Quabain-induced arrhythmias of single isolated myocardial cells and cell clusters cultured in vitro and their improvement by quinidine

Compensation of the AKT signaling by ERK signaling in transgenic mice hearts overexpressing TRIM72

The AKT and ERK signaling pathways are known to be involved in cell hypertrophy, proliferation, survival and differentiation. Although there is evidence for crosstalk between these two signaling pathways in cellulo, there is less evidence for cross talk in vivo. Here, we show that crosstalk between AKT and ERK signaling in the hearts of TRIM72-overexpressing transgenic mice (TRIM72-Tg) with alpha-MHC promoter regulates and maintains their heart size. TRIM72, a heart- and skeletal muscle-specific protein, downregulates AKT-mTOR signaling via IRS-1 degradation and reduces the size of rat cardiomyocytes and the size of postnatal TRIM72-Tg hearts. TRIM72 expression was upregulated by hypertrophic inducers in cardiomyocytes, while IRS-1 was downregulated by IGF-1. TRIM72 specifically regulated IGF-1-dependent AKT-mTOR signaling, resulting in a reduction of the size of cardiomyocytes. Postnatal TRIM72-Tg hearts were smaller than control-treated hearts with inhibition of AKT-mTOR signaling. However, adult TRIM72-Tg hearts were larger than of control despite the suppression of AKT-mTOR signaling. Activation of ERK, PKC-α, and JNK were observed to be elevated in adult TRIM72-Tg, and these signals were mediated by ET-1 via the ET receptors A and B. Altogether, these results suggest that AKT signaling regulates cardiac hypertrophy in physiological conditions, and ERK signaling compensates for the absence of AKT signaling during TRIM72 overexpression, leading to pathological hypertrophy.

Protective effects of coenzyme q(10) on decreased oxidative stress resistance induced by simvastatin

The effects of simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), on oxidative stress resistance and the protective effects of coenzyme Q (CoQ) were investigated. When simvastatin was administered orally to mice, the levels of oxidized and reduced CoQ₉ and CoQ₁₀ in serum, liver, and heart, decreased significantly when compared to those of control. The levels of thiobarbituric acid reactive substances induced by Fe²⁺-ascorbate in liver and heart mitochondria also increased significantly with simvastatin. Furthermore, cultured cardiac myocytes treated with simvastatin exhibited less resistance to oxidative stress, decreased time to the cessation of spontaneous beating in response to H₂O₂ addition, and decreased responsiveness to electrical field stimulation. These results suggested that oral administration of simvastatin suppresses the biosynthesis of CoQ, which shares the same biosynthesis pathway as cholesterol up to farnesyl pyrophosphate, thus compromising the physiological function of reduced CoQ, which possesses antioxidant activity. However, these undesirable effects induced by simvastatin were alleviated by coadministering CoQ₁₀ with simvastatin to mice. Simvastatin also reduced the activity of NADPH-CoQ reductase, a biological enzyme that converts oxidized CoQ to the corresponding reduced CoQ, while CoQ₁₀ administration improved it. These findings may also support the efficacy of coadministering CoQ₁₀ with statins.

Human eosinophil cationic protein enhances stress fiber formation in Balb/c 3T3 fibroblasts and differentiation of rat neonatal cardiomyocytes

We found that eosinophil cationic protein (ECP) stimulated the growth of mouse Balb/c 3T3 fibroblasts. ECP-treated 3T3 cells were more flattened and exhibited enhanced stress fiber formation. The enhancement of cytoskeleton after addition of recombinant ECP appeared stable and was able to inhibit disassembly of actin filaments that was induced by fibroblast growth factor-2. The ROCK inhibitor, Y-27632, abrogated this enhancement on stress fiber formation that was induced by ECP indicating the involvement of Rho/ROCK signaling pathway. The effect of ECP was assessed on the differentiation of primary cardiomyocytes derived from rat neonatal heart since the development of actin filaments is significantly related with organization of stress fibers. As the result, both beating rate and the expression of cardiac muscle specific markers such as atrial natriuretic factor were enhanced in the presence of ECP. Thus ECP may also function as a cardiomyocyte differentiation factor.

Different Effects of Calcium Channel Blockers on Matrix Metalloproteinase-2 Expression in Cultured Rat Cardiac Fibroblasts

The cardiac effects of calcium channel blockers (CCBs) related to cardiac remodeling are inconsistent. Matrix metalloproteinases (MMPs) contribute to tissue remodeling. Cardiac fibroblasts play an important role in the regulation of collagen degradation by MMPs. Using gelatin zymography, Western blotting, Griess reagent, and a calcium kit-fluo 3, we investigated the effects of nifedipine, verapamil, diltiazem, and amlodipine on MMP-2 expression and further elucidate the mechanisms in cultured rat cardiac fibroblasts. Nifedipine increased and amlodipine decreased the expression of MMP-2; however, neither verapamil nor diltiazem altered MMP-2 expression. Nifedipine also increased nitrite production, and this increase was blunted by a nitric oxide (NO) synthases inhibitor (L-NAME). Nifedipine-induced MMP-2 expression was also blunted by L-NAME. An NO donor (sodium nitroprusside) induced MMP-2 expression. Data indicated that nifedipine might increase MMP-2 expression through a possible NO-dependent pathway. Amlodipine had no influence on nitrite production. The amlodipine-induced decrease of MMP-2 expression was abolished by two protein tyrosine kinase inhibitors, genistein and herbimycin A, indicating that amlodipine might decrease MMP-2 expression through a possible protein tyrosine kinase pathway. None of the four CCBs could alter the fluoscence intensity of fluo 3, indicating that the effects of CCBs on MMP-2 expression were independent of the variation in intracellular C2+ concentration. Our findings revealed that different CCBs exerted different effects on MMP-2 expression in cardiac fibroblasts.

Cytokine-induced nitric oxide production inhibits mitochondrial energy production and impairs contractile function in rat cardiac myocytes

OBJECTIVES

The present study examined whether nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) can directly inhibit aerobic energy metabolism and impair cell function in interleukin (IL)-1beta,-stimulated cardiac myocytes.

BACKGROUND

Recent reports have indicated that excessive production of NO induced by cytokines can disrupt cellular energy balance through the inhibition of mitochondrial respiration in a variety of cells. However, it is still largely uncertain whether the NO-induced energy depletion affects myocardial contractility.

METHODS

Primary cultures of rat neonatal cardiac myocytes were prepared, and NO2-/NO3- (NOx) in the culture media was measured using Griess reagent.

RESULTS

Treatment with IL-1beta (10 ng/ml) increased myocyte production of NOx in a time-dependent manner. The myocytes showed a concomitant significant increase in glucose consumption, a marked increase in lactate production, and a significant decrease in cellular ATP (adenosine 5'-triphosphate). These metabolic changes were blocked by co-incubation with N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis. Sodium nitroprusside (SNP), a NO donor, induced similar metabolic changes in a dose-dependent manner, but 8-bromo-cyclic guanosine 3',5'-monophosphate (8-bromo-cGMP), a cGMP donor, had no effect on these parameters. The activities of the mitochondrial iron-sulfur enzymes, NADH-CoQreductase and succinate-CoQreductase, but not oligomycin-sensitive ATPase, were significantly inhibited in the IL-1beta, or SNP-treated myocytes. Both IL-1beta and SNP significantly elevated maximum diastolic potential, reduced peak calcium current (I(Ca)), and lowered contractility in the myocytes. KT5823, an inhibitor of cGMP-dependent protein kinase, did not block the electrophysiological and contractility effects.

CONCLUSIONS

These data suggest that IL-1beta-induced NO production in cardiac myocytes lowers energy production and myocardial contractility through a direct attack on the mitochondria, rather than through cGMP-mediated pathways.

Expression of kininogen genes by rat cardiomyocytes

To determine the existence of the kallikrein-kinin system in the heart, we have studied in vitro and in vivo whether rat heart expresses kininogens (KGNs). The reverse transcription-polymerase chain reaction (RT-PCR) for KGN mRNAs demonstrated that the cardiac tissue of adult male rats expresses T-KGN mRNA but not high-molecular-weight (H-) KGN mRNA. An intravenous injection of lipopolysaccharide (LPS) resulted in a significant increase in T-KGN mRNA levels of rat heart within 12 h. Polyacrylamide gel electrophoresis of cDNA products generated by RT-PCR from heart mRNA using primers specific for either T- or low-molecular-weigh (L-) KGN revealed that rat heart expressed not only T-KGN gene but also L-KGN gene, and that LPS injection exclusively stimulated the expression of T-KGN but not of L-KGN gene. T-KGN mRNA was also detected in cultured myocytes derived from fetal rat heart, and the expression was markedly enhanced by an addition of LPS to cultures. These results demonstrated that rat cardiomyocytes are the source of T- and L-KGNs but not of H-KGN, and that their expression of T-KGN mRNA is stimulated by LPS, probably via LPS-receptor CD14.

Different role of antioxidants in endotoxin-induced late myocardial protection

Previous studies have proposed that endogenous antioxidants play a protective role against cardiac ischemia-reperfusion injury in endotoxin pretreatment. However, the mechanism underlying this effect remains elusive. We therefore evaluated the role of endogenous antioxidants in delayed myocardial protection after different doses of endotoxin administration using cultured rat neonatal cardiomyocytes. Myocytes were treated with normal saline (control) or lipopolysaccharide (Escherichia coli, serotype O111) at doses of 40 and 80 microg/ml (ET40 and ET80). Also, antisense oligodeoxyribonucleotide (1.5 micromol/L) to manganese superoxide dismutase (Mn-SOD) and 3-amino-1,2,4-triazole (25 mg/ml) were added along with a 40 or 80 microg/ml endotoxin pretreatment in the IET40 and IET80 groups. Twenty-four hours later, Cells were subjected to hypoxia (pO2 < 1 kPa, 3 h) and reoxygenation (pO2: 19 kPa, 1 h). Compared with controls, cell viability enhanced significantly (65.3 +/- 5.9, 63.8 +/- 4.6, and 69.7 +/- 5.2% vs 47.2 +/- 4.3%, P < 0.05) and creatine kinase release decreased (7.34 +/- 1.76, 7.11 +/- 1.49, and 6.27 +/- 1.24 U/mg protein vs 11.23 +/- 2.49 U/mg protein, P < 0. 05) in ET40, IET40, and ET80 groups following reoxygenation. No statistically significant difference was found between the control and the IET80 groups. Furthermore, the levels of Mn-SOD (1.12 +/- 0. 31 vs 0.75 +/- 0.15 U/mg. protein, P < 0.05) and catalase activity (1265 +/- 109 vs 996 +/- 85 U/mg. protein, P < 0.05) were higher only in the ET80 group. The results suggest that at a dose of 40 microg/ml, cells were protected by mechanisms other than the augmentation of endogenous antioxidant activity which were more evident at a dose of 80 microg/ml. It seems that different doses of endotoxin pretreatment may induce delayed myocardial protection through various mechanisms.

Cardioprotective effect of angiotensin-converting enzyme inhibition against hypoxia/reoxygenation injury in cultured rat cardiac myocytes

BACKGROUND

Although ACE inhibitors can protect myocardium against ischemia/reperfusion injury, the mechanisms of this effect have not yet been characterized at the cellular level. The present study was designed to examine whether an ACE inhibitor, cilazaprilat, directly protects cardiac myocytes against hypoxia/reoxygenation (H/R) injury.

METHODS AND RESULTS

Neonatal rat cardiac myocytes in primary culture were exposed to hypoxia for 5.5 hours and subsequently reoxygenated for 1 hour. Myocyte injury was determined by the release of creatine kinase (CK). Both cilazaprilat and bradykinin significantly inhibited CK release after H/R in a dose-dependent fashion and preserved myocyte ATP content during H/R, whereas CV-11974, an angiotensin II receptor antagonist, and angiotensin II did not. The protective effect of cilazaprilat was significantly inhibited by Hoe 140 (a bradykinin B2 receptor antagonist), NG-monomethyl-L-arginine monoacetate (L-NMMA) (an NO synthase inhibitor), and methylene blue (a soluble guanylate cyclase inhibitor) but not by staurosporine (a protein kinase C inhibitor), aminoguanidine (an inhibitor of inducible NO synthase), or indomethacin (a cyclooxygenase inhibitor). Cilazaprilat significantly enhanced bradykinin production in the culture media of myocytes after 5.5 hours of hypoxia but not in that of nonmyocytes. In addition, cilazaprilat markedly enhanced the cGMP content in myocytes during hypoxia, and this augmentation in cGMP could be blunted by L-NMMA and methylene blue but not by aminoguanidine.

CONCLUSIONS

The present study demonstrates that cilazaprilat can directly protect myocytes against H/R injury, primarily as a result of an accumulation of bradykinin and the attendant production of NO induced by constitutive NO synthase in hypoxic myocytes in an autocrine/paracrine fashion. NO modulates guanylate cyclase and cGMP synthesis in myocytes, which may contribute to the preservation of energy metabolism and cardioprotection against H/R injury.

Metallothionein-overexpressing neonatal mouse cardiomyocytes are resistant to H2O2 toxicity

To study cellular and molecular events of cardiac protection by metallothionein (MT) from oxidative injury, a primary neonatal cardiomyocyte culture was established from a specific cardiac MT-overexpressing transgenic mouse model. Ventricular cardiomyocytes were isolated from 1- to 3-day-old neonatal mice and cultured in an Eagle's minimum essential medium supplemented with 20% fetal bovine serum under an atmosphere of 5% CO2-95% air at 37 degreesC. Forty-eight hours after plating was completed, the purity of such cultures was 95% myocytes, assessed by an immunocytochemical assay. Over 80% of the cardiomyocytes beat spontaneously on the first day of culture and synchronously in a confluent monolayer after the sixth day of culture. Cellular MT concentrations in the transgenic cardiomyocytes before culturing and on the sixth day postculturing were about seven- and twofold higher than nontransgenic controls, respectively. However, there were no significant differences in cell morphology, glutathione content, and antioxidant enzymatic activities between these two types of cardiomyocytes. When these cells were challenged by H2O2, the transgenic cardiomyocytes displayed a significant resistance to the toxic effect of this oxidant, as measured by cell viability, lactate dehydrogenase leakage, and morphological alterations. In addition, the transgenic cells were highly protected from H2O2-induced lipid peroxidation. These observations demonstrate that MT protects the cultured cardiomyocytes from H2O2 toxicity by preventing its interaction with macromolecules such as lipids, and this cultured primary neonatal mouse cardiomyocyte system provides a valuable tool to directly study cellular and molecular events of MT in cardiac protection against oxidative injury.

Inducible nitric oxide synthase augments injury elicited by oxidative stress in rat cardiac myocytes

The effects of nitric oxide (NO) produced by cardiac inducible NO synthase (iNOS) on myocardial injury after oxidative stress were examined: Interleukin-1 beta induced cultured rat neonatal cardiac myocytes to express iNOS. After induction of iNOS, L-arginine enhanced NO production in a concentration-dependent manner. Glutathione peroxidase (GPX) activity in myocytes was attenuated by elevated iNOS activity and by an NO donor, S-nitroso-N-acetyl-penicillamine (SNAP). Although NO production by iNOS did not induce myocardial injury, NO augmented release of lactate dehydrogenase from myocyte cultures after addition of H2O2 (0.1 mM, 1 h). Inhibition of iNOS with N omega-nitro-L-arginine methyl ester ameliorated the effects of NO-enhancing treatments on myocardial injury and GPX activity. SNAP augmented the myocardial injury induced by H2O2. Inhibition of GPX activity with antisense oligodeoxyribonucleotide for GPX mRNA increased myocardial injury by H2O2. Results suggest that the induction of cardiac iNOS promotes myocardial injury due to oxidative stress via inactivation of the intrinsic antioxidant enzyme, GPX.

Transplantation of genetically marked cardiac muscle cells

We examined the possibility that cardiomyocytes could be genetically marked or modified before being grafted to the heart under conditions applicable to the clinical setting. We used a replication-defective recombinant adenovirus carrying the beta-galactosidase reporter gene, and delivered it to cultured murine fetal cardiac myocytes. Virtually all fetal cardiomyocytes in a primary culture expressed beta-galactosidase 24 hours after recombinant adenovirus infection. These cells were transplanted to the hearts of syngenic adult recipient mice. Expression of the beta-galactosidase gene in the grafted cells was demonstrated by staining with 5-bromo-4-chloro-3-indoyl-beta-D-galactosidase, resulting in a blue color at the histochemical level and an electron-dense deposit on transmission electron microscopic analysis. Gene expression was recognized from 7 days to 12 weeks after transplantation. Implanted cardiomyocytes aligned themselves along the layers of the host myocardium. Formation of gap junctions was demonstrated by transmission electron microscopy. Neither inflammation nor fibrous scar tissue was detectable by histologic analysis. This study demonstrates that ex vivo gene transfer to the heart by means of the adenoviral vector is possible.

Induction of manganese superoxide dismutase in rat cardiac myocytes increases tolerance to hypoxia 24 hours after preconditioning

Manganese superoxide dismutase (Mn-SOD) is induced in ischemic hearts 24 h after ischemic preconditioning, when tolerance to ischemia is acquired. We examined the relationship between Mn-SOD induction and the protective effect of preconditioning using cultured rat cardiac myocytes. Exposure of cardiac myocytes to brief hypoxia (1 h) decreased creatine kinase release induced by sustained hypoxia (3 h) that follows when the sustained hypoxia was applied 24 h after hypoxic preconditioning (57% of that in cells without preconditioning). The activity and content of Mn-SOD in cardiac myocytes were increased 24 h after hypoxic preconditioning (activity, 170%; content, 139% compared with cells without preconditioning) coincidentally with the acquisition of tolerance to hypoxia. Mn-SOD mRNA was also increased 20-40 min after preconditioning. Antisense oligodeoxyribonucleotides corresponding to the initiation site of Mn-SOD translation inhibited the increases in the Mn-SOD content and activity and abolished the expected decrease in creatine kinase release induced by sustained hypoxia after 24 h of hypoxic preconditioning. Sense oligodeoxyribonucleotides did not abolish either Mn-SOD induction or tolerance to hypoxia. These results suggest that the induction of Mn-SOD in myocytes by preconditioning plays a pivotal role in the acquisition of tolerance to ischemia at a later phase (24 h) of ischemic preconditioning.

Polymorphonuclear leukocytes-induced injury in hypoxic cardiac myocytes

Growing evidence suggests that free radicals derived from polymorphonuclear leukocytes (PMNs) play an important role in myocardial ischemia-reperfusion injury. To elucidate the cellular mechanism by which activated PMNs exacerbate ischemic myocardial damage, we investigated the extent of cell injury, assessed by the morphological deterioration, free radical generation, and lipid peroxidation in mouse embryo myocardial cells coincubated with activated PMNs. The generation of PMN-derived free radicals was related to the extent of myocardial cell injury. When myocardial cell sheets were subjected to hypoxia and glucose-free media, myocardial cells were injured (cristalysis in the mitochondria and disruption of the sarcolemma) after adding various PMN activators, and the injury extended to the adjacent cells. Chemiluminescent emission and production of thiobarbituric acid-reactive substances in the coincubated cells increased markedly compared with myocardial cells or PMNs alone. The augmented lipid peroxidation coincided with the progression of myocardial cell injury. Catalase inhibited the myocardial cell injury by 52%, the chemiluminescence by 46%, and lipid peroxidation by 50%, whereas superoxide dismutase exhibited less pronounced inhibition. These results indicate that a chain reaction of lipid peroxidation in myocardial cells induced by PMN-derived free radicals closely correlates with membrane damage and contributes to the propagation of irreversible myocardial cell damage.

Contractile and morphological impairment of cultured fetal mouse myocytes induced by oxygen radicals and oxidants. Correlation with intracellular Ca2+ concentration

There is evidence that reperfusion injury of cardiac tissue may be caused by the generation of oxygen-derived free radicals and oxidants and by the induction of intracellular calcium overload, although the relation between these two mechanisms of injury is uncertain. In addition, the relation between the types of cellular injury and specific active species is unclear. In an attempt to resolve these problems, we investigated the effects of oxygen radicals and oxidants, which are purportedly generated during reperfusion after prolonged ischemia, and various antioxidants on contractility and morphology of cultured fetal mouse cardiac myocytes. Xanthine oxidase in the presence of xanthine, H2O2, HOCl, and NH2Cl induced cessation of spontaneous beating followed by cessation of electrical stimulation-elicited beating but did not induce an increase in [Ca2+]i. After prolonged incubation with xanthine oxidase + xanthine and H2O2, the cardiac myocytes showed morphological degeneration (at least 80% of the cells developed hypercontraction) with a concomitant increase in [Ca2+]i. These observations suggest that contractile impairment does not result in an increase of [Ca2+]i, but hypercontraction does. Catalase, but not superoxide dismutase, protected the cultured cardiac myocytes against xanthine oxidase + xanthine- and H2O2-induced contractile and morphological impairment. In the light of this observation, we hypothesize that the superoxide anion is not responsible for these types of impairment. Addition of dimethylthiourea (an .OH scavenger) and intracellular preloading with deferoxamine (an iron chelator) protected the myocytes against H2O2-induced contractile and morphological damage, but intracellular preloading with iron enhanced it. These observations led us to hypothesize that intracellularly generated .OH may be a mediator of H2O2-induced injury to cultured cardiac myocytes. In addition, we observed that H2O2 itself induced cessation of spontaneous but not electrical stimulation-elicited beating.

Properties of oscillatory afterpotentials in young embryonic chick hearts

Oscillatory afterpotentials in cardiac tissues are believed to result from an activation of calcium-operated cation channels of mixed selectivity (sodium, potassium), or perturbation of an electrogenic sodium-calcium exchange carrier, by an oscillatory release of calcium from sarcoplasmic reticulum. In the present experiments, the presence and properties of ouabain-induced oscillatory afterpotentials were examined in young (3-day-old) embryonic chick hearts, both fresh and organ cultured for 6-14 days. The hearts did not differentiate in organ culture, and the cells retained slowly rising spontaneous action potentials. To induce the oscillatory afterpotentials, automaticity was suppressed by elevating extracellular K+ ion concentration from 4 mM to 6 mM, and the preparations were electrically stimulated at a rate of 0.5 Hz. Stable oscillatory afterpotentials were induced with 1.3-7.5 microM ouabain. The oscillatory afterpotential amplitude was increased when the stimulation rate was increased to 3 and 4 Hz. The oscillatory afterpotentials were potentiated when extracellular Ca++ ion concentration was increased to 3.6 mM or by the addition of barium (0.1 mM). Low extracellular Na+ ion concentration (40-121 mM), strontium (4 mM), magnesium (2-4 mM), and manganese (1-4 mM) significantly depressed the oscillatory afterpotentials. These properties of the oscillatory afterpotentials are similar to those described for adult mammalian ventricular muscles and Purkinje fibers. Our results suggest that young embryonic chick hearts (which lack fast sodium channels and, instead, have slow sodium channels) possess the calcium-operated mixed cation channels at a very early developmental stage if the oscillatory afterpotentials result from the activation of the mixed cation channels by intracellular calcium.

Comparison of kinetic characteristics of Na+-Ca2+ exchange in sarcolemma vesicles and cultured cells from chick heart

The kinetic characteristics of Na+-Ca2+ exchange in isolated sarcolemma vesicles from new-borne chick heart, which contain about 70% of right-side-out vesicles, were compared with those of cultured embryonic chick heart cells. Na+-Ca2+ exchange was monitored as Nai-dependent Ca2+ uptake. Increase in the internal concentration of Na+ ([Na+]i) in these two preparations caused increase in both the initial rate and the saturation-level of Ca2+ uptake. Plots of the rate of Ca2+ uptake against [Na+]i showed similar saturation-kinetics in these two preparations. The apparent Michaelis constant (Km) (0.35 mM) for Ca2+ uptake by the intact cells was much higher than that (0.031 mM) for Ca2+ uptake by the vesicles. The degree of inhibition by Mg2+ was also higher in the cells than in the vesicles. Some possible reasons (age of the chicks used, membrane potential, etc.), for these differences were examined and are discussed.

Kinetic studies on sodium-dependent calcium uptake by myocardial cells and neuroblastoma cells in culture

Kinetic analyses were made on intracellular Na+-dependent Ca2+ uptake by myocardial cells and neuroblastoma cells (N-18 strain) in culture. Cells loaded with various concentrations of Na+ could be prepared by incubating them in Ca2+-free medium containing various concentrations of Na+. Cells pre-loaded with various concentrations of Na+ were incubated in medium containing Ca2+ and 45Ca. The resulting 45Ca uptake by the two types of cell depended greatly on the initial intracellular concentrations of Na+. Lineweaver-Burk plots of the initial rate of Ca2+ uptake against the external concentration of Ca2+ fitted well to straight lines obtained by linear regression (r greater than 0.95). This result shows that Ca2+ uptake by the two types of cell was achieved by a carrier-mediated transport system. This Na+-dependent Ca2+ uptake was accompanied by Na+ release and the ratio of Na+ release to Ca2+ uptake was close to 3 : 1. A comparison of the kinetic data between myocardial cells and N-18 cells suggested that N-18 cells possess a carrier showing the same properties as that of myocardial cells, i.e.: (1) a similar dependency on the intracellular concentration of Na+; (2) the coincidence of the apparent Michaelis constants for Ca2+ (0.1 mM); (3) the similarities of the Ki values for Co2+, Sr2+ and Mg2+ (Co2+ less than Sr2+ less than Mg2+) and (4) a similar dependency on pH. However, the maximal initial rate, V, of N-18 cells was about 1/100 that of myocardial cells. The rate of Na+-dependent Ca2+ uptake by non-excitable cells was much lower than that by myocardial cells.

Prostaglandins have limited actions on abnormalities of beating induced in cultured heart cells

Prostaglandins are antiarrhythmic in a variety of situations including ischaemic arrhythmias, but the mechanisms involved are not known. In view of this, the protective actions of prostaglandins A2, E2, F1 alpha, F2 beta, and I2 against abnormalities of beating induced in cultured heart cells were investigated. Abnormalities of beating were induced in single cells by variety of agents including ouabain Ca++, K+, dinitrophenol (DNP), and toxic material from the jellyfish Cyanea. Abnormalities were assessed in terms of rate, rate range, subjective arrhythmic behaviour and percent cells beating. The prostaglandins (at 10(-7)-10(-5) M) were added with the arrhythmogenic agent to test for their ability to modify agent-induced beating abnormalities and were compared with lidocaine and quinidine. Prostaglandins alone had minimal direct effects on the cells and only minimally reduced responses to arrhythmogenic agents. The most protective prostaglandins, PGE2 and PGF1 alpha, tended to normalise beating behaviour most noticeably in DNP-treated cells, unlike lidocaine and quinidine which were effective against Ca++-induced changes while worsening those of K+. Thus, a general ability to protect disturbed cardiac cells is not seen with high concentrations of prostaglandins.