Isolation of calcium-tolerant atrial and ventricular myocytes from adult rat heart

Collagen Production in Cardiac Fibroblasts During Inhibition of Aminopeptidase B

Objective. To determine whether the aminopeptidase B inhibitor, arphamenine A, could affect collagen production and expression in control and TGF-ß1-treated cardiac fibroblasts.

Design and Methods. Cardiac fibroblasts from passage 2 from normal male adult rats were cultured to confluency and incubated with and without 600 pmol/l TGF-ß1 for 2 days in serum-free Dulbecco's modified Eagle's medium and then incubated with 100 µmol/l arphamenine A for 1 day in this medium added ascorbic acid, ß-aminopropionitrile and titriated proline. Soluble collagen was measured in the conditioned medium and non-soluble collagen in the cell layer. Aminopeptidase activity was estimated by spectrophotometric determination of the liberation of p-nitroaniline from alanine- or arginine-p-nitroanilide. Matrix metalloproteinase (MMP) and lysyl oxidase activity were assayed in the conditioned medium. A semi-quantitative reverse transcriptase- polymerase chain reaction was used to examine the expression of lysyl oxidase and collagen type I and III. Results. Arphamenine A dose-dependently

inhibited basal and TGF-ß 1-stimulated aminopeptidase activity. Arphamenine A reduced soluble and non-soluble collagen production in control and TGF-ß1-treated cardiac fibroblasts, while it decreased collagen type I and III expression only in TGF-ß1-treated fibroblasts. Lysyl oxidase, MMP-1 and MMP-2 activity were inhibited by arphamenine A in the conditioned media of control and TGF-ß1treated cardiac fibroblasts.

Conclusions. Our data show that the specific aminopeptidase B inhibitor, arphamenine A, reduces collagen production in cardiac fibroblasts and that this reduction is accompanied by a pronounced inhibition of lysyl oxidase.

Activation of Notch signaling in cardiomyocytes during post-infarction remodeling

OBJECTIVE

Notch signaling is crucial for cell-to-cell interaction during cardiovascular development and may influence differentiation, proliferation, and apoptotic events. We investigated whether Notch signaling is activated during myocardial remodeling in heart failure (HF).

DESIGN

Myocardial gene expression and localization of Notch receptors (Notch1-4) and ligands (Jagged1-2, and Delta-like (Dll)-1 and 4) were investigated in rats with HF after induction of myocardial infarction and in humans with HF.

RESULTS

All Notch receptors and ligands investigated and Notch intracellular domain (NICD) were present in rat and human myocardial tissue and in cardiomyocytes with differences in their relative expression levels and altered expression levels in failing vs. non-failing myocardium. In isolated rat cardiomyocytes, Notch3 and Notch4 appeared to be the major Notch receptors, and Notch3 and Notch4 mRNA levels and NICD-3 and -4 in cardiomyocytes were upregulated in chronic HF (p < 0.05), indicating increased Notch3 and Notch4 signaling.

CONCLUSION

The Notch signaling system is present in the cardiomyocytes and activated in HF, indicating a role of Notch signaling during myocardial remodeling in HF.

α1-Agonists-induced Mg2+ efflux is related to MAP kinase activation in the heart

The stimulation of the alpha(1)-adrenergic receptor with phenylephrine results in the significant extrusion of Mg(2+) from the rat heart and cardiomyocytes. Phenylephrine-induced Mg(2+) extrusion is prevented by the removal of extracellular Ca(2+) or by the presence of Ca(2+)-channel blockers such as verapamil, nifedipine, or (+)BAY-K8644. Mg(2+) extrusion is almost completely inhibited by PD98059 (a MAP kinase inhibitor). The simultaneous addition of 5mM Ca(2+) and phenylephrine increases the extrusion of Mg(2+) from perfused hearts and cardiomyocytes. This Mg(2+) extrusion is inhibited by more than 90% when the hearts are preincubated with PD98059. ERKs are activated by perfusion with either phenylephrine or 5mM Ca(2+). This ERK activation is inhibited by PD98059. Overall, these results suggest that stimulating the cardiac alpha(1)-adrenergic receptor by phenylephrine causes the extrusion of Mg(2+) via the Ca(2+)-activated, Na(+)-dependent transport pathway, and the ERKs assists in Mg(2+) transport in the heart.

Effect of bestatin on angiotensin I-, II- and III-induced collagen gel contraction in cardiac fibroblasts

OBJECTIVE

The purpose of this investigation was to determine whether the aminopeptidase inhibitor with broad specificity, bestatin, affects angiotensin I (Ang I)-, angiotensin II (Ang II)- or angiotensin III (Ang III)-stimulated collagen gel contraction in cardiac fibroblasts.

DESIGN AND METHODS

Cardiac fibroblasts (from normal male adult rats) were cultured to confluency in Dulbeccos modified Eagles medium (DMEM) with 10% foetal bovine serum (FBS). These fibroblasts (100,000 cells) were then further incubated in a floating collagen gel lattice with the test products Ang I (1 micromol/L), Ang II (100 nmol/L), Ang III (100 nmol/L) and bestatin (100 micromol/L) for three days in DMEM without FBS. The area of the collagen gels embedded with cardiac fibroblasts was determined by a densitometric analysis. Aminopeptidase activity was estimated by spectrophotometric determination of the liberation of p-nitroaniline from alanine- or arginine-p-nitroanilide.

RESULTS

Ang I, II and III stimulated (p<0.05) collagen gel contraction by 30.4+/-4.8 (SEM)%, 27.1+/-3.1% and 15.4+/-3.6% respectively. Ang I- and II-induced stimulation of collagen gel contraction was of the same order but more pronounced (p<0.05) than Ang III- stimulated collagen gel contraction. The Ang I-, II- and III-stimulated collagen contraction was reduced by bestatin. Bestatin, however, did not affect basal collagen gel contraction in cardiac fibroblasts. Bestatin dose-dependently inhibited the hydrolysis of arginine- and alanine-p-nitroanilide in cardiac fibroblasts. When a neutralising antibody to transforming growth factor TGF-b1 was added to the collagen gel simultaneously with the angiotensins, the stimulated collagen contraction was not affected. Beta-aminoproprionitrile, an inhibitor of lysyl oxidase, completely abolished basal as well as Ang I-, II- and III-stimulated collagen contraction in cardiac fibroblasts.

RESULTS

Our data suggest that aminopeptidases are involved in the Ang I-, II- and III-induced stimulation of collagen contraction in cardiac fibroblasts.

Streptozotocin-induced diabetes impairs Mg2+ homeostasis and uptake in rat liver cells

Male Sprague-Dawley rats rendered diabetic by streptozotocin injection presented 10 and 20% decreases in total hepatic Mg2+ content at 4 and 8 wk, respectively, following diabetes onset. This decrease was associated with a parallel decrease in K+ and ATP content and an increase in Na+ level. In diabetic liver cells, the Mg2+ extrusion elicited by alpha1-adrenoceptor stimulation was markedly reduced compared with nondiabetic livers, whereas that induced by beta-adrenoceptor stimulation was unaffected. In addition, diabetic hepatocytes did not accumulate Mg2+ following stimulation of protein kinase C pathway by vasopressin, diacylglycerol analogs, or phorbol 12-myristate 13-acetate derivates despite the reduced basal content in cellular Mg2+. Experiments performed in purified plasma membrane from diabetic livers located the defect at the level of the bidirectional Na+/Mg2+ exchanger operating in the basolateral domain of the hepatocyte cell membrane, which could extrude but not accumulate Mg2+ in exchange for Na+. The impairment of Mg2+ uptake mechanism, in addition to the decrease in cellular ATP level, can contribute to explaining the decrease in liver Mg2+ content observed under diabetic conditions.

Transforming growth factor-beta 1-mediated collagen gel contraction by cardiac fibroblasts

OBJECTIVE

Myofibroblasts and transforming growth factor-beta(1) (TGF-beta(1)) are key elements of cardiac tissue fibrosis development. The aim of this study was to determine whether the ability of TGF-beta(1) to affect the contractile activity of cardiac fibroblasts depends on their differentiation into myofibroblasts.

METHODS

Cardiac fibroblasts (from male adult Wistar rats) from passage two were cultured to confluency and incubated on a hydrated collagen gel with and without TGF-beta(1) (0, 20, 40, 100, 200, 400 or 600 pmol/L) for one, two and three days in a Dulbecco's Modified Eagle's Medium without foetal bovine serum.

RESULTS

TGF-beta(1) dose-dependently increased the contraction of collagen gel mediated by cardiac fibroblasts, reaching a maximal effect at 100 pmol/L TGF-beta(1). TGF-beta(1) also stimulated 3(H)-thymidine incorporation and total protein content in cardiac fibroblasts in the collagen gel lattice. TGF-beta(1) dose-dependently induced an increase in beta-smooth muscle actin, a marker of myofibroblasts. The TGF-beta(1)-induced reduction of area of the collagen gel was negatively correlated to the TGF-beta(1)-evoked appearance of a-smooth muscle actin in the collagen gel matrix.

CONCLUSION

Our data demonstrate that TGF-beta(1) increased the contractile activity of adult rat cardiac fibroblasts and their ability to differentiate into myofibroblasts. Because contractile activity was correlated with differentiation, the influence of TGF-beta(1) on cardiac fibroblast-induced collagen gel contraction might depend on the promotion of myofibroblast differentiation.

Electrical coupling of single cardiac rat myocytes to field-effect and bipolar transistors

A novel bipolar transistor for extracellular recording the electrical activity of biological cells is presented, and the electrical behavior compared with the field-effect transistor (FET). Electrical coupling is examined between single cells separated from the heart of adults rats (cardiac myocytes) and both types of transistors. To initiate a local extracellular voltage, the cells are periodically stimulated by a patch pipette in voltage clamp and current clamp mode. The local extracellular voltage is measured by the planar integrated electronic sensors: the bipolar and the FET. The small signal transistor currents correspond to the local extracellular voltage. The two types of sensor transistors used here were developed and manufactured in the laboratory of our institute. The manufacturing process and the interfaces between myocytes and transistors are described. The recordings are interpreted by way of simulation based on the point-contact model and the single cardiac myocyte model.

Stimulation of collagen gel contraction by angiotensin II and III in cardiac fibroblasts

OBJECTIVE

The aim of the present study was to investigate whether angiotensin II (Ang II), angiotensin III (Ang III) or Ang II (2-8), angiotensin IV (Ang IV) or Ang II (3-8) and Ang II (1-7), Ang II (4-8), Ang II (5-8) and Ang II (1-4) can stimulate collagen gel contraction in cardiac fibroblasts in serum-free conditions.

METHODS

Cardiac fibroblasts (from male adult Wistar rats) from passage 2 were cultured to confluency and added to a hydrated collagen gel in a Dulbecco's Modified Eagle's Medium, with or without foetal bovine serum, for one, two or three days. The area of the collagen gels embedded with cardiac fibroblasts was determined by a densitometric analysis. Collagen gel contraction was characterised by a decrease in the gel area.

RESULTS

Ang II dose-dependently stimulated the contraction of collagen mediated by cardiac fibroblasts after one, two or three days of incubation in a serum-free medium. Telmisartan completely blocked the Ang II-induced collagen contraction by cardiac fibroblasts. PD 123319 and des-Asp(1)-Ile(8)-Ang II had no effect on the Ang II-induced collagen contraction by cardiac fibroblasts. Ang III also stimulated the contraction of collagen mediated by cardiac fibroblasts after one, two or three days of incubation in a serum-free medium. des-Asp(1)-Ile(8)-Ang II and telmisartan completely blocked the Ang III-induced collagen gel contraction by cardiac fibroblasts. des-Asp(1)-Ile(8)-Ang II, however, had no effect on the Ang II-induced collagen gel contraction by cardiac fibroblasts. Ang IV and Ang II (4-8), (5-8), (1-7) and (1-4), however, had no effect on collagen gel contraction by cardiac fibroblasts. Addition of telmisartan, PD 123319 or des-Asp(1)-Ile(8)-Ang II alone did not affect collagen gel contraction by cardiac fibroblasts.

CONCLUSION

Our data demonstrate that the effects of Ang II on the collagen gel contraction by adult rat cardiac fibroblasts in serum-free conditions are Ang II type 1(AT(1))-receptor- mediated, because they are abolished by the specific AT(1)-receptor antagonist, telmisartan, and not by the AT(2)-receptor antagonist PD 123319 or by the Ang III antagonist des-Asp(1)-Ile(8)-angiotensin. The Ang III- stimulated contraction of collagen by cardiac fibroblasts is completely blocked by the Ang III receptor antagonist, des-Asp(1)-Ile(8)-angiotensin II, and by telmisartan.

Myocardial distribution and regulation of GRK and beta-arrestin isoforms in congestive heart failure in rats

Myocardial G protein-coupled receptor kinase 2 (GRK2) has been shown to be involved in the pathophysiology of congestive heart failure (CHF). However, the cellular distribution of this isoform, as well as the other isoforms of the GRK-arrestin system, has not been studied in myocardial tissue. Thus myocardial expression and cellular distribution of the different GRK and arrestin isoforms were investigated in a rat model of CHF. Rats subjected to ligation of the left coronary artery or sham operation were euthanized 2, 7, or 42 days after the surgical procedure. Myocardial GRK2, GRK5, beta-arrestin-1, and beta-arrestin-2 mRNA levels, but not that of GRK3, were induced in the failing hearts. Consistently, Western blot analysis of tissue extracts from the nonischemic region of the left ventricle revealed 3.0-, 2.6-, and 1.5-fold elevations of GRK2, GRK5, and beta-arrestin-1, respectively, 7 days after induction of myocardial infarction compared with the sham-operated rats (P < 0.05). Immunohistochemical analysis of myocardial tissue sections and Western blot analysis of isolated cells revealed localization of GRK2 and beta-arrestin-1 predominantly in endothelial cells. Conversely, GRK3 was confined to cardiac myocytes. GRK5 immunostaining appeared to be homogeneously distributed in the cellular elements of the myocardium. In conclusion, myocardial mRNA and protein levels of GRK2, GRK5, and beta-arrestin-1 are induced in postinfarction failure in rats. The immunohistochemical analysis suggests that GRK2 and beta-arrestin-1 may act as primary regulators of endothelial function. Conversely, the cellular distribution of GRK3 and GRK5 implicates these isoforms as putative regulators of cardiac myocyte function.

Angiotensin II-induced stimulation of collagen secretion and production in cardiac fibroblasts is mediated via angiotensin II subtype 1 receptors

The possible contributions of the angiotensin receptor subtypes 1 (AT1) and 2 (AT2) to angiotensin II (Ang II)-induced changes in collagen secretion and production were studied using the specific angiotensin AT1- and AT2-receptor antagonists telmisartan and P-186, respectively. Cardiac fibroblasts (from normal male adult rats) from passage 2 were cultured to confluency and incubated in the presence of 10(-10) to 10(-6) M Ang II in serum-free Dulbecco's MEM medium for 24 hours. Collagen production and secretion were assayed by'H-Proline incorporation; non-collagen production and secretion were also calculated. Ang II dose-dependently increased collagen secretion and production in rat adult cardiac fibroblasts in culture. Non-collagen secretion and production were also concentration-dependently increased by Ang II. Addition of 100 nmol/l Ang II increased (p<0.01) collagen secretion and production bv 75+/-6 (SEM)% and 113+/-23%, respectively, and non-collagen secretion and production by 65+/-6% and 57+/-16%, respectively. Pretreatment of cardiac fibroblasts with telmisartan completely blocked the Ang II-induced increase in collagen secretion (p<0.001) and production(p<0.05) and in non-collagen secretion (p<0.01) and production (p<0.01). P-186 had no effect on the Ang II-induced increase in collagen secretion and production. Addition of telmisartan and P-186 did not affect collagen secretion and production in basal cardiac fibroblasts. Our data demonstrate that the effects of Ang II on collagen secretion and production in adult rat cardiac fibroblasts in culture are AT1-receptor mediated, since they were abolished by the specific AT1-receptor antagonist, telmisartan, but not by the specific AT2-receptor antagonist, P-186.

Volatile anaesthetic effects on Na+-Ca2+ exchange in rat cardiac myocytes

We examined the influence of two clinically relevant concentrations (1 and 2 MAC (minimum alveolar concentration)) of halothane and sevoflurane on both efflux and reverse modes of Na+-Ca2+ exchange (NCX) in enzymatically dissociated adult rat cardiac myocytes. We hypothesised that a volatile anaesthetic-induced decrease in myocardial contractility is mediated by a reduction in intracellular calcium concentration ([Ca2+]i) via inhibition of NCX. Cells were exposed to cyclopiazonic acid and zero extracellular Na+ and Ca2+ to block sacroplasmic reticulum (SR) re-uptake and NCX efflux, respectively. As [Ca2+]i increased under these conditions, extracellular Na+ was rapidly (< 300 ms) reintroduced in the presence or absence of a volatile anaesthetic to selectively promote Ca2+ efflux via NCX. Other cells exposed to cyclopiazonic acid and ryanodine to inhibit SR Ca2+ re-uptake and release were Na+ loaded in zero extracellular Ca2+. The reintroduction of extracellular Ca2+ was used to selectively activate Ca2+ influx via NCX. Compared to controls, both 1 and 2 MAC halothane as well as sevoflurane reduced NCX-mediated efflux. The reduction in NCX-mediated influx was concentration dependent, but comparable between the two anaesthetics. Both anaesthetics at each concentration also shifted the relationship between extracellular Na+ (or extent of Na+ loading) and NCX-mediated efflux (or influx) to the right. These data indicate that despite inhibition of NCX-mediated Ca2+ efflux, volatile anaesthetics produce myocardial depression. However, the inhibition of NCX-mediated Ca2+ influx may contribute to decreased cardiac contractility. The overall effect of volatile anaesthetics on the [Ca2+]i profile is likely to be determined by the relative contributions of influx vs. efflux via NCX during each cardiac cycle.

Transforming growth factor-beta(1) induces angiotensin-converting enzyme synthesis in rat cardiac fibroblasts during their differentiation to myofibroblasts

OBJECTIVES

Appearance of angiotensin-converting enzyme (ACE) in fibrotic tissue can be the result of the action either of one particular growth factor or of cross-talk between multiple factors. Transforming growth factor-beta1 (TGF-beta(1)) is an effective inducor of the differentiation of cultured fibroblasts to myofibroblasts, which are heterogeneous cells with different phenotypes. The present study investigated whether TGF-beta(1) is able to induce, in vitro, the differentiation of cultured fibroblasts to myofibroblasts with a phenotype containing ACE.

DESIGN

Adult rat cardiac ventricular fibroblasts were obtained from hearts perfused with collagenase. Cells from second passage were always used. Rat cardiac ventricular fibroblasts were incubated with TGF-beta(1) (10 ng/ml) for seven days. Cell characterisation was performed using light microscopy and indirect immunostaining. Presence of vimentin, desmin, factor VIII, alpha-smooth muscle actin, and ACE was checked with both immunostaining and Western blotting. ACE activity was measured fluorometrically with hippuryl-histidyl-leucine as substrate. Synthesis of DNA was measured as (3)H-thymidine incorporation.

RESULTS

Fibroblasts contained two types of activity of hip-his-leu degradation, namely a lisinopril-dependent activity (ACE activity) and a lisinopril-independent activity ('ACE-like' activity) which is performed by peptidase(s) other than ACE. The ACE activity constituted approximately 30% of the total activity. TGF-beta(1) induced an increase in both ACE activity and ACE protein (detected by immunoblotting) by 4.5 +/- 0.9- and 6.9 +/- 2.0-fold, respectively (p<0.05). This induction of ACE was accompanied by a profound modification of the fibroblasts phenotype, which consisted of a change in cell morphology, an enlargement of cell volume and an increase in cell protein content. TGF-beta(1) profoundly inhibited (3)H-thymidine incorporation and the number of cells in growing cultures. The induction of alpha-smooth muscle actin by TGF-beta(1) (6.8 +/- 2.8-fold increase, p<0.05) simultaneously with these modifications in cell morphology and proliferation indicates the appearance of myofibroblasts. These myofibroblasts did not contain desmin.

CONCLUSION

TGF-beta(1) is able to induce the appearance of ACE in cultures of adult rat cardiac ventricular fibroblasts. The appearance of the enzyme is accompanied by the differentiation of fibroblasts to myofibroblasts.

cADP ribose and [Ca(2+)](i) regulation in rat cardiac myocytes

cADP ribose (cADPR)-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) responses were assessed in acutely dissociated adult rat ventricular myocytes using real-time confocal microscopy. In quiescent single myocytes, injection of cADPR (0.1-10 microM) induced sustained, concentration-dependent [Ca(2+)](i) responses ranging from 50 to 500 nM, which were completely inhibited by 20 microM 8-amino-cADPR, a specific blocker of the cADPR receptor. In myocytes displaying spontaneous [Ca(2+)](i) waves, increasing concentrations of cADPR increased wave frequency up to approximately 250% of control. In electrically paced myocytes (0.5 Hz, 5-ms duration), cADPR increased the amplitude of [Ca(2+)](i) transients in a concentration-dependent fashion, up to 150% of control. Administration of 8-amino-cADPR inhibited both spontaneous waves as well as [Ca(2+)](i) responses to electrical stimulation, even in the absence of exogenous cADPR. However, subsequent [Ca(2+)](i) responses to 5 mM caffeine were only partially inhibited by 8-amino-cADPR. In contrast, even under conditions where ryanodine receptor (RyR) channels were blocked with ryanodine, high cADPR concentrations still induced an [Ca(2+)](i) response. These results indicate that in cardiac myocytes, cADPR induces Ca(2+) release from the sarcoplasmic reticulum through both RyR channels and via mechanisms independent of RyR channels.

Parallel stimulation of glucose and Mg(2+) accumulation by insulin in rat hearts and cardiac ventricular myocytes

The stimulation of beta-adrenoceptors in cardiac cells results in a rapid loss of cellular Mg(2+). Because insulin physiologically counteracts several of the cellular effects mediated by the activation of beta-adrenoceptors and the elevation of cytosolic cAMP levels, we investigated whether insulin administration could prevent Mg(2+) mobilization from rat hearts and ventricular myocytes. Rat hearts were perfused in a retrograde Langendorff system, and the changes in extracellular Mg(2+) were measured by atomic absorbance spectrophotometry. Pretreatment of the hearts with 6 nmol/L insulin completely prevented the Mg(2+) extrusion induced by the beta-adrenergic agonist isoproterenol. Furthermore, the administration of insulin per se induced an accumulation of Mg(2+) by the heart. This accumulation was small but detectable in the presence of 25 to 35 micromol/L [Mg(2+)](o) and increased in proportion to [Mg(2+)](o). Insulin-mediated Mg(2+) accumulation was not observed in hearts perfused with a medium devoid of glucose or with a medium containing the inhibitors of glucose transport, cytochalasin B and phloretin. Insulin-stimulated [(3)H]2-deoxyglucose accumulation was measured in collagenase-dispersed cardiac ventricular myocytes in the presence of varying levels of [Mg(2+)](o). Glucose transport was not observed below 25 micromol/L [Mg(2+)](o), and it also increased in proportion to [Mg(2+)](o). Taken together, these results indicate the presence of a major uptake of Mg(2+) into cardiac cells that is stimulated by insulin and may require the insulin-induced operation of a glucose transporter. Hence, extracellular and/or intracellular Mg(2+) may modulate glucose transport and/or utilization.

Elevated myocardial cytosolic calcium impairs insulin-like growth factor-1-stimulated protein synthesis in chronic renal failure

Rats and humans with chronic renal failure (CRF) are reported to have resistance to recombinant human insulin-like growth factor-1 (rhIGF-1). Because basal cytosolic calcium ([Ca2+]i), a second messenger, may be increased in CRF, this study was conducted to examine whether elevated basal [Ca2+]i may cause resistance to IGF-1. Cardiomyocytes from four groups of rats were studied: untreated CRF, CRF with parathyroidectomy (PTX), CRF with the calcium channel blocker felodipine (F), and sham operation of the kidney (SO). CRF was created by ligation of two-thirds of the left renal artery and contralateral nephrectomy. Rats from each group were pair-fed the same diet for 20 to 22 d. Basal [Ca2+]i in cardiomyocytes (nM) in the CRF rats (102.0 +/- 2.8; SEM), was significantly higher than in each of the CRF-PTX, CRF-F, and SO groups (65.2 +/- 1.9, 63.8 +/- 2.6, and 63.5 +/- 2.0, respectively; P < 0.01). rhIGF-1 increased cardiomyocyte [Ca2+]i in all four groups of rats. The rise in [Ca2+]i was significantly diminished in the CRF rats (P < 0.05) and did not differ among the CRF-PTX, CRF-F, and SO rats. Protein synthesis after incubation with 0, 50, 100, 200, or 400 ng/ml rhIGF-1 was lower in cardiomyocytes from CRF rats than in each of the other three groups (P < 0.05) and was significantly less in the CRF-F rats compared with SO animals. IGF-1 receptor mRNA and IGF-1 receptor number and affinity were not different among the four groups. These findings suggest that cardiomyocytes from CRF rats display elevated basal [Ca2+]i and attenuated rhIGF-1-induced increase in [Ca2+]i; basal protein synthesis is decreased, and IGF-1-stimulated protein synthesis is impaired; elevated basal [Ca2+]i seems to contribute to this diminished response to rhIGF-1.

Characterization of two Mg2+ transporters in sealed plasma membrane vesicles from rat liver

The plasma membrane of mammalian cells possesses rapid Mg2+ transport mechanisms. The identity of Mg2+ transporters is unknown, and so are their properties. In this study, Mg2+ transporters were characterized using a biochemically and morphologically standardized preparation of sealed rat liver plasma membranes (LPM) whose intravesicular content could be set and controlled. The system has the advantages that it is not regulated by intracellular signaling machinery and that the intravesicular ion milieu can be designed. The results indicate that 1) LPM retain trapped intravesicular total Mg2+ with negligible leak; 2) the addition of Na+ or Ca2+ induces a concentration- and temperature-dependent efflux corresponding to 30-50% of the intravesicular Mg2+; 3) the rate of flux is very rapid (137.6 and 86.8 nmol total Mg2+ . micrometer -2 . min-1 after Na+ and Ca2+ addition, respectively); 4) coaddition of maximal concentrations of Na+ and Ca2+ induces an additive Mg2+ efflux; 5) both Na+- and Ca2+-stimulated Mg2+ effluxes are inhibited by amiloride, imipramine, or quinidine but not by vanadate or Ca2+ channel blockers; 6) extracellular Na+ or Ca2+ can stimulate Mg2+ efflux in the absence of Mg2+ gradients; and 7) Mg2+ uptake occurs in LPM loaded with Na+ but not with Ca2+, thus indicating that Na+/Mg2+ but not Ca2+/Mg2+ exchange is reversible. These data are consistent with the operation of two distinct Mg2+ transport mechanisms and provide new information on rates of Mg2+ transport, specificity of the cotransported ions, and reversibility of the transport.

High glucose concentration causes a rise in [Ca2+]i of cardiac myocytes

Diabetes mellitus is associated with an elevation in the basal levels of cytosolic calcium ([Ca2+]i) of cardiac myocytes. This may be due in part to a glucose-induced elevation in [Ca2+]i. The present study examined this issue and explored the cellular pathways responsible for such a phenomenon. A total of 30 mM glucose, mannitol or choline chloride, but not urea, induced a time- and dose-dependent rise in the [Ca2+]i of cardiac myocytes. G protein inhibition by GDP beta S or pertussis toxin produced significant inhibition (> or = 80%) in the rise in [Ca2+]i. Incubation of cardiac myocytes in a calcium free medium or in media containing verapamil, nifedipine or amlodipine almost completely abolished the rise in [CA2+], while ryanodine produced only small reduction (10%) in the glucose-induced rise in [Ca2+]i. Rp-cAMP or H-89, inhibitors of the cAMP-protein kinase A pathway, produced a modest decrease in the rise in [Ca2+]i, while staurosporine (an inhibitor of PKC) and HOE 694 (an inhibitor of the Na(+)-H+ exchanger) had no effect on the rise in [Ca2+]i. The results indicate that the osmotic activity of glucose (cell shrinkage) activates G protein(s), most likely through a stretch receptor, which in turn stimulates calcium channels inhibitable by verapamil, nifedipine and amlodipine, thus permitting a calcium influx into the cardiac myocytes. The increased calcium entry may stimulate a calcium release from intracellular stores by a calcium-induced calcium release process. Thus, in cardiac myocytes direct activation of calcium channels, and to a small extent activation of the cAMP-protein kinase A, and calcium-induced calcium release mediate the high glucose-induced acute rise in their [Ca2+]i.

Calcium-antagonist effects of norbormide on isolated perfused heart and cardiac myocytes of guinea-pig: a comparison with verapamil

1. Cardiac effects on norbormide and verapamil were compared in single ventricular myocytes, right atria, and Langendorff perfused hearts isolated from guinea-pigs. 2. In ventricular myocytes, norbormide 50 microM inhibited the peak calcium current (ICa) by 49.6 +/- 3.9% without altering the shape of the current-voltage relationship; verapamil 1 microM inhibited ICa by 83.2 +/- 3.3%. Neither norbormide nor verapamil affected ICa at the first beat after a 3 min quiescence period; during repeated depolarizations, both drugs cumulatively blocked ICa (use-dependence), with time constants of 23.0 +/- 7.0 s for norbormide and 91.3 +/- 8.4 s for verapamil. 3. In constant-flow perfused hearts electrically driven at 2.5 Hz or 3.3 Hz, both norbormide and verapamil concentration-dependently decreased ventricular contractility (dP/dtmax), atrio-ventricular (AV) conduction velocity and coronary pressure. Intraventricular conduction velocity was slightly decreased by norbormide but not by verapamil. At an equivalent change in AV conduction, norbormide depressed heart contractility less than verapamil. The effects of norbormide on AV conduction, intraventricular conduction, and contractility were frequency-dependent. Furthermore, the curves correlating the mechanical and electrical effects of norbormide at the two frequencies used were apparently coincident, while those of verapamil were clearly separated. 4. In spontaneously beating right atria, norbormide and verapamil decreased the frequency of sinus node (SA) in a concentration-dependent way. At an equivalent effect on the AV conduction, norbormide exerted a greater effect on sinus frequency than verapamil. 5. These results indicate that in guinea-pig heart norbormide has the pharmacological profile of a Ca-antagonist with strong electrophysiological properties. In comparison with verapamil, norbormide is more selective on SA and AV node tissues and exerts a weaker negative inotropic effect on ventricles. In principle, this pattern of effects may be an advantage in treating supraventricular tachyarrhythmias in patients with heart failure. The effect of norbormide on intraventricular conduction may represent an additional antiarrhythmic mechanism.

Mobilization of Mg2+ from rat heart and liver mitochondria following the interaction of thyroid hormone with the adenine nucleotide translocase

The in vitro addition of thyroid hormone to isolated rat heart or liver mitochondria induces the extrusion of approximately 2-4 nmol Mg2+/mg protein from both mitochondria preparations. The mobilization of Mg2+ is not accompanied by extrusion of matrix ATP or K+, or by mitochondria swelling, thus excluding that the phenomenon occurs through the nonspecific opening of the mitochondrial permeability transition pore. Moreover, the Mg2+ extrusion is completely prevented by bongkrekic acid, a membrane-permeant inhibitor of the adenine nucleotide translocase (AdNT), and by cyclosporine, which has also been reported to inhibit AdNT in a bongkrekate-like manner, operating at the matrix site of the translocase. By contrast, atractyloside, another specific inhibitor of AdNT that operates at the cytosolic site of the AdNT, only partially affects the Mg2+ mobilization (< 30% inhibition). These findings and the binding of 125I-labeled thyroid hormone to both the dimeric and monomeric moiety of AdNT support the hypothesis that AdNT can operate as a specific receptor for thyroid hormone in the mitochondria, and suggest that thyroid hormone operates at the matrix site of the translocase. In addition, these observations may imply that some of the so called "nongenomic effects" exerted by thyroid hormone on mitochondrial metabolism could occur through changes in the matrix content of Mg2+.

Elevation of cytosolic calcium of rat cardiac myocytes in phosphate depletion

Phosphate depletion is associated with a rise in cytosolic calcium ([Ca2+]i) of cells and such a derangement is responsible in major part for organ dysfunction in phosphate depletion (PD). Cardiac function is impaired in PD, and it is possible that PD is also associated with rise in [Ca2+]i of cardiac myocytes. The present study examined the effect of PD on [Ca2+]i of cardiac myocytes and explored the mechanisms that may lead to the rise in their [Ca2+]i. The [Ca2+]i of cardiac myocytes began to rise and ATP content began to fall at the third week of PD. After six weeks of PD, the values of [Ca2+]i were significantly higher (P < 0.01) and those of ATP content were significantly lower (P < 0.01) than in control (PW) rats. The Vmax of Ca2(+)-ATPase and Na+,K(+)-ATPase as well as the Na(+)-Ca2+ exchange were significantly lower (P < 0.01) in PD than in PW animals. The data of the present study are consistent with the notion that the rise in [Ca2+]i of cardiac myocytes of PD rats is due to a decrease in calcium efflux out of them.

Distinct patterns of calcium transients during early and delayed afterdepolarizations induced by isoproterenol in ventricular myocytes

BACKGROUND

The relation between early afterdepolarizations (EADs) and changes in intracellular Ca2+ concentration ([Ca2+]i) is still unclear. In the present study, we compared spatiotemporal changes in [Ca2+]i related to EADs and delayed afterdepolarizations (DADs) induced by isoproterenol.

METHODS AND RESULTS

Isolated patch-clamped guinea pig ventricular myocytes, loaded with fluo-3 acetoxymethyl ester (fluo-3 AM), were paced at 0.1 to 2 Hz. Isoproterenol (100 nmol/L) caused alterations in both phase 2 and phase 4 of the action potential (AP), consistent with EADs and DADs, respectively. During EADs (n = 16), similar to driven APs, increases in [Ca2+]i occurred simultaneously throughout the cell, whereas during DADs (n = 25), they originated in discrete cell sites and propagated as a wave. This difference was confirmed by analysis of eight EADs and DADs coupled to the same beat. Ca2+ transient linked to EADs reached a peak relative fluorescence level (expressed as percentage of the maximal level reached during the last stimulated beat) that was always higher than that reached during the DADs (77 +/- 3% versus 64 +/- 2%, P < .001). Spatial heterogeneity of Ca2+ transients was assessed by the maximal time interval between peaks monitored in different cell regions; this time lag was always greater during DADs than during EADs (290 versus 40 milliseconds, P = .006).

CONCLUSIONS

The present study had two main findings. First, even very modest notches occurring during the plateau of the AP may be accompanied by a marked secondary increase in [Ca2+]i. Second, these Ca2+ transients occurring during EADs are synchronous throughout the cell and differ significantly from those observed under identical conditions during DADs.

Down-regulation of PTH-PTHrP receptor of heart in CRF: role of [Ca2+]i

The mRNA of PTH-PTHrP receptor in the kidney and liver of CRF rats is down-regulated. It is not known whether this is a generalized phenomenon and the signals that mediate such down-regulation are not evident. Excess PTH in CRF induces a rise in basal levels of cytosolic calcium ([Ca2+]i), and the high [Ca2+]i is implicated in the genesis of cell dysfunction in CRF. Therefore, it is reasonable to propose that the PTH-induced rise in [Ca2+]i provides a negative feedback control system for the down-regulation of the mRNA of the PTH-PTHrP in order to protect the cells from the harmful effects of progressive rise in blood levels of PTH in CRF. The heart contains the mRNA for this receptor and it is a target organ for PTH action. We examined whether the message of the PTH-PTHrP receptor is down-regulated in the heart of CRF animals and evaluated the role of [Ca2+]i in this process. The expression of the mRNA of the PTH-PTHrP was significantly reduced in the heart of CRF rats as compared to normal animals. Also, the CRF rats had elevated blood levels of PTH and high [Ca2+]i of cardiac myocytes. The parathyroidectomy of CRF rats prevented the rise in blood PTH levels and normalized [Ca2+]i of cardiac myocytes and returned the mRNA of their PTH-PTHrP receptor towards normal levels. The treatment of CRF rats with verapamil normalized [Ca2+]i of cardiac myocytes and returned the mRNA of their PTH-PTHrP receptor towards normal levels, despite marked elevation of blood levels of PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Improved isolation of cardiomyocytes by trypsination in addition to collagenase treatment

The present study was undertaken to develop an improved and stabilized method for isolating cardiomyocytes from perfused rat heart. Different lots of the commercial collagenases used for isolating cardiomyocytes give variable results both with respect to the total cell yield and the percentage of elongated cells obtained. When trypsin was present both before and during collagenase treatment of the tissue, the performance of the collagenases was improved and stabilized, and a high and stable cell yield (7.5 x 10(6) cells per heart), and a high percentage of elongated cells (about 70%) was regularly obtained. The cells possessed alpha 1-adrenergic binding sites with binding properties (Bmax = 43.5 fmol/mg protein and Kd = 125.5 pmol/l) in agreement with values previously reported. The cells were able to respond functionally, as the cellular uptake of 86Rb+ increased by 18% after alpha 1-adrenoceptor stimulation with phenylephrine. These criteria indicate that the cells were well preserved during the isolation procedure.

Altered cytosolic calcium homeostasis in rat cardiac myocytes in CRF

Chronic renal failure (CRF) is associated with an increase in calcium content of heart. This was attributed to the secondary hyperparathyroidism of CRF, since PTH augments entry of calcium into cardiac myocytes. At present, it is not known whether the increase in calcium content of heart reflects a rise in basal levels of cytosolic calcium ([Ca2+]) of cardiac myocytes. Further, in order for the PTH-induced entry of calcium into cardiac myocytes to raise their basal levels of [Ca2+]i, calcium extrusion out of these cells should be impaired as well. The present study examined the effect of CRF with and without excess PTH (PTX) and of the treatment of CRF rats with verapamil on basal levels of [Ca2+]i and ATP content of cardiac myocytes and on the activities of the pumps that are directly (Ca(2+)-ATPase and Na(+)-Ca2+ exchanger) and indirectly (Na(+)-K+ ATPase) responsible for calcium extrusion out of these cells. The basal levels of [Ca2+]i of cardiac myocytes increased (P < 0.01) and their ATP content decreased (P < 0.01) as the duration of CRF advanced. CRF was associated with significant decrement in Vmax of Ca2+ ATPase and Na(+)-K+ ATPase and in Na(+)-Ca2+ exchange. These derangements were prevented by prior PTX of the CRF rats or by their treatment with verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of action of equinatoxin II, a cytolysin from the sea anemone Actinia equina L. belonging to the family of actinoporins

Actinia equina equinatoxin II (EqT-II) is a representative of a family of pore-forming, basic, polypeptide toxins from sea anemones, now called actinoporins. This family comprises at least 27 members, which are all hemolytic at rather low concentrations. Red blood cell (RBC) hemolysis by EqT-II is the result of a colloid-osmotic shock caused by the opening of toxin-induced pores. Using osmotic protectants of different size the functional radius of the lesion was estimated to be approximately 1.1 nm. These pores are most probably constituted by oligomeric aggregates of cytolysin molecules, whose presence on the membrane of lysed RBC was directly demonstrated by polyacrylamide gel electrophoresis (PAGE) after covalent cross-linking. EqT-II is active also against a variety of mammalian cells including leukocytes, platelets and cardiomiocytes. An increased permeability of the plasma membrane after Eq-II attack is compatible with the notion that the toxin forms pores also on these cells. Eq-II permeabilises even purely lipidic model membranes, suggesting a protein receptor is not necessary. Using calcein-loaded unilamellar vesicles (UVs) comprised of phosphatydylcholine (PC) mixed with other lipids we observed that the rate and extent of permeabilization greatly increases when sphingomyelin (SM) or the ganglioside GM1 were introduced, particularly in the case of large UVs (which are more sensitive to the toxin than small UVs). PAGE indicated that the increased effect of Eq-II on SM containing vesicles is due to an increased level of toxin binding to such vesicles. The formation of cation-selective channels by EqT-II was directly demonstrated using planar lipid membranes where the toxin induced discrete increases of the film conductivity. The conductance of the channel was consistent with the estimated size of the lesion formed in RBC. Several factors can affect toxin activity: serum, low pH, low ionic strength and multivalent cations are potent inhibitors. pH Dependence is bell shaped, optimum activity being between pH 8 and 9. Similarly the action of Ca2+ is also bivalent: up to a concentration of approximately 2 mM it stimulates hemolysis, but above this concentration it inhibits (with 50% inhibition occurring at approximately 10 mM). When the known amino acid sequences of actinoporins are examined a common trait emerges; the presence of a well conserved, amphiphilic, putative alpha-helix at the N-terminus, which might be involved in the insertion of EqT-II in lipid membranes.

Regulation of magnesium uptake and release in the heart and in isolated ventricular myocytes

Perfused rat hearts release or accumulate approximately 10% of total Mg2+ content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+ by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+ transport using 28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+ efflux and influx are independent of the concentration of extracellular Mg2+ (0 to 1.2 mM). Mg2+ release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+ uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+ or Ca2+ decreases or abolishes Mg2+ efflux from myocytes. Cd2+ or other Ca2+ channel blockers also inhibit Mg2+ efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+ with Sr2+ or with Mn2+ decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of 85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+ is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+ fluxes can be explained by the transport of Sr2+ rather than Mg2+ through the transport(s) systems. By contrast, replacement of extracellular Ca2+ with Ba2+ inhibits stimulated Mg2+ uptake but not Mg2+ release. These results indicate that cardiac myocytes have a major pool of Mg2+ that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+ are quantitatively similar and appear to be independent of the extracellular Mg2+ concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.

Regulation of Mg2+ uptake in isolated rat myocytes and hepatocytes by protein kinase C

A large Mg2+ cell uptake against concentration gradients is stimulated in collagenase-dispersed rat myocytes by carbachol and in hepatocytes by carbachol or vasopressin. The signalling pathway(s) responsible for this stimulation of Mg2+ uptake was investigated by using various activators or inhibitors of protein kinase C (PKC) and by correlating Mg2+ uptake with cell PKC activity and cAMP content. In both cell preparations, the direct stimulation of PKC by diacylglycerol analogs or phorbol esters reproduce the same pattern of Mg2+ uptake as that induced by carbachol or vasopressin. These data indicate that the activation of PKC is responsible for a stimulation of Mg2+ uptake by myocytes or hepatocytes, whereas increase in cAMP in these cells stimulates Mg2+ release.

Diamide: positive inotropic effect in isolated atria and inhibition of Na+/Ca2+ exchange in cardiomyocytes

The influence of frequency of stimulation and external calcium on the positive inotropic response of guinea-pig left atria to diamide and the inhibitory action on Na+/Ca2+ exchange activity of rat cardiomyocytes by this oxidant of sulphhydryl groups have been investigated. Diamide (50-500 microM) induces a concentration-dependent positive inotropic effect which is more pronounced when atria are driven at 1.0 Hz rather than at 0.5 and 0.1 Hz, and are bathed in 2.72 mM rather than in 1.36 mM external calcium. A decrease in the positive inotropic effect at 35 degrees C with respect to 29 degrees C is also observed. In addition, diamide in positive inotropic concentrations (100-300 microM) significantly reduces Na+/Ca2+ exchange activity and cytoplasmic glutathione levels in adult rat cardiomyocytes. The thiol reducing agent dithiothreitol either reverses or prevents diamide effects both in isolated atria and cardiomyocytes, suggesting that the actions of diamide are correlated to its property to oxidize sulphhydryl groups to disulphides. In view of the functional importance of Na+/Ca2+ exchange in myocardial contractility, it is proposed that diamide may increase the heart force of contraction by an inhibition of the sarcolemmal Na+/Ca2+ exchange activity.

Hormonal control of Mg2+ transport in the heart

Magnesium is abundant in the mammalian body and the second most abundant cation in cells. Because the concentration of intracellular free Mg2+ is relatively high (0.2-1 mM), Mg2+ is unlikely to act as a second messenger, like Ca2+, by rapidly changing its cytosolic concentration. But changes in Mg2+ do have profound effects on cellular metabolism, structure and bioenergetics. Key enzymes or metabolic pathways, mitochondrial ion transport, Ca2+ channel activities in the plasma membrane and intracellular organelles, ATP-requiring reactions, and structural properties of cells and nucleic acids are modified by changes in Mg2+ concentration. Yet, although some information is available from giant cells and bacteria, little is known about the regulation of intracellular Mg2+ in mammalian cells. Here we report a new transport mechanism for Mg2+ across the sarcolemma of cardiac cells in both intact hearts and dissociated myocytes. We show that noradrenaline, through beta-adrenergic stimulation and increase of cyclic AMP, stimulates a large efflux of Mg2+ from cardiac cells. This transport is of major dimensions and can move up to 20% of total cellular Mg2+ within a few minutes.