioid receptor stimulation on[Ca²⁺]_iresponses during extracellular acidosis in isolated single ratventricular myocytes, by a spectrofluorometric method. U-50488H (10-30 µM), a selective -opioid receptor agonist, dosedependently decreased the electrically induced[Ca²⁺]_itransient, which results from the influx ofCa²⁺ and the subsequentmobilization of Ca²⁺ from thesarcoplasmic reticulum (SR). U-50488H (30 µM) also increased theresting[Ca²⁺]_iand inhibited the[Ca²⁺]_itransient induced by caffeine, which mobilizesCa²⁺ from the SR, indicating thatthe effects of the -opioid receptor agonist involved mobilization ofCa²⁺ from its intracellular poolinto the cytoplasm. The Ca²⁺responses to 30 µM U-50488H were abolished by 5 µMnor-binaltorphimine, a selective -opioid receptorantagonist, indicating that the event was mediated by the -opioidreceptor. The effects of the agonist on[Ca²⁺]_iand the electrically induced[Ca²⁺]_itransient were significantly attenuated when the extracellular pH(pH_e) was loweredto 6.8, which itself reduced intracellular pH(pH_i) and increased[Ca²⁺]_i.The inhibitory effects of U-50488H were restored during extracellular acidosis in the presence of 10 µM ethylisopropyl amiloride, a potentNa⁺/H⁺exchange blocker, or 0.2 mM Ni²⁺,a putativeNa⁺/Ca²⁺exchange blocker. The observations indicate that acidosismay antagonize the effects of -opioid receptor stimulation viaNa⁺/H⁺andNa⁺/Ca²⁺exchanges. When glucose at 50 mM, known to activate theNa⁺/H⁺exchange, was added, both the resting[Ca²⁺]_iand pH_i increased. Interestingly,the effects of U-50488H on [Ca²⁺]_iand the electrically induced[Ca²⁺]_itransient during superfusion with glucose were significantly attenuated; this mimicked the responses during extracellular acidosis. When a high-Ca²⁺ (3 mM) solutionwas superfused, the resting[Ca²⁺]_iincreased; the increase was abolished by 0.2 mMNi²⁺, but thepH_i remained unchanged. Like theresponses to superfusion with high-concentration glucose andextracellular acidosis, the responses of the[Ca²⁺]_iand electrically induced[Ca²⁺]_itransients to 30 µM U-50488H were also significantly attenuated. Results from the present study demonstrated for the first time thatextracellular acidosis antagonizes the effects of -opioid receptorstimulation on the mobilization ofCa²⁺ from SR. Activation of bothNa⁺/H⁺andNa⁺/Ca²⁺exchanges, leading to an elevation of[Ca²⁺]_i,may be responsible for the antagonistic action of extracellular acidosis against -opioid receptor stimulation.

     

Partial inhibition of SERCA is responsible for extracellular Ca2+ dependence of AlF-4-induced [Ca2+]i oscillations in rat pancreatic

AlF₄^-is known to generate oscillations in intracellular Ca²⁺ concentration ([Ca²⁺]_i) by activating G proteins in many cell types. However, in rat pancreatic acinar cells, AlF₄^--evoked [Ca²⁺]_i oscillations were reported to be dependent on extracellular Ca²⁺, which contrasts with the [Ca²⁺]_i oscillations induced by cholecystokinin (CCK). Therefore, we investigated the mechanisms by which AlF₄^- generates extracellular Ca²⁺-dependent [Ca²⁺]_i oscillations in rat pancreatic acinar cells. AlF₄^--induced [Ca²⁺]_i oscillations were stopped rapidly by the removal of extracellular Ca²⁺ and were abolished on the addition of 20 mM caffeine and 2 µM thapsigargin, indicating that Ca²⁺ influx plays a crucial role in maintenance of the oscillations and that an inositol 1,4,5-trisphosphate-sensitive Ca²⁺ store is also required. The amount of Ca²⁺ in the intracellular Ca²⁺ store was decreased as the AlF₄^--induced [Ca²⁺]_i oscillations continued. Measurement of ⁴⁵Ca²⁺ influx into isolated microsomes revealed that AlF₄^-directly inhibited sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA). The activity of plasma membrane Ca²⁺-ATPase during AlF₄^- stimulation was not significantly different from that during CCK stimulation. After partial inhibition of SERCA with 1 nM thapsigargin, 20 pM CCK-evoked [Ca²⁺]_i oscillations were dependent on extracellular Ca²⁺. This study shows that AlF₄^- induces [Ca²⁺]_i oscillations, probably by inositol 1,4,5-trisphosphate production via G protein activation but that these oscillations are strongly dependent on extracellular Ca²⁺ as a result of the partial inhibition of SERCA. cholecystokinin; plasma membrane adenosine 5'-triphosphatase; G proteins; caffeine     

Evidence for Na+/Ca2+ exchange in intact single skeletal muscle fibers from the mouse

The myoplasmic free Ca²⁺concentration([Ca²⁺]_i)was measured in intact single fibers from mouse skeletal muscle withthe fluorescent Ca²⁺ indicatorindo 1. Some fibers were perfused in a solution in which theconcentration of Na⁺ was reducedfrom 145.4 to 0.4 mM (low-Na⁺solution) in an attempt to activate reverse-modeNa⁺/Ca²⁺exchange (Ca²⁺ entry in exchangefor Na⁺ leaving the cell). Undernormal resting conditions, application oflow-Na⁺ solution only increased[Ca²⁺]_iby 5.8 ± 1.8 nM from a mean resting[Ca²⁺]_iof 42 nM. In other fibers,[Ca²⁺]_iwas elevated by stimulating sarcoplasmic reticulum (SR)Ca²⁺ release with caffeine (10 mM)and by inhibiting SR Ca²⁺ uptakewith2,5-di(tert-butyl)-1,4-benzohydroquinone(TBQ; 0.5 µM) in an attempt to activate forward-modeNa⁺/Ca²⁺exchange (Ca²⁺ removal from thecell in exchange for Na⁺ influx).These two agents caused a large increase in[Ca²⁺]_i,which then declined to a plateau level approximately twice the baseline[Ca²⁺]_iover 20 min. If the cell was allowed to recover between exposures tocaffeine and TBQ in a solution in whichCa²⁺ had been removed, theincrease in[Ca²⁺]_iduring the second exposure was very low, suggesting thatCa²⁺ had left the cell during theinitial exposure. Application of caffeine and TBQ to a preparation inlow-Na⁺ solution produced a large,sustained increase in[Ca²⁺]_iof ~1 µM. However, when cells were exposed to caffeine and TBQ in alow-Na⁺ solution in whichCa²⁺ had been removed, a sustainedincrease in[Ca²⁺]_iwas not observed, although[Ca²⁺]_iremained higher and declined slower than in normalNa⁺ solution. This suggests thatforward-modeNa⁺/Ca²⁺exchange contributed to the fall of[Ca²⁺]_iin normal Na⁺ solution, but whenextracellular Na⁺ was low, aprolonged elevation of[Ca²⁺]_icould activate reverse-modeNa⁺/Ca²⁺exchange. The results provide evidence that skeletal muscle fibers possess aNa⁺/Ca²⁺exchange mechanism that becomes active in its forward mode when [Ca²⁺]_iis increased to levels similar to that obtained during contraction.

     

Gender-specific reduction in contractility and [Ca(2+)](i) in vascular smooth muscle cells of female rat

The hypothesisthat vascular protection in females and its absence in males reflectsgender differences in [Ca²⁺]_i andCa²⁺ mobilization mechanisms of vascular smooth musclecontraction was tested in fura 2-loaded aortic smooth muscle cellsisolated from intact and gonadectomized male and female Wistar-Kyoto(WKY) and spontaneously hypertensive (SHR) rats. In WKY cells incubated in Hanks' solution (1 mM Ca²⁺), the resting length and[Ca²⁺]_i were significantlydifferent in intact males (64.5 ± 1.2 µm and 83 ± 3 nM) than inintact females (76.5 ± 1.5 µm and 64 ± 7 nM). In intact male WKY,phenylephrine (Phe, 10⁵ M) caused transient increasein [Ca²⁺]_i to 428 ± 13 nMfollowed by maintained increase to 201 ± 8 nM and 32% cellcontraction. In intact female WKY, the Phe-induced [Ca²⁺]_i transient was notsignificantly different, but the maintained [Ca²⁺]_i (159 ± 7 nM) and cellcontraction (26%) were significantly less than in intact male WKY. InCa²⁺-free (2 mM EGTA) Hanks', Phe and caffeine (10 mM)caused transient increases in[Ca²⁺]_i and contraction that werenot significantly different between males and females. Membranedepolarization by 51 mM KCl caused 31% cell contraction and increased[Ca²⁺]_i to 259 ± 9 nM in intactmale WKY, which were significantly greater than a 24% contraction and214 ± 8 nM [Ca²⁺]_i in intactfemale WKY. Maintained Phe- and KCl-stimulated cell contraction and[Ca²⁺]_i were significantly greaterin SHR than WKY in all groups of rats. Reduction in cell contractionand [Ca²⁺]_i in intact femalescompared with intact males was significantly greater in SHR (~30%)than WKY (~20%). No significant differences in cell contraction or[Ca²⁺]_i were observed betweencastrated males, ovariectomized (OVX) females, and intact males, orbetween OVX females with 17-estradiol implants and intact females.Exogenous application of 17-estradiol (10⁸ M) tocells from OVX females caused greater reduction in Phe- and KCl-inducedcontraction and [Ca²⁺]_i in SHR thanWKY. Thus the basal, maintained Phe- and depolarization-induced [Ca²⁺]_i and contraction of vascularsmooth muscle triggered by Ca²⁺ entry from theextracellular space exhibit differences depending on gender and thepresence or absence of female gonads. Cell contraction and[Ca²⁺]_i due to Ca²⁺release from the intracellular stores are not affected by gender or gonadectomy. Gender-specific reduction in contractility and [Ca²⁺]_i in vascular smoothmuscle of female rats is greater in SHR than WKY rats.

     

Effect of Cytoplasmic Ca2+ on the Membrane Potential and Membrane Resistance of Chara Plasmalemma

Effects of cytoplasmic Ca²⁺ on the electrical properties ofthe plasma membrane were investigated in tonoplast-free cellsof Chara australis that had been internally perfused with media,containing either 1 mM ATP to fuel the electrogenic pump orhexokinase and glucose to deplete the ATP and stop the pump. In the presence of ATP, cytoplasmic Ca²⁺ up to 2.5?10^–5M did not affect the membrane potential (about -190 mV), butmembrane resistance decreased uniformly with increasing [Ca²⁺]_i.In the absence of ATP, the membrane potential, which was onlyabout -110 mV, was depolarized further by raising [Ca²⁺]_i from1.4?10^–6 to 2.5?10^–5 M. Membrane resistance, whichwas nearly the twofold that of ATP-provided cells, decreasedmarkedly with an increase in [Ca²⁺]_i from zero to 1.38?10^–6M, but showed no change for further increases. Internodal cellsof Nitellopsis obtusa were more sensitive to intracellular Ca²⁺with respect to membrane potential than were those of Charaaustralis, reconfirming the results obtained by Mimura and Tazawa(1983). The effect of cytoplasmic Ca²⁺ on the ATP-dependent H⁺ effluxwas measured. No marked difference in H⁺ effluxes was detectedbetween zero and 2.5?10^–5 M [Ca²⁺]_i; but, at 10^–4M the ATP-dependent H⁺ efflux was almost zero. Ca²⁺ efflux experimentswere done to investigate dependencies on [Ca²⁺]_i and [ATP]_i.The efflux was about 1 pmol cm^–2 s^–1 at all [Ca²⁺]_iconcentrations tested (1.38?10^–6, 2.5?10^–5, 10^–4M).This value is much higher than the influx reported by Hayamaet al. (1979), and this efflux was independent of [ATP]_i. Thepossibility of a Ca²⁺-extruding pump is discussed. ¹ Present address: Botanisches Institut der Universit?t Bonn,Venusbergweg 22, 5300 Bonn, F.R.G. (Received September 22, 1984; Accepted February 19, 1985)     

Role of cyclic ADP-ribose in the regulation of [Ca2+]i in porcine tracheal smooth muscle

The purpose ofthe present study was to determine whether cyclic ADP-ribose (cADPR)acts as a second messenger forCa²⁺ release through ryanodinereceptor (RyR) channels in tracheal smooth muscle (TSM). Freshlydissociated porcine TSM cells were permeabilized with -escin, andreal-time confocal microscopy was used to examine changes inintracellular Ca²⁺ concentration([Ca²⁺]_i).cADPR (10 nM-10 µM) induced a dose-dependent increase in [Ca²⁺]_i,which was blocked by the cADPR receptor antagonist 8-amino-cADPR (20 µM) and by the RyR blockers ruthenium red (10 µM) and ryanodine (10 µM), but not by the inositol 1,4,5-trisphosphate receptor blockerheparin (0.5 mg/ml). During steady-state[Ca²⁺]_ioscillations induced by acetylcholine (ACh), addition of 100 nM and 1 µM cADPR increased oscillation frequency and decreased peak-to-troughamplitude. ACh-induced[Ca²⁺]_ioscillations were blocked by 8-amino-cADPR; however, 8-amino-cADPR didnot block the[Ca²⁺]_iresponse to a subsequent exposure to caffeine. These results indicatethat cADPR acts as a second messenger forCa²⁺ release through RyR channelsin TSM cells and may be necessary for initiating ACh-induced[Ca²⁺]_ioscillations.

     

Palytoxin disrupts cardiac excitation-contraction coupling through interactions with P-type ion pumps

Palytoxin is a coral toxin that seriously impairs heart function, but its effects on excitation-contraction (E-C) coupling have remained elusive. Therefore, we studied the effects of palytoxin on mechanisms involved in atrial E-C coupling. In field-stimulated cat atrial myocytes, palytoxin caused elevation of diastolic intracellular Ca²⁺ concentration ([Ca²⁺]_i), a decrease in [Ca²⁺]_i transient amplitude, Ca²⁺ alternans followed by [Ca²⁺]_i waves, and failures of Ca²⁺ release. The decrease in [Ca²⁺]_i transient amplitude occurred despite high sarcoplasmic reticulum (SR) Ca²⁺ load. In voltage-clamped myocytes, palytoxin induced a current with a linear current-voltage relationship (reversal potential 5 mV) that was blocked by ouabain. Whole cell Ca²⁺ current and ryanodine receptor Ca²⁺ release channel function remained unaffected by the toxin. However, palytoxin significantly reduced Ca²⁺ pumping of isolated SR vesicles. In current-clamped myocytes stimulated at 1 Hz, palytoxin induced a depolarization of the resting membrane potential that was accompanied by delayed afterdepolarizations. No major changes of action potential configuration were observed. The results demonstrate that palytoxin interferes with the function of the sarcolemmal Na⁺-K⁺ pump and the SR Ca²⁺ pump. The suggested mode of palytoxin toxicity in the atrium involves the conversion of Na⁺-K⁺ pumps into nonselective cation channels as a primary event followed by depolarization, Na⁺ accumulation, and Ca²⁺ overload, which, in turn, causes arrhythmogenic [Ca²⁺]_i waves and delayed afterdepolarizations. atrial myocytes; intracellular calcium     

Regulation of cytoplasmic calcium levels by two nitric oxide receptors

We examined the effects of dissolved nitric oxide (NO) gas oncytoplasmic calcium levels ([Ca²⁺]_i) in C6glioma cells under anoxic conditions. The maximum elevation (27 ± 3 nM) of [Ca²⁺]_i was reached at 10 µM NO. Asecond application of NO was ineffective if the first was >0.5 µM.The NO donor diethylamine/NO mimicked the effects of NO. Acute exposureof the cells to low calcium levels was without effect on the NO-evokedresponse. Thapsigargin (TG) increased [Ca²⁺]_iand was less effective if cells were pretreated with NO. Hemoglobin inhibited the effects of NO at a molar ratio of 10:1. 8-Bromo-cGMP waswithout effect on the NO-evoked response. If cells were pretreated withTG or exposed chronically to nominal amounts of calcium, NO decreased[Ca²⁺]_i. The results suggest that C6 gliomacells have two receptors for NO. One receptor (NO_A)elevates [Ca²⁺]_i and resides on theendoplasmic reticulum (ER). The other receptor (NO_B)decreases [Ca²⁺]_i and resides on theplasmalemma or the ER. The latter receptor dominates when the level ofcalcium within intracellular stores is diminished.

     

Long-lasting muscle fatigue: partial disruption of excitation-contraction coupling by elevated cytosolic Ca2+ concentration during contractions

The repeated elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_i) above resting levels during contractile activity has been associated with long-lasting muscle fatigue. The mechanism underlying this fatigue appears to involve elevated [Ca²⁺]_i levels that induce disruption of the excitation-contraction (E-C) coupling process at the triad junction. Unclear, however, are which aspects of the activity-related [Ca²⁺]_i changes are responsible for the deleterious effects, in particular whether they depend primarily on the peak [Ca²⁺]_i reached locally at particular sites or on the temporal summation of the increased [Ca²⁺] in the cytoplasm as a whole. In this study, we used mechanically skinned fibers from rat extensor digitorum longus muscle, in which the normal E-C coupling process remains intact. The [Ca²⁺]_i was raised either by applying a set elevated [Ca²⁺] throughout the fiber or by using action potential stimulation to induce the release of sarcoplasmic reticulum Ca²⁺ by the normal E-C coupling system with or without augmentation by caffeine or buffering with BAPTA. Herein we show that elevating [Ca²⁺]_i in the physiological range of 2–20 µM irreversibly disrupts E-C coupling in a concentration-dependent manner but requires exposure for a relatively long time (1–3 min) to cause substantial uncoupling. The effectiveness of Ca²⁺ released via the endogenous system in disrupting E-C coupling indicates that the relatively high [Ca²⁺]_i attained close to the release site at the triad junction is a more important factor than the increase in bulk [Ca²⁺]_i. Our results suggest that during prolonged vigorous activity, the many repeated episodes of relatively high triadic [Ca²⁺] can disrupt E-C coupling and lead to long-lasting fatigue. skeletal muscle; low-frequency fatigue; ryanodine receptor; skinned fiber