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Biomedical subjects

G A Langer

Publications and source records attributed to G A Langer.

At least 73 records · Page 4Linked to original sources
PubMed1987

Cardiac ischemia. Part I--Metabolic and physiologic responses.

In the ischemic state, which leads to myocardial infarction, there is a gradation of cardiac muscle injury and a sequence of functional loss. On coronary occlusion an immediate cellular leak of potassium occurs and the rate of relaxation declines. Within 1 to 2 minutes there is complete loss of contraction followed by the onset of contracture in 7 to 10 minutes in isolated preparations. The major problem of this initial period, if the occlusion zone is not too great, is electrical dysfunction. The next 1 to 6 hours is the period of variable reversible injury. Positron emission tomography technique and fatty acid and carbohydrate tracers quantitatively assess regions that are metabolically competent. The problem is to maintain and improve the competence of these regions during reperfusion, whether by thrombolytic therapy (streptokinase) or tissue plasminogen activator, percutaneous transluminal angioplasty, acute coronary artery bypass graft or by total vented bypass and regional reperfusion without thoracotomy (procedure under development).

Animals↗
PubMed1986

Compartmentation of cellular calcium in rabbit ventricle dependent upon its transsarcolemmal route.

Augmentation of calcium influx may be buffered in the myocardium by slowly exchangeable compartments. Myocardial calcium influx was enhanced in rabbit interventricular septa by interventions which affect either calcium channel current (BAY k 8644) or Na/Ca exchange (ouabain, low Na0). The effect of these interventions on slowly exchangeable calcium was measured by performing 45Ca washouts. In addition, beating was resumed following a period of prolonged quiescence and the effect on slowly exchangeable calcium measured. Previous work had shown that at a Ca0 of 1.0 mM, low sodium perfusion induced a significant increment in slowly exchangeable calcium. The present investigation demonstrates that ouabain and resumption of stimulation following quiescence also augment this calcium. At 1.0 mM Ca0, as few as 15 beats administered in a 10 min interval were found to induce a significant increment in calcium content, which could be inhibited by the mitochondrial inhibitor CCCP but not by ryanodine. In contrast, BAY k 8644 did not augment slowly exchangeable calcium, although it produced at 0.5 mM Ca0 the same inotropic response as ouabain and 75 mM Na0. In conclusion, slowly exchangeable calcium is not affected by augmentation of calcium channel current, but is enhanced by ouabain, resumption of stimulation, and low Na0. A common characteristic shared by those interventions which enhance slowly exchangeable calcium is an increased Nai/Na0 ratio, which results in an augmented calcium uptake via on Na/Ca exchange.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1986

Role of sarcolemmal-bound calcium in regulation of myocardial contractile force.

A rapidly exchangeable component of cellular calcium plays a significant role in the control of force development in heart muscle. Recently completed studies indicate that a large fraction of this calcium is bound to sites on the sarcolemma: The curve that relates extracellular calcium ([Ca]0)(from 50 microM to 10 mM) to force development and the one that relates [Ca]0 to calcium bound to a highly purified sarcolemmal fraction are superimposable. The ability of a series of cations (lanthanum, cadmium, manganese, magnesium) to uncouple excitation from contraction is the same as their relative ability to displace calcium from sarcolemma. Polymyxin B, a highly charged cationic amphiphilic peptidolipid, specifically competes for calcium-binding sites on anionic and zwitterionic phospholipid. It is a potent displacer of calcium from myocardial cells and purified sarcolemma and a potent uncoupler. Phospholipase D cleaves the nitrogenous base from sarcolemmal phospholipid with production of anionic phosphatidic acid. Phospholipase D treatment increases calcium bound to cells and to purified sarcolemma and increases force development of ventricular tissue from both neonatal rats and adult rabbits. Insertion of charged amphiphiles in the sarcolemma as phospholipid analogues modulates interaction of calcium with myocardial sarcolemma. Anionic dodecylsulfate increases calcium bound to sarcolemmal vesicles by more than 80% and increases force development in rabbit papillary muscle by 100%. Cationic dodecyltrimethylamine produces a negative effect on binding and decreases contractile force by 70%. Neutral laurylacetate produces negligible effects on binding and force development. The effect of pH variation on calcium binding to phospholipid extracted from sarcolemma indicates that the latter accounts for at least 75% of the binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Augmentation of sarcolemmal Ca by anionic amphiphile: contractile response of three ventricular tissues.

The anionic amphiphile dodecyl sulfate was used at the concentration of 100 microM, below the concentration at which membrane permeability is compromised. In rat myocardial tissue culture dodecyl sulfate induced a large increase in Ca uptake in the intact cells, of which 84% was distributed in a rapidly exchangeable (t1/2 less than 18 s) and 16% in a slowly exchangeable (t1/2 = 17 min) compartment. Dodecyl sulfate induced a large increase in Ca bound to isolated sarcolemmal membrane of these cells. This increase in membrane binding and the distribution in whole cells is consistent with insertion of dodecyl sulfate in the sarcolemma (SL) with subsequent Ca binding to its anionic head group. With perfusate extracellular calcium concentration ([Ca]o) equal to 1 mM, dodecyl sulfate produced greater than 60% increase in active force in ventricular tissue from adult rabbit and neonatal rat but virtually no increase in adult rat ventricle. Preperfusion with 0.1 mM [Ca]o or 10(-6) M ryanodine markedly increased the relative response of adult rat ventricle to dodecyl sulfate. After a quiescent period in rabbit ventricle, dodecyl sulfate caused a progressive increase of force of each beat compared with control; i.e., the treppe response was increased at each successive beat. This did not occur in adult rat ventricle. These results further clarify the different quantitative role of SL-bound versus sarcotubular Ca in the hearts of different species.

Age Factors↗
PubMed1985

Passive calcium-buffering capacity of a rabbit ventricular homogenate preparation.

This study characterizes the passive Ca2+-binding capacity of a crude homogenate fraction prepared from rabbit ventricular tissue. The soluble and particulate fractions of the homogenate were separated by centrifugation, and the Ca2+-binding capacity of each fraction was measured separately with Ca2+-selective electrodes and then summed to obtain the Ca2+-binding capacity of the total tissue homogenate. Qualitatively, the Ca2+ bound to the homogenate exhibited both relatively high- (Km less than 1 microM) and low-affinity (Km greater than 10 microM) sites over the physiological range of free Ca2+ concentrations. The total homogenate bound 72, 128, and 308 mumol Ca2+/kg wet wt tissue at 1, 10, and 50 microM free Ca2+ concentrations, respectively. The Ca2+ bound to the homogenate was mostly La3+ displaceable, which suggests that it represents a largely noncompartmentalized, rapidly exchangeable fraction of Ca2+. Myofibrillar ATPase activity was examined as a function of free Ca2+ concentration in an ionic medium similar to that used in the Ca2+-binding experiments. At free Ca2+ concentrations that stimulated myofibrillar ATPase activity half maximally, the measured rise in Ca2+ bound to the homogenate above resting values was greater than 100 mumol/kg wet wt. These data provide base-line values for the passive Ca2+-buffering capacity of the myocardium, although relevance to the physiological setting must be interpreted with caution.

Adenosine Triphosphatases↗
PubMed1985

Charged amphiphiles regulate heart contractility and sarcolemma-Ca2+ interactions.

We have used charged amphiphiles as phospholipid analogues to modulate the interaction of Ca2+ with myocardial sarcolemma. The amphiphiles were dodecyl sulfate, dodecyltrimethylamine, and lauryl acetate; these are anionic, cationic, and neutral molecules, respectively. The hydrophobic alkyl chain is identical in each case; only the hydrophilic head group is varied. The anionic dodecyl sulfate (50-100 microM) augmented the Ca2+ binding and Na+-Ca2+ exchange activity of sarcolemmal vesicles by approximately 80% and increases the contractility of rabbit papillary muscle. Both developed force and its derivative were increased by approximately 100% with no increase in rest tension. Cationic dodecyltrimethylamine (20-100 microM) produced nearly opposite effects. Neutral lauryl acetate (100 microM) had little apparent effect on any measurement. The charged amphiphiles should be useful tools for further elucidation of excitation-contraction coupling processes in a variety of contractile tissues.

Acetates↗
PubMed1985

Contribution of myocardial diffuse double-layer calcium to contractile function.

The role of diffuse double-layer calcium in cardiac excitation-contraction coupling was examined using rabbit interventricular septa, cultured neonatal rat myocardial cells, and gas-dissected sarcolemmal membranes. The diffuse double layer refers to the space directly adjacent to the sarcolemma where the ionic composition of the media is a direct function of the membrane surface potential. The divalent cation dimethonium was used as a specific probe for the diffuse double layer. According to Gouy-Chapman theory, replacement of sodium with sucrose should increase the amount of calcium located in this compartment. Dimethonium (10 mM) was found to decrease calcium uptake and contractility during low-sodium (33 mM) perfusion when the perfusate contained sucrose but not LiCl. Dimethonium did not decrease calcium uptake or contractility during control perfusion. The results suggest that calcium present in the myocardial diffuse double layer can be augmented or reduced in accordance with Gouy-Chapman theory. Changes in diffuse double-layer calcium are accompanied by small (7.8%) but significant changes in contractility.

Animals↗
PubMed1985

Phospholipase D produces increased contractile force in rabbit ventricular muscle.

Inclusion of phospholipase D (Streptomyces chromofuscus, 5 U/ml) in the medium perfusing rabbit papillary muscles increased peak force development by 78% and peak dF/dt by 89%. The maximal contractile response occurred 40-50 minutes after addition of the enzyme to the perfusate. As peak contractile response developed, aftercontractions appeared in most muscles. The inotropic response was slowly reversible, with disappearance of aftercontractions, upon discontinuation of the enzyme. Phospholipase D produces a specific increase of phosphatidic acid in the sarcolemma and, therefore, an increase in net anionic charge on the membrane. It has been shown previously that phospholipase D induces a large increase in sarcolemmal calcium binding. We propose, on the basis of the present study, that phospholipid-bound calcium may play a significant role in the control of contractile force in mammalian myocardium.

Animals↗
PubMed1985

The effect of pH on cellular and membrane calcium binding and contraction of myocardium. A possible role for sarcolemmal phospholipid in EC coupling.

Calcium binding to cultured whole cell monolayers, to their membranes made by "gas dissection," and to sarcolemmal vesicles from canine heart, is measured as a function of pH from 5.5-8.5. The effects on binding are compared to the contraction response of the cultured cells over the same pH range. All responses to pH, including calcium binding and contraction, are well described (r = 0.98-0.99) by a relation in which calcium binding depends primarily upon the extent of ionization of sarcolemmal binding sites. The effect of pH on calcium binding to phospholipid extracted from sarcolemmal vesicles suggests that the phospholipid accounts for at least 75% of the binding. The extent of ionization of the sites is pH-dependent and predicted by a form of the Henderson-Hasselbach relation with pK of the putative sites between 6.60 and 7.15. As pH increases from 5.5-8.5, membranous calcium binding, cellular calcium uptake, and contraction amplitude increase proportionally. Cellular calcium uptake increases by 4.4 mmol/kg dry weight cells (0.75 mmol/kg wet weight). Sixty percent to 65% of this increment is rapidly exchangeable and lanthanum displaceable, indicative of sarcolemmal localization. The remainder enters a slowly exchangeable (t1/2 = 26 minutes) compartment not directly related to support of contraction. The study supports the hypothesis that calcium bound to sarcolemmal sites plays a critical role in control of myocardial contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Surface structures of the heart cell and their role in contractile control.

Studies which focus on calcium (Ca) at the cellular surface indicate: Ca in the vascular and interstitial spaces is in rapid equilibrium with that bound within the sarcolemmal-glycocalyx complex of the cell. At [Ca]o of 1.0-1.5 mM 80-85% of this Ca is bound to phospholipid of the sarcolemmal bilayer and amounts to 500-700 mumoles/kg wet wt. tissue. Phospholipids of net anionic charge (phosphatidylserine, phosphatidylinositol) or zwitterionic structure (phosphatidylethanolamine) are likely to play a predominant role in the binding. Augmentation of net anionic charge (formation of phosphatidate by phospholipase D, addition of dodecylsulfate) of the sarcolemma augments Ca binding and is positively inotropic. Conversely, insertion of a cationic amphiphile (Polymyxin B, dodecyltrimethylamine) displaces Ca and is negatively inotropic. Augmentation of anionic Ca binding sites induces a significant increase of Na-Ca exchange in sarcolemmal vesicles which may contribute to the associated positive inotropy found in the intact myocardium.

Animals↗
PubMed1984

Effects of low sodium perfusion on cardiac caffeine sensitivity and calcium uptake.

Myocardial compartmentation of calcium was investigated in the arterially perfused rabbit interventricular septum under conditions of augmented calcium uptake. Reduction of perfusate sodium concentration (100-36 mM [Na]0) produced the expected increased active force development and an increased myocardial calcium content that was inversely proportional to [Na]0. Caffeine was used to inhibit calcium uptake by the sarcoplasmic reticulum (SR) and to stimulate SR calcium release. The diastolic tension response to caffeine was also inversely proportional to [Na]0: at [Na]0 of 139 mM, 10 mM caffeine increased diastolic tension by 20%; whereas at 36 mM [Na]0 diastolic tension increased by 205%. The increase in diastolic tension in response to caffeine was considered a reflection of increased cytosolic calcium. The increase in diastolic tension with caffeine required that reduced [Na]0 be present at the time caffeine was administered. Caffeine sensitivity (measured by an increase in diastolic tension) and active force development declined to control levels within 3 minutes after the end of a 40 minute period of low [Na]0 perfusion despite the presence of an additional 1 mmole calcium per kg dry wt in the muscle at the 3 minute mark when caffeine was added. The results indicate that low [Na]0 perfusion induces an increment in myocardial calcium content, a major fraction of which is neither related directly to contractility nor involved in the response to caffeine.

Animals↗
PubMed1984

Calcium compartmentation in cardiac tissue culture: the effects of extracellular sodium depletion.

The effect of extracellular sodium depletion upon cellular calcium distribution in myocardial tissue culture is studied with the use of sucrose or lithium chloride as substitutes for sodium. With sucrose substitution more than 50% of the increase in calcium at the cellular surface secondary to [Na]0 depletion is probably localized to a screening layer with the remainder bound to the sarcolemma of the cell. Calcium distribution (sucrose substituted) is studied under three different perfusion conditions: (1) In HEPES-buffered perfusate (pH = 7.1-7.35) the 113% gain in Ca at the cellular surface upon 33 mM [Na]0 perfusion is rapidly exchangeable, completely lanthanum (La) displaceable and more than 90% reversible. 5 X 10(-5) verapamil does not affect the response. (2) In 10 mM Pi (phosphate) solution at pH = 7.15, in which a slowly exchangeable mitochondrial Ca compartment is added, the gain in Ca after low [Na]0 is again rapidly exchangeable and 85% reversible that is, similar to the response in HEPES-buffered solution. (3) In 10 mM Pi solution at pH = 7.35 in which slow phase calcium uptake in 133 mM [Na]0 is increased, the initial rapid Ca response to low [Na]0 is similar to that in (1) and (2) but the slow phase uptake rate is further increased by 3.75 times. The major portion of this uptake is not reversible upon return to 133 mM [Na]0. The results distinguish two markedly different effects of [Na]0 depletion in tissue culture dependent upon the pre Na-depletion state of Ca compartmentation: (1) Addition of almost all the calcium to rapidly exchangeable sites at the cellular surface when exchange is limited to these sites and to mitochondria. (2) Further addition of calcium to more slowly exchangeable cellular site(s) when slow phase uptake is increased, by elevation of Pi at pH = 7.35, prior to low [Na]0 perfusion.

Animals↗
PubMed1984

Calcium at the sarcolemma.

Experiments in mammalian ventricle in which calcium (Ca) is removed from and replaced to arterial perfusate indicate that a component of Ca important in contractile control is bound to rapidly exchangeable cellular sites. These sites appear to be in rapid equilibrium with extracellular Ca and are replenished, beat-to-beat, from the extracellular source. The binding sites appear to be anionic phospholipids of the sarcolemma. They are visualized to represent a store of sarcolemmal Ca which, upon membrane excitation, supplies the systems responsible for transsarcolemmal movement--the Ca channel and the Na-Ca exchanger. The amount of Ca bound to sarcolemma is an important factor in control of the amount of Ca which enters the cell via the channel or exchanger. Conversely, there is evidence that suggests that the activity of (at least) the exchanger determines, in part, the quantity of Ca bound. Increased activity of the Na-Ca exchanger is associated with increased Ca-binding to sarcolemmal sites.

Animals↗
PubMed1984

Phospholipase D increases cell surface Ca2+ binding and positive inotropy in rat heart.

An increase in the content of anionic phospholipid (AP) in the sarcolemma may lead to increased Ca2+ binding and a concomitant inotropic response. To test this hypothesis we treated cultured neonatal rat myocardial cells with phospholipase D (PLD), an enzyme that converts membrane phospholipids to phosphatidic acid. PLD treatment resulted in an increase in total exchangeable Ca2+ of 36%, or 1.56 +/- 0.27 mmol Ca2+/kg dry wt, 79 +/- 3% of which remained displaceable by La3+. The quantity of Ca2+ displaced by polymyxin B, a drug that displaces Ca2+ preferentially from anionic phospholipid sites, increased by 85% from a predicted 0.74 +/- 0.07 to 1.37 +/- 0.19 mmol Ca2+/kg dry wt. Simultaneously the contractility of neonatal rat ventricular tissue increased by 1.7- to 2.5-fold. Spontaneous electrical activity of treated cells was not significantly altered. Thus PLD treatment, which increases the quantity of AP in the membrane, resulted in an 85% increase in Ca2+ bound to AP and concomitantly resulted in a significant increase in contractility. Present results, in association with published results on the effect of PLD on Na+-Ca2+ exchange (J. Biol. Chem. 259: 16-19, 1984), indicate that Ca2+ bound to anionic sarcolemmal phospholipids plays a major role in the control of transsarcolemmal Ca2+ flux and force development.

Animals↗
PubMed1984

Transsarcolemmal calcium movements in arterially perfused rabbit right ventricle measured with extracellular calcium-sensitive dyes.

Activation-dependent changes of mean extracellular calcium concentrations were monitored spectrophotometrically in arterially perfused right ventricle of rabbit via extracellular application of the calcium-sensitive absorption dyes, antipyrylazo III and tetramethylmurexide. After rest periods of 3 minutes or longer, 10% of total dye-accessible calcium (300-500 microM) is depleted cumulatively from the extracellular space by four to eight beats at 2 Hz stimulation. During continued stimulation, or during quiescence following a depletion response, mean extracellular calcium concentrations return toward the prestimulatory level in the course of 2-4 minutes. A single stimulation placed 2-30 seconds subsequent to rapid pacing results in a potentiated beat with an accompanying net increment of mean extracellular calcium. Total dye-accessible calcium can increase by at least 4% at such a beat, and depletion responses can be repeated immediately thereafter. During a sequence of such responses, extracellular calcium lost cumulatively during three to five rapid beats is replenished, for the most part, by a single post-stimulatory beat. These results demonstrate that most of the "activator pool" of cellular calcium can turn over to the extracellular space during a single contraction cycle in rabbit myocardium, and suggest that, in mammalian myocardium, diastolic calcium efflux may be quantitatively negligible in relation to efflux during excitation-contraction coupling.

Animals↗
PubMed1983

Ca2+ displacement by Polymyxin B from sarcolemma isolated by 'gas dissection' from cultured neonatal rat myocardial cells.

Amphiphilic, cationic Polymyxin B is shown to displace Ca2+ from 'gas dissected' cardiac sarcolemma in a dose-dependent, saturable fashion. The Ca2+ displacement is only partially reversible, 57% and 63%, in the presence of 1 mM or 10 mM Ca2+, respectively. Total Ca2+ displaced by a non-specific cationic probe, lanthanum (La3+), at maximal displacing concentration (1 mM) was 0.172 +/- 0.02 nmol/microgram membrane protein. At 0.1 mM, Polymyxin B displaced 42% of the total La3+-displaceable Ca2+ or 0.072 +/- 0.01 nmol/microgram protein. 5 mM Polymyxin displaced Ca2+ in amounts equal to those displaced by 1 mM La3+. Pretreatment of the membranes with neuraminidase (removal of sialic acid) and protease leads to a decrease in La3+-displaceable Ca2+ but to an increase in the fraction displaced by 0.1 mM Polymyxin from 42% to 54%. Phospholipase D (cabbage) treatment significantly increased the La3+-displaceable Ca2+ to 0.227 +/- 0.02 nmol/microgram protein (P less than 0.05), a gain of 0.055 nmol. All of this phospholipid specific increment in bound Ca2+ was displaced by 0.1 mM Polymyxin B. The results suggest that Polymyxin B will be useful as a probe for phospholipid Ca2+-binding sites in natural membranes.

Animals↗