Structure of the freeze-fractured sarcolemma in the normal and anoxic rabbit myocardium

Caveolins as Regulators of Stress Adaptation

Caveolins have been recognized over the past few decades as key regulators of cell physiology. They are ubiquitously expressed and regulate a number of processes that ultimately impact efficiency of cellular processes. Though not critical to life, they are central to stress adaptation in a number of organs. The following review will focus specifically on the role of caveolin in stress adaptation in the heart, brain, and eye, three organs that are susceptible to acute and chronic stress and that show as well declining function with age. In addition, we consider some novel molecular mechanisms that may account for this stress adaptation and also offer potential to drive the future of caveolin research.

Massive palmitoylation-dependent endocytosis during reoxygenation of anoxic cardiac muscle

In fibroblasts, large Ca transients activate massive endocytosis (MEND) that involves membrane protein palmitoylation subsequent to mitochondrial permeability transition pore (PTP) openings. Here, we characterize this pathway in cardiac muscle. Myocytes with increased expression of the acyl transferase, DHHC5, have decreased Na/K pump activity. In DHHC5-deficient myocytes, Na/K pump activity and surface area/volume ratios are increased, the palmitoylated regulatory protein, phospholemman (PLM), and the cardiac Na/Ca exchanger (NCX1) show greater surface membrane localization, and MEND is inhibited in four protocols. Both electrical and optical methods demonstrate that PTP-dependent MEND occurs during reoxygenation of anoxic hearts. Post-anoxia MEND is ablated in DHHC5-deficient hearts, inhibited by cyclosporine A (CsA) and adenosine, promoted by staurosporine (STS), reduced in hearts lacking PLM, and correlates with impaired post-anoxia contractile function. Thus, the MEND pathway appears to be deleterious in severe oxidative stress but may constitutively contribute to cardiac sarcolemma turnover in dependence on metabolic stress.

DOI:http://dx.doi.org/10.7554/eLife.01295.001

Many people who survive a stroke or heart attack experience substantial tissue damage when the blood supply is restored. Much of this damage can be caused by the mitochondria inside the cells releasing a protein called cytochrome c that can cause cells to die in a process called apoptosis. The cytochrome c is released as the outer membrane of the mitochondria becomes permeable and pores called permeability transition pores open up in the inner membrane. Now Lin et al. explore if additional molecules released from the mitochondria might also initiate important cellular responses during the reoxygenation of oxygen-deprived tissue.

Lin and co-workers recently showed that the mitochondria of some cells can release a small enzyme cofactor, coenzyme A, which then promotes a cellular response called massive endocytosis. This process can cause up to 70% of the cell surface membrane to be absorbed into the interior of the cell in the form of membrane vesicles. Most forms of endocytosis involve a much smaller fraction of the cell membrane and employ a set of well-known endocytic proteins that are not involved in massive endocytosis. Now, Lin et al. investigate the role of massive endocytosis in cardiac muscle.

Electrical and optical measurements reveal that massive endocytosis occurs as cardiac cells that have been deprived of oxygen are reoxygenated. Lin et al. also find that an enzyme called DHHC5 must be present to allow endocytosis to take place during reoxygenation. DHHC5 is an enzyme that catalyzes a process called acylation – the transfer of acyl groups to proteins at the cell surface. Moreover, the deletion of DHHC5 has a beneficial impact on the performance of cardiac muscle after oxygen deprivation, which implies that molecules that inhibit protein acylation might protect the heart from damage during reoxygenation. Together, these results establish new pathological and physiological roles for the acylation, which is one of the most common biochemical modifications made to membrane proteins after they are synthesized.

DOI:http://dx.doi.org/10.7554/eLife.01295.002

Massive endocytosis triggered by surface membrane palmitoylation under mitochondrial control in BHK fibroblasts

Large Ca transients cause massive endocytosis (MEND) in BHK fibroblasts by nonclassical mechanisms. We present evidence that MEND depends on mitochondrial permeability transition pore (PTP) openings, followed by coenzyme A (CoA) release, acyl CoA synthesis, and membrane protein palmitoylation. MEND is blocked by inhibiting mitochondrial Ca uptake or PTP openings, depleting fatty acids, blocking acyl CoA synthesis, metabolizing CoA, or inhibiting palmitoylation. It is triggered by depolarizing mitochondria or promoting PTP openings. After mitochondrial MEND blockade, MEND is restored by cytoplasmic acyl CoA or CoA. MEND is blocked by siRNA knockdown of the plasmalemmal acyl transferase, DHHC5. When acyl CoA is abundant, transient H2O2 oxidative stress or PKC activation initiates MEND, but the immediate presence of H2O2 prevents MEND. The PTP inhibitor, NIM811, significantly increases plasmalemma in normally growing cells. Thus, the MEND pathway may contribute to constitutive as well as pathological plasmalemma turnover in dependence on mitochondrial stress signaling. DOI: http://dx.doi.org/10.7554/eLife.01293.001.

Caveolin-1 deficiency exacerbates cardiac dysfunction and reduces survival in mice with myocardial infarction

Caveolin (Cav)-1 has been involved in the pathogenesis of ischemic injuries. For instance, modulations of Cav-1 expression have been reported in animal models of myocardial infarction and cerebral ischemia-reperfusion. Furthermore, ablation of the Cav-1 gene in mice has been shown to increase the extent of ischemic injury in models of cerebral and hindlimb ischemia. Cav-1 has also been suggested to play a role in myocardial ischemic preconditioning. However, the role of Cav-1 in myocardial ischemia (MI)-induced cardiac dysfunction still remains to be determined. We determined the outcome of a permanent left anterior descending coronary artery (LAD) ligation in Cav-1 knockout (KO) mice. Wild-type (WT) and Cav-1 KO mice were subjected to permanent LAD ligation for 24 h. The progression of ischemic injury was monitored by echocardiography, hemodynamic measurements, 2,3,5-triphenyltetrazolium chloride staining, β-binding analysis, cAMP level measurements, and Western blot analyses. Cav-1 KO mice subjected to LAD ligation display reduced survival compared with WT mice. Despite similar infarct sizes, Cav-1 KO mice subjected to MI showed reduced left ventricular (LV) ejection fraction and fractional shortening as well as increased LV end-diastolic pressures compared with their WT counterparts. Mechanistically, Cav-1 KO mice subjected to MI exhibit reduced β-adrenergic receptor density at the plasma membrane as well as decreased cAMP levels and PKA phosphorylation. In conclusion, ablation of the Cav-1 gene exacerbates cardiac dysfunction and reduces survival in mice subjected to MI. Mechanistically, Cav-1 KO mice subjected to LAD ligation display abnormalities in β-adrenergic signaling.

Cardiac-specific overexpression of caveolin-3 induces endogenous cardiac protection by mimicking ischemic preconditioning

BACKGROUND

Caveolae, lipid-rich microdomains of the sarcolemma, localize and enrich cardiac-protective signaling molecules. Caveolin-3 (Cav-3), the dominant isoform in cardiac myocytes, is a determinant of caveolar formation. We hypothesized that cardiac myocyte-specific overexpression of Cav-3 would enhance the formation of caveolae and augment cardiac protection in vivo.

METHODS AND RESULTS

Ischemic preconditioning in vivo increased the formation of caveolae. Adenovirus for Cav-3 increased caveolar formation and phosphorylation of survival kinases in cardiac myocytes. A transgenic mouse with cardiac myocyte-specific overexpression of Cav-3 (Cav-3 OE) showed enhanced formation of caveolae on the sarcolemma. Cav-3 OE mice subjected to ischemia/reperfusion injury had a significantly reduced infarct size relative to transgene-negative mice. Endogenous cardiac protection in Cav-3 OE mice was similar to wild-type mice undergoing ischemic preconditioning; no increased protection was observed in preconditioned Cav-3 OE mice. Cav-3 knockout mice did not show endogenous protection and showed no protection in response to ischemic preconditioning. Cav-3 OE mouse hearts had increased basal Akt and glycogen synthase kinase-3beta phosphorylation comparable to wild-type mice exposed to ischemic preconditioning. Wortmannin, a phosphoinositide 3-kinase inhibitor, attenuated basal phosphorylation of Akt and glycogen synthase kinase-3beta and blocked cardiac protection in Cav-3 OE mice. Cav-3 OE mice had improved functional recovery and reduced apoptosis at 24 hours of reperfusion.

CONCLUSIONS

Expression of caveolin-3 is both necessary and sufficient for cardiac protection, a conclusion that unites long-standing ultrastructural and molecular observations in the ischemic heart. The present results indicate that increased expression of caveolins, apparently via actions that depend on phosphoinositide 3-kinase, has the potential to protect hearts exposed to ischemia/reperfusion injury.

The role of hypoxia in vascular injury and repair

Although the terms ischemia and hypoxia are often used interchangeably, they represent distinct processes that result in different modulatory effects at the cellular level. Hypoxia is a reduction in oxygen delivery below tissue demand, whereas ischemia is a lack of perfusion, characterized not only by hypoxia but also by insufficient nutrient supply. Hypoxia can be either acute or chronic, and both are centrally regulated by hypoxia-inducible factor, a transcription factor that governs the expression of key response genes such as vascular endothelial growth factor and erythropoietin. Whereas severe chronic hypoxia can cause cell death, less-severe hypoxia can protect against subsequent damage, a phenomenon known as hypoxic conditioning. Several important processes are characterized by hypoxia, including ischemia-reperfusion, tumor growth and progression, inflammation, myocardial ischemia, and a number of ocular pathologies.

Regional myocardial ischemia-induced activation of MAPKs is associated with subcellular redistribution of caveolin and cholesterol

Ischemia-reperfusion activates ERK and p38 MAPK in cardiac membranes, but the role of caveolae in MAPK signaling during this stress has not been studied. The purpose of this study was to determine the effect of in vivo myocardial ischemia-reperfusion on the level and distribution of caveolin-1 and -3 and cholesterol as well as MAPK activation in caveolin-enriched fractions. Adult male rats were subjected to in vivo regional myocardial ischemia induced by 25 min of coronary artery occlusion and 10 min ( n = 5) or 2 h ( n = 4) of reperfusion. Another group of rats served as appropriate nonischemic time controls ( n = 4). A discontinuous sucrose density gradient was used to isolate caveolae/lipid rafts from ischemic and nonischemic heart tissue. Caveolin-1 and -3, as well as cholesterol, were enriched in the light fractions. A redistribution of caveolin-3 and a reduction in caveolin-1 and cholesterol levels in the light fractions occurred after 10 min of reperfusion. The ERKs were activated in ischemic zone light and heavy fractions by 10 min of reperfusion. p44 ERK was activated after 2 h of reperfusion only in the light fractions, whereas p42 ERK phosphorylation was increased in the light and heavy fractions. Although no p38 MAPK activation occurred after 10 min of reperfusion, 2 h of reperfusion caused significant activation of p38 MAPK in nonischemic zone light and heavy fractions. These results show the importance of caveolar membrane/lipid rafts in MAPK signaling and suggest that subcellular compartmentation of p44/p42 ERKs and p38 MAPK may play distinct roles in the response to myocardial ischemia-reperfusion.

Structure of caveolae

The introduction of the electron microscope to the study of the biological materials in the second half of the last century has dramatically expanded our view and understanding of the inner workings of cells by enabling the discovery and study of subcellular organelles. A population of flask-shaped or spherical invaginations of the plasma membrane were described and named plasmalemmal vesicles or caveolae. Until the discovery of caveolin-1 as their first molecular marker in early 1990s, the study of caveolae was the exclusive domain of electron microscopists that demonstrated caveolae at different surface densities in most mammalian cells with few exceptions. Electron microscopy techniques in combination with other approaches have also revealed the structural features of caveolae as well as some of their protein and lipid residents. This review summarizes the data on the structure and components of caveolae and their stomatal diaphragms.

Aggregation of myocardial sarcolemmal transmembrane proteins is not hindered by an interaction with the cytoskeleton. Possible implications for ischemia and reperfusion

Heart myocytes subjected to ischemia show a clustering of the sarcolemmal proteins. In the erythrocyte membrane, a system in which intramembranous particle (IMP) aggregation is extensively studied, it is found that an IMP aggregation can in principle only occur upon removal of the membrane skeleton of spectrin and actin by rather drastic experimental conditions. With regard to phospholipid composition and topology the sarcolemma and the erythrocyte membrane show large similarities and therefore it was proposed that a loss of the interaction of the IMPs and the cytoskeleton is also a prerequisite for the sarcolemmal IMP aggregation (Verkleij et al., 1990). Freezing myocardial tissue, both from adult and neonatal rat, from temperatures lower than 37 degrees C resulted in an aggregation of the sarcolemmal IMPs. The aggregation is proportional to the degree of lowering of the temperature at which the tissue is cryofixed. This in contrast to the erythrocyte membrane, where lowering the temperature only induces moderate IMP aggregation. The IMP aggregation in the sarcolemma is reversible upon a subsequent increase in incubation temperature. The results clearly demonstrate that the interaction between the sarcolemmal proteins does not hinder aggregation of the IMPs, as proposed previously, and suggest that loosening of this complex does not have to proceed the aggregation of the sarcolemmal intramembranous particles during ischemia.

Protective activity of nifedipine and R 58735 upon damage caused by global ischemia in the guinea pig heart-lung preparation

In the guinea pig heart-lung preparation, the protective effects of nifedipine and R 58735 on cardiovascular alterations following mild (35 min) and severe (60 min) ischemia and reperfusion (30 min) were studied. Nifedipine and R 58735 were equi-protective against the effects of mild ischemia with respect to functional (LVP, dp/dt, and cardiac output) and biochemical (ATP, CrP, and adenylate charge) parameters. A clear difference, however, was observed between nifedipine and R 58735 upon severe ischemia, where R 58735 produced a significantly greater protection of functional, but not of biochemical parameters. Since no significant differences between the two compounds were found with respect to the concentrations of high energy phosphates after 35 and 60 min of ischemia before reperfusion, an energy sparing effect is not likely to be responsible for the difference between nifedipine and R 58735 in severe ischemia. An additional protective effect of R 58735 upon reperfusion in severe ischemia experiments may explain the difference between the two compounds.

Membrane structure in ultrarapidly frozen, unpretreated, freeze-fractured myocardium

Ultrarapid freezing has been applied to monitor the structure of the freeze-fractured myocardial sarcolemma. Our two goals were to demonstrate that large areas of membrane can be preserved free of visible ice crystal damage and, thus, be amenable to quantitative analysis and to compare the structure of directly frozen myocardial membranes with conventionally prepared tissue. The E face was most affected by lack of chemical pretreatment. First, our laboratory reported an increase in E face particle density from 379 +/- 30/micron 2 in conventional fixed tissue to 489 +/- 18/micron 2 in unpretreated tissue. Discrete arrays of 12-15 nm particles on the E face were a striking feature of the unfixed sarcolemma. However, P face intramembrane particle (IMP) density remained unchanged from previous estimates in fixed tissue. Specialized regions of the sarcolemma were enhanced in ultrarapidly frozen tissue. Particle domains of the adherens junctions were very prominent in forming a cap alongside the gap junctions. Both the P and E faces of the gap junctions were highly ordered into hexagonal arrays. Caveolae in the membrane were infrequent in both P and E faces.

Effects of aspirin on tight junction structure of the canine gastric mucosa

The effects of aspirin on the canine gastric mucosal barrier were examined using the freeze-fracture and extracellular tracer techniques. Aspirin treatment (3, 20, or 40 min) resulted in alterations in tight junction complex morphology and permeability. Discontinuities in the apical occluding complex, hyperplastic tight junctions (extensions of the apical tight junction strands radiating over the lateral plasma membrane), and a variability in the number of strands (1-20) comprising the complex were observed. A concurrent increase in lanthanum permeability between nonnecrotic surface mucous epithelial cells was also demonstrated. The results of these experiments may suggest that aspirin-induced impairment of the tight junction complexes between viable gastric mucosal epithelial cells may be a major contributing factor in the etiology of stomach disorders.

The ultrastructure of the myocyte in different regions of experimental infarcts in the cat heart

The ultrastructure of myocytes was studied in the left ventricular myocardium of the cat heart after 3 h of LAD ligation. The ischemic, borderline, and normally perfused myocardium was defined by in vivo injection of fluorescein and by regional myocardial blood flow measurements with 15.5 microns radio-labeled microspheres. A semiquantitation of the number of irreversible injured cells in per cent of total counted in the three different zones showed that 53%-63% were irreversibly injured in the ischemic zone, 7%-26% in the borderline area, while none were irreversibly injured in the normally perfused myocardium. The interior excitation-contraction couplings in the normally perfused myocardium comprise interior dyads, triads, reversed triads, and encircling couplings. While the couplings in general were structurally resistant to ischemia, injured interior couplings were apparent in severely damaged cells of the hypoperfused tissue. Such injuries comprise a widening of the junctional gap and a disintegration of the junctional processes.

Caveolae preservation in the characterization of human neuromuscular disease

We have examined freeze-fracture replicas and conventional thin-section images of rat myofibers prepared by perfusion and by conventional immersion fixation protocols, and myofibers of normal and dystrophic human myofibers prepared by similar immersion fixation methods. In both rat and human myofibers, the size and distribution of caveolae was found to differ substantially according to the method of glutaraldehyde exposure, the depth of the myofiber from the surface exposed to the fixative, and if surgically bisected, the distance from the cut end of the myofiber. Conventional immersion fixation resulted in unavoidable but predictable alterations in sarcolemmal caveolae. These reproducible artifacts of fixation technique substantially complicate the use of caveolae as reliable markers for the characterization of human neuromuscular disease.

Structure-function studies of canine cardiac sarcolemmal membranes. II. Structural organization of the sarcolemmal membrane as determined by electron microscopy and lamellar X-ray diffraction

The morphological and ultrastructural properties of highly purified canine cardiac sarcolemmal vesicles, prepared by a modification (Colvin, R.A., Ashavaid, T.F. and Herbette, L.G. (1985) Biochim. Biophys. Acta 812, 601-608) of the method of Jones et al. (Jones L.R., Madlock, S.W. and Besch, H.R. (1980) J. Biol. Chem. 255, 9971-9980), were examined by several techniques. Thin-section electron microscopy showed predominantly intact unilamellar vesicles with little staining beyond the lipid bilayer boundaries. Freeze-fracture electron microscopy demonstrated that the majority of particles are approx. 90 A diameter and present at a density of 780 +/- 190 micrometers-2 (+/- S.D.). If it is assumed that some of these particles represent the (Na+ + K+)-ATPase, the finding that they are largely confined to the convex fracture face suggests a predominant right-side-out orientation of these sarcolemmal vesicles that is consistent with biochemical assays. The sarcolemmal membrane width measured by electron microscopy (unhydrated membrane width of 50-70 A) is consistent with the unit cell dimensions of 56-77 A determined by lamellar X-ray diffraction (hydrated membrane width). A unit cell dimension of 56-62 A was also found by X-ray diffraction for sarcolemmal lipids extracted from these preparations, indicating that the isolated sarcolemmal preparations do not contain a significant surface coat (glycocalyx). As both cardiac and skeletal sarcoplasmic reticulum membranes have a 80-100 A membrane width, these findings demonstrate that the purified sarcolemmal membrane is structurally distinct from both cardiac and skeletal sarcoplasmic reticulum. In contrast to the protein-rich skeletal sarcoplasmic reticulum membrane, which contains a single essential protein responsible for the regulation of cytosolic Ca2+ concentration, the sarcolemma is a lipid-rich membrane that contains a variety of proteins associated with many regulatory functions served by this membrane in cardiac muscle.

Ischemia-induced changes in sarcolemmal (Na+, K+)-ATPase, K+-pNPPase, sialic acid, and phospholipid in the dog and effects of the nisoldipine and chlorpromazine treatment

This work was undertaken to study functional and structural changes of the cardiac sarcolemmal membrane which was isolated from the ischemic lesion in the dog. The sarcolemmal fraction was prepared, by adopting the method devised by Reeves and Sutko , from the right ventricle and the subendocardial and subepicardial layers of the left ventricle. Ischemic lesion was produced by occlusion of a branch of the left anterior descending coronary artery for a period of 1.5 hr in the thoracotomized dog, followed by release of the occlusion for 3 hr. Nisoldipine, 5 micrograms/kg, was given twice intravenously, and chlorpromazine was infused at a rate of 10 micrograms/kg X min, in addition to the administration of twice bolus doses of 400 micrograms/kg each. Nisoldipine significantly decreased the incidence of premature ventricular contractions and microvascular hemorrhage. Sarcolemmal purity was monitored by using enzyme and chemical markers; the results indicated that the membrane preparation was tenfold purified over the homogenate. Although the activities of ouabain-sensitive (Na+, K+)-ATPase and ouabain-sensitive K+-p-nitrophenylphosphatase ( pNPPase ) of the sarcolemmal preparation isolated from the subendocardial layer were similar to those from the subepicardial layer in the nonischemic left ventricle, a significant decrease in these activities was observed only when the sarcolemmal fraction isolated from the subendocardial layer of ischemic area was compared with that from the subendocardial layer of nonischemic area. In contrast, the sialic acid content of the sarcolemma from the ischemic subendocardial layer was significantly increased compared to that of the nonischemic subendocardial layer. No such changes occurred in sarcolemma prepared from the ischemic subepicardial layer. The total phospholipid content as well as phosphatidylcholine and -ethanolamine contents of the sarcolemmal membrane prepared from the subendocardial layer of ischemic area were significantly decreased compared to nonischemic area. Nisoldipine prevented the ischemia-induced alterations in sarcolemmal (Na+, K+)-ATPase, pNPPase , sialic acid and phospholipids of the subendocardial layer. Chlorpromazine showed a less consistent effect than did Nisoldipine under our experimental conditions. Our study thus demonstrates that the lipid component and function of cardiac sarcolemmal membrane are altered in the early ischemic lesion and that these alterations are nonuniform in distribution and are alleviated by some pharmacological intervention.

Alterations in the morphology of rabbit skeletal muscle plasma membrane during membrane isolation

This study describes changes in morphology of plasmalemma from fast skeletal muscle in the course of tissue disruption and isolation. We find that conditions used to solubilize muscle contractile elements, in the isolation of plasmalemma, including the use of 0.6 M KCl or 0.4 M LiBr in the cold (0-4 degrees C), lead to altered plasmalemma morphology. The intramembrane particles, as revealed by freeze-fracture electron microscopy, become aggregated, leaving large domains devoid of particles. The square arrays in the P face and the complementary "pits" in the E face also become aggregated, sometimes forming sizeable aggregates of square arrays. Thin-section electron microscopy using tannic acid enhancement reveals plasma membrane associated components, on both cytoplasmic and extracellular faces, are largely reduced by the salt treatment. Pyrophosphate and magnesium at lower concentrations, sometimes used instead of high salt, also resulted in particle aggregation, although less pronounced than with concentrated salt solutions. The plasma membrane-associated proteins on both plasma membrane surfaces were likewise decreased by this treatment. Pyrophosphate treatment also separated the basal lamina from the plasma membrane. Incubation of muscle in isoosmotic sucrose does not alter the morphology of the plasmalemma with regard to particle aggregation, diminution of membrane associated components, or separation of the basal lamina. Our observations suggest that membrane-associated protein and/or cytoskeleton constrains the mobility of components in the plane of the membrane and that removal of this constraint leads to aggregation of intramembrane particles.

The role of calcium in cellular dysfunction

In order to study the effects of substances with selective Ca2+ entry blocking properties which enhance the tolerance to ischemia by preventing a toxic calcium overload, attempts are made to localize calcium ultrastructurally. Under normoxic conditions, a mobile pool of Ca2+ is localized in synaptic vesicles and in mitochondria of brain cells; in mitochondria of cardiac, skeletal and vascular smooth muscle; and in the junctional SR of fast skeletal muscle. A plasmalemmabound pool of Ca2+ is present in cardiac and slow skeletal muscle. Ischemia or hypoxia induce marked shifts in calcium of both the mobile and the plasmalemma-bound pools. Cardiac and skeletal muscle mitochondria scavenge huge amounts of calcium, especially during the reperfusion period following prolonged circulatory arrest. The membrane-bound Ca2+ is lost under these conditions. In the hypoxic brain, the amount of intracellular calcium clearly parallels the degree of damage. Observations made on peripheral and brain blood vessels show that high amounts of precipitated Ca2+ cover the myofilaments upon induction of spasm. The antispasmodic effect of the selective Ca2+-entry blockers flunarizine and lidoflazine is morphologically characterized by the absence of Ca2+ over the myofilaments in muscle treated as such whereas the extracellular Ca2+ remained the same. This observation indicates that the entry of Ca2+ is blocked at the level of the plasma membrane. In the heart and brain these drugs preserve the structural integrity of the plasmalemma-surface coat complex and thereby protect the cells against the devastating consequences of cellular calcium overload.

The effect of a calcium channel antagonist, Nisoldipine, on the ischemia-induced change of canine sarcolemmal membrane

Ischemic injury was produced in the dog heart by occluding the left anterior descending coronary artery just below the second diagonal branch for a duration of 1.5 h followed by the release of occlusion. Nisoldipine, 3.5 micrograms/kg was injected intravenously 10 min before the occlusion and again 10 min before the commencement of reperfusion. The activity of serum creatine phosphokinase greatly increased after the reperfusion, and this increase was significantly suppressed by Nisoldipine. This drug, in addition, prevented ischemia-induced myocardial hemorrhage and premature ventricular contraction. Sarcolemmal membrane vesicles were prepared from an ischemic and non-ischemic portions of the myocardium 3 h after the commencement of reflow. The fraction was purified approximately 12-fold with respect to ouabain-sensitive (Na+K+)-ATPase as an indicator; contamination of mitochondria was minimum with cytochrome c oxidase as an indicator. Without treatment of Nisoldipine, the total amount of sarcolemmal phospholipid obtained from the ischemic area, as well as the amounts of phosphatidyl-choline and phosphatidyl-ethanolamine, were significantly decreased as compared with those obtained from the non-ischemic area. Nisoldipine treatment abolished the decrease in the sarcolemmal phospholipids, total as well as phosphatidyl-choline and -ethanolamine, induced by ischemia plus reperfusion. Therefore, our work indicates that the Ca++ channel antagonist, Nisoldipine, suppresses the ischemia-induced increase in phospholipid breakdown of cardiac sarcolemma probably through its inhibitory effect on the Ca++-mediated activation of membrane phospholipase, through its vasodilatory action, or both.

Permeability and structural studies of heart cell gap junctions under normal and altered ionic conditions

The permeability and ultrastructure of communicating junctions of cultured neonatal rat ventricular cells are examined under control conditions and during treatments which raise intracellular Ca2+. Lucifer Yellow (487 mol wt) is used to examine junctional permeability. Under normal ionic conditions dye transfer from an injected muscle cell to neighboring muscle cells occurs rapidly (in less than 6 sec) while transfer to neighboring fibroblasts occurs more slowly. Application of monensin, which results in a partial contracture with superimposed asynchrony, or A23187, which results in a partial contracture, do not inhibit the transfer of dye between the muscle cells. A23187 did result in junctional blockade between muscle cells and fibroblasts. Freeze-fractured gap junctions from control and monensin-treated cells exhibit no distinguishable differences. Center-to-center spacing was not significantly different, 9.0 nm +/- 1.4 SD versus 9.2 nm +/- 1.3 SD, respectively; and particle diameters were virtually unchanged, 8.69 nm +/- 0.9 SD versus 8.61 nm +/- 1.07 SD, respectively. These results suggest that concentrations of intracellular Ca2+ sufficient to support a partial contracture and asynchronous contractile activity do not result in a block of intercellular junctions in cultured myocardial cells. These results are discussed in terms of intracellular Ca2+ -buffering and junctional sensitivity to Ca2+.

Electrical coupling among heart cells in the absence of ultrastructurally defined gap junctions

Cells from the ventricles of 7-day chick embryos were aggregated into spheroidal clusters by 48 hr of culture on a gyratory platform. All aggregates beat spontaneously and rhythmically. Microelectrode impalement of widely separated cells within aggregates indicated that they were coupled, as evidenced by a mean coupling ratio (delta V2/ delta V1) of 0.81 +/- 0.09, and by simultaneity of intrinsic electrical activity (action potentials and subthreshold voltage fluctuation). In freeze-fracture preparations, the cell surfaces contained numerous small groups of intramembrane protein (IMP) particles, arranged in macular clusters, and linear and circular arrays. Using the criterion of 4 clustered IMP particles to defined a minimal gap junction, 0.27% of the total P-face examined was devoted to gap junctional area. Within such clusters particles were packed at about 8200/micrometer2; in nonjunctional regions, particles were scattered at a density of about 2000/micrometer2. When exposed to cycloheximide (CHX: 50 micrograms/ml) for 24--48 hr, coupling ratio declined to 0.44. This decrease could be attributed largely to leakiness of the nonjunctional membrane. Aggregates continued to beat rhythmically and in a coordinated fashion even after 72 hr in inhibitor. However, between 3--21 hr in CHX gap junctional area declined to 0.10%, and all particle clusters disappeared from the P-faces of aggregates in CHX for 24 or 48 hr. Neither macular nor linear particle arrays were seen. We conclude that organized gap junctions are unnecessary for electrotonic coupling between embryonic heart cells. These findings support the idea that low-resistance cell-to-cell pathways may exist as isolated channels scattered throughout the area of closely apposed plasma membranes.

Oxygen requirements, morphology, cell coat and membrane permeability of calcium-tolerant myocytes from hearts of adult rats

The morphological, functional, and biochemical properties of freshly isolated heart muscle cells were examined. A reproducible method for the separation and purification of such cells isolated from adult rat heart was developed. It yields an average of 5 X 10(6) striated rectangular cells which retain normal morphology (range 2.5 to 11 X 10(6) and 4 X 10(6) calcium-tolerant cells (range 2.5 to 5.5 X 10(6) per heart. After purification, 85 to 95% of the cells retain normal morphology in solutions of calcium ion activity equal to 10 microM, and 65 to 79% of the cells are rectangular in solutions of calcium ion activity equal to 1 mM. Under the light microscope we were able to identify functionally intact individual cells that are calcium-tolerant and contract only in response to electrical stimulations, as well as dying myocytes that beat spontaneously. The examination of such cells under the electron microscope permitted us to address the question: What is the sequence of structural changes in a dying cell? The sarcomere lengths measured both in the living state and after preparation for electron microscopy are in the physiological range. In steady states of oxygen tension, respiration of the intact cells is undiminished from 50 torr to 2 torr. The oxygen tension for half maximal respiration is 0.15 torr. Therefore, the limitation of oxygen diffusion to the mitochondria of isolated heart muscle cells must be remarkably small.

The role of postischemic recirculation in the development of ischemic neuronal injury following complete cerebral ischemia

The neuronal response to complete cerebral ischemia (CCI) of 5-15 min duration was evaluated at the light and electron microscopic level subsequent to postischemic recirculation periods of up to 60 min. Following postischemic reperfusion, the homogeneous neuronal changes characteristic of permanent CCI were modified into a heterogeneous pattern of selectively vulnerable neuronal responses. Four basic types of neuronal injury were represented within this heterogeneous neuronal population. The Type I neuronal response was most numerous and consisted of chromatin clumping, nucleolar condensation and a breakdown of polysomes. This response may represent a reversal of some of the neuronal changes observed after permanent CCI. In addition to the above changes, Type II neurons contained swollen mitochondria and Golgi saccules which appeared as microvacuoles under the light microscope. Type III neurons displayed varying degrees of neuronal shrinkage and numerous swollen mitochondria. Type IV neurons were markedly shrunken and electron-dense with few identifiable subcellular structures. The distribution of Type I neurons was random but the other neuronal responses occurred in "selectively vulnerable" brain regions. The number of Type II, III, and IV neurons increased with extended insult durations but were unaffected by the length of recirculation. Ten minutes of CCI represented the threshold for a significant increase in the number of severely altered neurons. These findings suggest that considerable neuronal injury may be present after 10-15 min of CCI, and the lack of a recirculation period following CCI appears to afford the brain parenchyma an extensive degree of structural protection.