The effects of insulin and anoxia on the metabolism of isolated mature rat cardiac myocytes

Use of Isolated Adult Myocytes to Evaluate Cardiotoxicity. II. Preparation and Properties*

The preparation and properties of isolated adult cardiac myocytes are reviewed, with the goal being to evaluate their usefulness as a model system for measuring cardiotoxicity. Some important factors in cell isolation methodology which impact on the quality of the preparation are identified, along with criteria for assessing the quality of cells after isolation. By all criteria, myocytes isolated by good procedures appear to largely retain their original properties. Moreover, the distinctive behavior of adult myocytes under metabolic stress endows them with a particular usefulness as monitors of toxicity. Overall, we conclude that the art of adult heart cell isolation and culture is now sufficiently advanced for either freshly isolated cells in suspension or cells in culture to be a useful model system for toxicity studies.

Compartmentalization of non-adenine nucleotides in anoxic cardiac myocytes

Loss of 5'-nucleotides from cardiac myocytes is a distinguishing feature of myocardial ischemia. Previous work has documented dislocations of metabolic processes mediated by both purine and pyrimidine nucleotides, especially the adenine nucleotides. This study was designed to establish the extent of anoxia-induced depletion of non-adenine nucleotides in the cytosolic compartment of heart muscle cells. Cardiac myocytes were incubated aerobically (O(2)) or anoxically (N(2)) for 30 or 60 min; anoxic cells at both time points were reoxygenated for 10 min. Roughly 85-90% of cytosine triphosphate (CTP) and uridine triphosphate (UTP) were cytosolic under aerobic conditions, compared with 62% of guanosine triphosphate (GTP) and 90% of adenosine triphosphate (ATP) under similar conditions. Similarly, the total cytidine and uridine nucleotide pool of aerobic myocytes was 70-90% cytosolic vs. 61% of total guanine nucleotides and 78% of total adenine nucleotides. After the onset of anoxia, cytosolic nucleotides (principally the triphosphate forms) were quickly degraded. Reoxygenation of anoxic myocytes for 10 min allowed some recovery of ATP, GTP, and CTP, but very little recovery of UTP. The recovered nucleotide appeared almost exclusively in the cytosol. These results support the concept that non-adenine nucleotides could reach critically low levels in anoxic or ischemic heart in advance of adenine nucleotides. The importance of the depletion of non-adenine nucleotides is discussed in terms of the energetic needs of the myocyte, and the need for the cell to drive G-protein-coupled reactions, lipid synthesis, and glycogenesis.

Participation of glucose transporters on atrial natriuretic peptide-induced glucose uptake by adult and neonatal cardiomyocytes under oxygenation and hypoxia

Natriuretic peptides, beside their endocrine actions, have paracrine functions which include regulating glucose uptake and metabolism. Atrial natriuretic peptide (ANP) actions are mediated by cGMP which is implicated in the metabolic adaptation of glucose metabolism to oxygen deprivation in the heart. Although, it has been reported that ANP increases glucose uptake, cGMP decreases it. The aim of the present paper was to evaluate the role of the glucose transporters 1 and 4 (GLUTS), in glucose uptake produced by ANP in fatty acid-dependent adult cardiomyocytes and glucose-dependent neonatal cardiomyocytes under oxygenation and hypoxia, which reverts adult metabolism to glucose-dependent. We also explored if the calcium-calmodulin complex participates in ANP-induced increase in glucose uptake. Neonatal cells had a higher glucose uptake than adult cells and GLUT 1 participated in basal uptake in both cell types. Hypoxia increased glucose uptake in adult cardiomyocytes but not in neonatal cells and this increase in glucose uptake was mediated by GLUT4. ANP increased glucose uptake in both adult and neonatal myocytes, under oxygenation and hypoxia, and GLUT4 favored this increase. Neonatal cells were less sensitive to ANP. Trifluoperazine, a calcium-calmodulin blocker, inhibited the ANP-induced increase in glucose uptake. This suggests that ANP promotes GLUT 4 calcium-mediated recruitment to the cell membrane. In conclusion, glucose uptake regulation is one of the paracrine metabolic effects of ANP in adult and neonatal cardiomyocytes under oxygenation and hypoxia. This effect of this peptide could explain the beneficial effects found in the internal medicine and surgical fields.

Effects of polarizing solution on glucose uptake of rat oxygenated or hypoxic ventricular myocytes

1. Although adult cardiac metabolism depends on fatty acids rather than on carbohydrates, hypoxic hearts consume glucose and infusion with a glucose-insulin-potassium solution (GIK) helps in the recovery of the pressure register, the heart rate and electrocardiogram of rat hypoxic whole hearts. 2. Rat myocytes obtained by enzymatic disaggregation captured significant amounts of glucose in Tyrode's solution (9.8 +/- 2.05% during oxygenation and 16.26 +/- 2.76% during hypoxia) after 60 min incubation. Cells incubated with GIK showed a significantly increased glucose uptake (to 30.04 +/- 1.99% during oxygenation and to 37.06 +/- 1.97% during hypoxia). 3. Glucose was the component of the Tyrode's solution that most enhanced its own entry, increasing glucose entry to 45.42 +/- 2.64% with oxygen and 41.42 +/- 2.69% under hypoxia. 4. Antibodies against glucose transporters 1 and 4 (GLUT-1 and -4, respectively) decreased glucose transport during GIK treatment under hypoxia. 5. We conclude that GIK has beneficial effects mediated by GLUT-1 and -4 during hypoxia, inducing recovery of the damaged tissue.

The study of oxidative stress in freshly isolated Ca(2+)-tolerant cardiomyocytes from the adult rat

Cardiotoxicity studies using isolated heart cells are becoming increasingly advocated as a supplement to, and sometimes as a replacement for, whole heart or whole animal experimentation. In fact, the use of isolated cardiomyocytes has the great advantage of enabling mechanistic and comparative studies of compounds, which are directly toxic to cardiomyocytes. Since the 1970s, different procedures have been developed in order to obtain Ca(2+)-tolerant cardiomyocytes. The advances in this field will be reviewed and an optimised method to obtain freshly isolated Ca(2+)-tolerant cardiomyocytes from the adult rat for use in toxicological studies will be described. With this procedure, a high number of rod-shaped cells can be obtained (6-7 x 10(6)/heart corresponding to 70% of total number of cells). It is also possible to maintain cell viability, glutathione content and enzymatic activity of glutathione reductase (GR), glutathione peroxidase (GPX) and glutathione S-transferase (GST) in acceptable levels for 4 hours. Cardiotoxicity studies performed with isoproterenol (ISO) in the presence of copper and with the model toxic substance tert-butylhydroperoxide (t-BHP) demonstrate the importance of oxidative stress as a cardiotoxic mechanism elicited by these molecules. The results obtained are also good indicators for future applications of this methodology to other cardiotoxicity studies.

Isolation and cultivation of Ca2+ tolerant cardiomyocytes from the adult rat: improvements and applications

1. Primary cultures of cardiomyocytes provide a valuable tool for the study of the pharmacological and toxicological properties of drugs and chemicals, but for several technical reasons cardiomyocytes from adult animals are not routinely used in long-term culture. Because of significant advances in cardiovascular research, tissue engineering and cell transplantation, the need to isolate primary cells from adult animal and/or human tissue is likely to increase in the future. 2. The most common protocols for the isolation and cultivation of cardiomyocytes have been reviewed and the various approaches have been compared. The recent advances in cell culture techniques and the use of the cytoprotective agent, e.g. 2,3-butanedione monoxime greatly increases cell yield and cell viability of isolated and cultured cardiomyocytes. New concepts emerge that enabled an assessment of cellular differentiation in cultured cardiomyocytes and certain specific nuclear transcription factors may play a pivotal role in this process.

Oxygen sensitivity of mitochondrial metabolic state in isolated skeletal and cardiac myocytes

In striated muscle the coupling of blood flow to changes in tissue metabolism is hypothesized to be dependent in part on release of vasodilating metabolic by-products generated when mitochondrial metabolism becomes O2 limited. Cytochrome oxidase, the terminal step in oxidative phosphorylation, is half-maximally saturated at < 1 mmHg PO2 in isolated mitochondria. However, blood flow is regulated at tissue PO2 of approximately 20 mmHg. If the affinity of mitochondrial respiration for O2 were higher in vivo than in vitro, O2 limitation of mitochondrial metabolism near mean tissue levels could occur. In the present study the PO2 at which mitochondrial metabolism becomes inhibited (critical PO2) was measured for cardiac myocytes in suspension (1.1 +/- 0.15 mmHg) and single cells (1.0 +/- 0.22 and 1.25 +/- 0.22 mmHg in cardiac myocytes and rat spinotrapezius cells, respectively). These measurements are consistent with those from isolated mitochondria, indicating that vasodilators produced when oxidative phosphorylation becomes inhibited may be important for regulating blood flow only in highly glycolytic muscles or under conditions of severe O2 limitation.

Deformation of the Bowditch staircase in Ca(2+)-overloaded mammalian cardiac tissue--a calcium phenomenon?

Concentrations of 1-4 mumol l-1 isoproterenol cause both in right ventricular papillary muscles and in enzymatically isolated myocytes of the guinea-pig a Ca2+ overload-induced state which is functionally characterized by biphasic (multiphasic) twitches and biphasic (multiphasic) intracellular calcium transients, respectively, during excitation-contraction coupling. This state was stabilized in the in vitro experiments for some hours by a co-ordination of the interstimulus interval, the temperature of the superfusion fluid and the addition of calcium agonists. The functional stability is the precondition for the reproducibility of the experimental results particularly after the application of long-lasting stimulation programmes. Changes in the shape of biphasic contractions were compared with changes in the time course of biphasic intracellular calcium transients using three manipulations of a different kind: (1) the interruption of the steady pacing rhythm, (2) the variation of the interstimulus interval, (3) the addition of ryanodine. It was shown that: (1) The BOWDITCH staircase in calcium overloaded multicellular preparations is changed in that each individual component of the twitch passes through its own staircase. A homologous behaviour can be observed in the configuration of the phasic and tonic component of biphasic intracellular calcium transients. (2) At different driving frequencies the relative proportion of the two components of a biphasic twitch corresponds to the time integrals of the two components of biphasic intracellular calcium transients. (3) Ryanodine suppresses both the first component of the biphasic twitch and the phasic component of the biphasic intracellular calcium transient. The SR Ca(2+)-ATPase inhibitor thapsigargin increases the second component of the biphasic calcium transient. This supports the hypothesis that the size of the tonic component is in part determined by intracellular calcium reuptake. The results of both kinds of experiments would be compatible with the assumption that in calcium overloaded mammalian cardiac cells calcium reaches the contractile system directly as well as via two intracellular stores ('extended two-Ca-store concept').

Some growth factors stimulate cultured adult rabbit ventricular myocyte hypertrophy in the absence of mechanical loading

Cultured adult rabbit cardiac myocytes treated with recombinant growth factors display enhanced rates of protein accumulation (ie, growth) in response to insulin and insulin-like growth factors (IGFs), but epidermal growth factor, acidic or basic fibroblast growth factor, and platelet-derived growth factor failed to increase contractile protein synthesis or growth of the heart cells. Insulin and IGF-1 increased growth rates by stimulating anabolic while simultaneously inhibiting catabolic pathways, whereas IGF-2 elevated growth modestly by apparently inhibiting lysosomal proteolysis. Neutralizing antibodies directed against either IGF-1 or IGF-2 or IGF binding protein 3 blocked protein accumulation. A monoclonal antibody directed against the IGF-1 receptor also inhibited changes in protein turnover provoked by recombinant human IGF-1 but not IGF-2. Of the other growth factors tested, only transforming growth factor-beta 1 increased the fractional rate of myosin heavy chain (MHC) synthesis, with beta-MHC synthesis being elevated and alpha-MHC synthesis being suppressed. However, the other growth factors were able to modestly stimulate the rate of DNA synthesis in this preparation. Bromodeoxyuridine labeling revealed that these growth factors increased DNA synthesis in myocytes and nonmyocytes alike, but the heart cells displayed neither karyokinesis or cytokinesis. In contrast, cocultures of cardiac myocytes and nonmyocytes and nonmyocyte-conditioned culture medium failed to enhance the rate of cardiac MHC synthesis or its accumulation, implying that quiescent heart cells do not respond to "conditioning" by cardiac nonmyocytes. These findings demonstrated that insulin and the IGFs promote passively loaded cultured adult rabbit heart cells to hypertrophy but suggest that other growth factors tested may be limited in this regard.

Evaluation of chloroform cardiotoxicity utilizing a modified isolated rat cardiac myocytes

An improved technique for the isolation of cardiac myocytes from adult rat is described. This method produced a high yield of isolated cardiomyocytes (30-35 x 10(6) cells/heart) with viability over 88%. The rod-shaped cells represented about 70% of the viable cardiac myocytes. The leakage of creatine phosphokinase (CPK) in the incubation medium from the control was about 37% at zero time. This model was used to investigate chloroform (CHCl3) cardiotoxicity by measuring dye exclusion, change the viable cardiac myocytes shape and CPK leakage. As a result of CHCl3 exposure, there was a significant decrease in Trypan blue exclusion after 30 min and during the entire time course. The percentage of rod-shaped cells decreased significantly during the whole period of the study. Also, a significant elevation in CPK leakage after treatment with 1000 ppm CHCl3 was observed. The data show that the present technique for cardiomyocytes isolation from adult rats produced high percentage of viable cells. Furthermore, the toxic effect of CHCl3 on cardiac myocytes was time dependent.

Guanosine metabolism in adult rat cardiac myocytes: ribose-enhanced GTP synthesis from extracellular guanosine

The metabolic fate of transported guanosine was examined in adult rat cardiac myocytes. Freshly isolated cells were incubated with 10 microM or 100 microM [3H]guanosine and the nucleotide products extracted and examined for radiolabel distribution. The data presented show significant incorporation of guanosine into the 5'-nucleotide pool, and a marked stimulation of that incorporation by ribose. An average of 233 pmol/mg cell protein extracellular guanosine was incorporated into the cellular 5'-nucleotides over 90 min at both 10 microM and 100 microM external nucleoside. This appeared primarily as GTP (approx. 204 pmol/mg cell protein in 90 min). Only guanine nucleotides contained radiolabel; adenine nucleotides and IMP remained unlabelled even after 90 min incubation of the cells with [3H]guanosine. Addition of 5 mM ribose to the medium stimulated guanosine incorporation into 5'-nucleotides 1.6-fold (380 pmol/mg protein vs 234 pmol/mg over 90 min at 10 microM guanosine), but did not enhance the amount of guanosine transported into the cells. Intracellular guanosine concentrations exceeded those of the incubation medium at both external guanosine concentrations studied. More [3H]guanosine was salvaged at 100 microM than at 10 microM external guanosine (562 vs 380 pmol/mg protein in 90 min), but only if ribose was present in the medium. We conclude from these studies that guanosine is salvaged by heart muscle, and that at high guanosine levels the rate of guanosine salvage appears dependent on the availability of phosphoribosylpyrophosphate within the cells. At lower guanosine levels in the presence of ribose, cell guanine concentrations limit the rate of guanosine incorporation into 5'-nucleotides.

Why does halothane relax cardiac muscle but contract malignant hyperthermic skeletal muscle?

We have studied the question of the possible role of sarcoplasmic reticulum (SR) in the interaction of volatile anesthetics (such as halothane, enflurane and isoflurane) with muscle. We used two cardiac muscle models, i.e., isolated rat myocytes and Langendorff perfused rat hearts. We compared the results with those for skeletal muscle SR from rabbits, rats and pigs susceptible to malignant hyperthermia (MH). In both skeletal and cardiac muscle SR, volatile anesthetics enhanced the calcium release from the SR. In cardiac muscle, these agents are known to decrease contractility (negative inotropism). We found that caffeine, a well-known agent which releases calcium from the SR, also had a negative inotropic effect in cardiac muscle, raising the possibility of an unexpected link between the potentiation of calcium release and mechanism underlying the observed negative inotropism. Current understanding of anesthetic mechanisms does not include this possibility. We further found that both volatile anesthetics and caffeine decrease the content of calcium in the SR, suggesting that the increase of calcium permeability results in the decrease of calcium ions in the SR which are available for excitation-contraction (E-C) coupling. In MH-susceptible skeletal muscle, a similar increase in calcium permeability does not cause a decrease of contractility, but rather may contribute to a fatal syndrome of temperature increase provoked by abnormal contracture. This difference may be because in skeletal myoplasm calcium ions recycle internally, while in the cardiac muscle cell they are in dynamic equilibrium with extracellular calcium ions.

Electrical properties of gap junction channels in guinea-pig ventricular cell pairs revealed by exposure to heptanol

Cell pairs were isolated from adult guinea pig ventricles to study the electrical properties of gap junction channels. The experiments involved a double voltage-clamp approach and whole-cell, tight-seal recording. Heptanol decreased the intracellular current, In, in a dose-dependent fashion. Before complete uncoupling, In showed fluctuations suggesting the operation of gated channels. In the presence of 3 mM heptanol, In showed quantal steps arising from spontaneous opening and closing of single channels. The IV-relationship of the channels was linear (range: +/- 95 mV). Analysis of current records revealed the following single-channel conductances, gamma n: Mean value = 37 pS; median value = 33 pS. gamma n was insensitive to the non-junctional membrane potential (range: -90 to +10 mV). 3 mM ATP4- in the pipette solution had no effects on gamma n, 6 mM ATP4- produced a small decrease, and 6 mM ATP + 0.1 mM cAMP- an increase in gamma n. Channel transitions from closed to open state were variable (range of apparent time constants: 2.5-32 ms; mean: 11 ms).

Na+-H+ exchange in isolated myocytes from adult rat heart

The intracellular pH (pHi) in ventricular myocytes isolated from adult rat heart was measured in suspension using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Steady-state pHi in bicarbonate-free media [extracellular pH (pHo) = 7.4] was 7.16 +/- 0.11 at 37 degrees C. With the use of the ammonium chloride prepulse technique, pHi was acidified, and the rate of return to resting pHi was determined. Initial rate analysis of the recovery was used to characterize the kinetics of proton net efflux via the Na+-H+ exchanger. At pHo = 7.4, proton extrusion was stimulated by extracellular sodium with a K1/2 = 58 +/- 16 mM and a maximal rate of recovery of 55 +/- 7 mmol/(1 cell H2O.min). Amiloride, which inhibited greater than 90% of the observed proton movements, was a competitive inhibitor with respect to Nao, with an inhibition constant of 3.5 microM. Proton net efflux was also inhibited by extracellular protons, with a maximal flux occurring above pHo = 8 and no net efflux occurring below pHo = 6.0. Efflux was stimulated by intracellular protons over a much narrower range (pHi = 6.6-7.1). This steep dependence indicates the involvement of at least one additional proton binding site besides the intracellular transport site, in accordance with the kinetic behavior observed in other cell systems.

Protection of the heart from ischemia by a new oligomeric derivative of prostaglandin E1

A lipophilic oligomeric ester was synthesized from prostaglandin E1. The compound was found to protect a Langendorff perfused rat heart from ischemic insult. After 15 minutes cessation of perfusion, the flow was restarted, and the recovery of contraction was measured. In control experiments, the recovery was 16.1 +/- 7.3%. In the case of pre-ischemic addition of the compound (10 micrograms/ml at 10 minutes before ischemic insult), the recovery was 31 +/- 13.2% (n = 7; the difference was not significant). In post-ischemic addition (10 micrograms/ml), the recovery was 75.9 +/- 9.0% (n = 7; p less than 0.01). The compound was also effective in protecting rat heart myocytes against a 60 minutes anoxia/15 minutes reoxygenation injury as judged by the loss of "rod-shaped" intact myocytes. At a 10 micrograms/ml concentration, the compound protected against the loss of rod-shaped myocytes by 30% in pre-anoxia addition and 35% in post-anoxia addition. The levels of significance in these experiments were p less than 0.001. Possible mechanisms of action of this compound are discussed.

Some determinants of quality and yield in the isolation of adult heart cells from rat

We investigated the effect of changes in perfusate substrate and Ca content on the quality and yield of isolated adult rat heart cells. When 1 mM Ca was added to the recirculating perfusate 15 min after collagenase addition, the ATP level of cells in the heart 15 min later, and their morphology in histological section, was no different from when no Ca was added back. The cells subsequently isolated were of similar yield, but a greater percentage were rod-shaped, compared with cells isolated without Ca restoration to the perfusate. Increased yield could be obtained by including substrates in the perfusate in addition to glucose. Either fatty acids or amino acids were effective. We conclude that: (1) all cells in the heart are Ca tolerant at the end of enzyme perfusion; (2) the presence of substrates in addition to glucose can help cells survive the isolation process.

Antibody-mediated enhancement of calcium permeability in cardiac myocytes

Our study shows that antibodies, specific to the ADP/ATP carrier of the inner mitochondrial membrane, crossreact with the cell surface of cardiac myocytes, where the calcium channel seems to be the antigenic determinant. The antibodies enhanced the calcium current and suppressed its inactivation. Affinity-purified antibodies (IgG) exhibit an acute cytotoxic effect, which required extracellular calcium and was prevented by calcium channel blockers. Our findings suggest that antibody-mediated cytotoxicity results secondary to calcium overload caused by enhanced cellular calcium permeability, requiring no complement-dependent process.

Modal gating behavior of cardiac sodium channels in cell-free membrane patches

Single voltage-activated Na+ channel currents were obtained from membrane patches of isolated ventricular cells of guinea pig hearts. The currents were compared when measured from cell-attached patches and from the same patch but at least 20 minutes after manual excision. The averaged currents showed a distinctly delayed decay in the excised patches due to the appearance of long lasting openings or bursts of openings. In contrast to control patches, the open time distribution in excised patches requires at least two exponentials. A short mean open time was voltage independent for cell-attached patches (0.38 ms +/- 0.07 ms between -60 and -20 mV, 6 cell-attached patches; and 0.41 +/- 0.1 ms, 7 excised patches). The long mean open time found in excised patches was clearly voltage dependent and increased from 0.48 +/- 0.14 ms (-80 mV) to 2.87 +/- 0.35 ms (-20 mV, regression coefficient +0.88, 7 patches). Sweeps with long openings appeared in clusters. The clustering of records with long openings, short openings, or without openings (nulls) was quantified by a runs analysis which showed a highly significant nonrandom ordering. The results show that in excised patches inactivation is temporally hibernating.

Calcium block of guinea-pig heart sodium channels with and without modification by the piperazinylindole DPI 201-106

1. External Ca2+ block of Na+ channels was studied by a gigaohm-seal patch clamp technique in single cardiac ventricular cells from guinea-pig. Single-channel currents were recorded from cell-attached patches. 2. Increasing external Ca2+ concentrations in the patch pipette from 0.1 to 20 mM reduced the single-channel conductance of normal Na+ channels from 27 to 14 pS without causing flickering (obtained from linear regression, eight patches). 3. Exposed to external Ca2+ concentrations of 20 mM, the single-channel currents decreased at potentials negative to -60 mV in spite of an increased driving force for inward Na+ currents. 4. An external concentration of 35 mM-Mg2+, which is supposed to exert a screening of surface charges nearly equal to that of 20 mM-Ca2+ (Hille, Woodhull & Shapiro, 1975), reduced the single-Na+-channel conductance only from 26 (1 mM-Mg2+) to 20 pS (linear regression, eight patches). A weaker voltage-dependent block at potentials negative to -50 mV was observed in 35 mM-Mg2+ than in 20 mM-Ca2+. Therefore, surface charge effects cannot explain the obvious reduction of the conductance of single Na+ channels found when the external Ca2+ concentration was increased. 5. Single Na+-channel currents increased with an increase in the external Na+ concentration [( Na+]o) but showed saturation. The Na+o-single-channel current relationship could be described by i = imax/(1 + kd/[Na+]o) with imax = 5.4 pA and kd = 359 mM (seventeen patches). 6. The mean open time of Na+ channels varied between 0.18 and 0.59 ms (potentials between -80 and -20 mV). No significant changes in the mean open time could be obtained when Ca2+ was varied between 0.1 and 20 mM. 7. The piperazinylindole compound DPI 201-106 was used as a tool to prolong the open time of single Na+ channels. If the external Ca2+ concentration was increased from 0.1 to 20 mM the currents through the modified channels were reduced. The reduction of single-channel currents was accentuated at potentials negative to -60 mV (20 mM-Ca2+) similar to the control channels. 8. In contrast to non-modified Na+ channels, the mean open time of DPI 201-106-modified channels proved extremely voltage and Ca2+ dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of calcium channel currents in guinea-pig single ventricular heart cells by the dihydropyridine Bay K 8644

1. A single glass micropipette voltage clamp technique with intracellular dialysis was used to study Ba2+ currents in isolated ventricular cells from guinea-pig hearts. Effects of the 1,4-dihydropyridine Bay K 8644 on whole-cell currents were evaluated at 37 degrees C. 2. Bay K 8644 increased the Ba2+ peak currents at test potentials between -50 and +20 mV and shifted the current-voltage relationships towards hyperpolarizing potentials (leftward shift for Ca2+ channel activation, 13.8 +/- 4.1 mV; n = 9; Bay K 8644, 5 mumol/l). 3. The peak times of the Ba2+ currents were diminished over the voltage range tested between -40 and +20 mV after Bay K 8644 in parallel with a shortening of the time constant of activation that was estimated from fits of the recorded currents with a d2f model. 4. The decay of the Ba2+ currents was fitted with two exponentials including a pedestal. The compound Bay K 8644 accelerated the fast decay over the whole voltage range. The amplitude of the rapidly inactivated component of the Ba2+ currents was strikingly increased after application of Bay K 8644. 5. The steady-state inactivation using a 0.5 or 5 s pre-pulse was shifted towards hyperpolarizing potentials (leftward shift 10.3 +/- 5.2 mV; n = 4; Bay K 8644, 5 mumol/l). 6. The change in the time course of Bay K 8644-modified Ba2+ currents cannot be described solely by a decrease of the backward rate coefficient from an open to a closed state of the Ca2+ channel (Sanguinetti, Krafte & Kass, 1986). The described effects of Bay K 8644 on the inactivation can be both qualitatively and quantitatively described by a model of current-dependent inactivation (Standen & Stanfield, 1982), assuming a lower affinity of an internal binding site for Ba2+ than for Ca2+.

A laser diffraction system with improved sensitivity for long-time measurements of sarcomere dynamics in isolated cardiac myocytes

To measure the mechanical activity of enzymatically isolated mammalian myocytes the principle of laser light diffraction was used. Since the viability of isolated cardiac myocytes showed a marked dependence on the laser power used, an opto-electronic system with improved light sensitivity and low susceptibility to optical noise was developed. The high sensitivity was achieved by a novel approach in the detection of diffraction patterns, that provides a significant reduction of the amount of laser power required. This improvement rendered possible the application of laser diffraction during extended experiments including pharmacological interventions. The static performance of the system, as assessed by means of calibration gratings, showed a resolution in the order of 5 nm for small changes in sarcomere length in the range from 1.2 microns to 2.0 microns. Examples of measurements on resting and contracting cells are presented, and the limitations of the application of the system to biological specimens are discussed.

Mechanical parameters determined in dispersed ventricular heart cells

A high-resolution laser diffraction system suitable for studying the basic mechanical properties of small contractile single cells has been developed. This method was used to establish the mechanical behavior of 95 ventricular cells isolated from adult guinea pig hearts. During contraction, the sarcomere length shortened from 1828 +/- 43 nm (mean +/- SD) to 1518 +/- 99 nm. The maximal velocities were 1.98 +/- 0.64 micron/s for shortening and 1.93 +/- 0.54 micron/s for re-lengthening. The twitch duration from 20% shortening to 80% re-lengthening was 622 +/- 120 ms.

Evidence for succinate production by reduction of fumarate during hypoxia in isolated adult rat heart cells

It has been demonstrated that perfusion of myocardium with glutamic acid or tricarboxylic acid cycle intermediates during hypoxia or ischemia, improves cardiac function, increases ATP levels, and stimulates succinate production. In this study isolated adult rat heart cells were used to investigate the mechanism of anaerobic succinate formation and examine beneficial effects attributed to ATP generated by this pathway. Myocytes incubated for 60 min under hypoxic conditions showed a slight loss of ATP from an initial value of 21 +/- 1 nmol/mg protein, a decline of CP from 42 to 17 nmol/mg protein and a fourfold increase in lactic acid production to 1.8 +/- 0.2 mumol/mg protein/h. These metabolite contents were not altered by the addition of malate and 2-oxoglutarate to the incubation medium nor were differences in cell viability observed; however, succinate release was substantially accelerated to 241 +/- 53 nmol/mg protein. Incubation of cells with [U-14C]malate or [2-U-14C]oxoglutarate indicates that succinate is formed directly from malate but not from 2-oxoglutarate. Moreover, anaerobic succinate formation was rotenone sensitive. We conclude that malate reduction to succinate occurs via the reverse action of succinate dehydrogenase in a coupled reaction where NADH is oxidized (and FAD reduced) and ADP is phosphorylated. Furthermore, by transaminating with aspartate to produce oxaloacetate, 2-oxoglutarate stimulates cytosolic malic dehydrogenase activity, whereby malate is formed and NADH is oxidized. In the form of malate, reducing equivalents and substrate are transported into the mitochondria where they are utilized for succinate synthesis.

Cell pairs isolated from adult guinea pig and rat hearts: effects of [Ca2+]i on nexal membrane resistance

Cell pairs isolated from adult rat and guinea pig ventricles were used to study the resistance of the nexal membrane, rn. Each cell of a cell pair was connected to a voltage-clamp circuit to obtain simultaneous whole-cell, tight-seal recordings. With this technique, rn was determined under experimental conditions aimed at primarily modifying [Ca2+]i. Moderate changes in [Ca2+]i (produced by trains of depolarizing voltage-clamp pulses activating the slow inward current, or alterations in [Ca2+]o from 0.5 to 10 mM), resulted in no change in rn for normally coupled cell pairs (rn = 5 M omega), but small and reversible changes in slightly uncoupled preparations (rn greater than or equal to 50 M omega). Large increases in rn developed with substantial elevations in [Ca2+]i (secondary to [Na+]o-withdrawal, exposure to strophanthidin in conjunction with isi, or Ca2+-dialysis). Increases in rn brought about via elevation in [Ca2+]i always were accompanied by cell shortening consistent with a sustained contracture. The current-voltage relationship of the nexal membrane was ohmic regardless of whether rn was low (control) or elevated (after increasing [Ca2+]i).

Omega-conotoxin: direct and persistent blockade of specific types of calcium channels in neurons but not muscle

Blockade of Ca2+ channels by omega-conotoxin GVIA, a 27 amino acid peptide from the venom of the marine snail Conus geographus, was investigated with patch-clamp recordings of whole-cell and unitary currents in a variety of cell types. In dorsal root ganglion neurons, the toxin produces persistent block of L- and N-type Ca2+ channels but only transiently inhibits T-type channels. Its actions appear to be neuron-specific, since it blocks high-threshold Ca2+ channels in sensory, sympathetic, and hippocampal neurons of vertebrates but not in cardiac, skeletal, or smooth muscle cells. Block occurs through direct interaction of the toxin with an external site closely associated with the Ca2+ channel, without apparent involvement of a second messenger or dependence on channel gating. The tissue and channel-type specificity and the directness and slow reversibility of the block are features that favor use of omega-conotoxin as a tool for purifying particular neuronal Ca2+ channels and defining their physiological function.

Preliminary evaluation of cocarboxylase on myocardial protection of the rat heart

The presence of cocarboxylase (CC) is essential for the oxidation of pyruvate to acetylcoenzyme A (acetyl-CoA) and its subsequent degradation by means of the Krebs cycle. We compared the effects of various concentrations of CC in a cardioplegic solution on the survival and hemodynamic and metabolic recovery of 23 isolated, working rat hearts subjected to 60 minutes of hypothermic (23 degrees C) ischemic arrest. Group 1 (N = 6) consisted of hearts infused with the basic cardioplegic solution (Tyers' solution with glucose), to which no CC was added. In group 2 (N = 6) CC was added at 0.1 ml/L to the cardioplegic solution. In group 3 (N = 5) CC was added at 1 ml/L, and in group 4 (N = 6) CC was added at 10 ml/L. The cardioplegic infusions were performed at a pressure of 40 mm Hg for 2 minutes just before arrest; 30 minutes later they were performed again for 1 minute. Only two hearts (33.3%) recovered in group 1 whereas five recovered in group 2, five (100%) in group 3, and five (83.3%) in group 4. The recovery of hemodynamic performance as a percentage of preischemic control values showed marked improvement in the CC groups, especially group 3, when compared with group 1. The metabolic variables in the CC groups were also markedly improved, with significantly (p less than .05) decreased levels of tissue lactate and increased levels of creatine phosphate compared with those in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic free calcium concentration and glucose transport in isolated cardiac myocytes

The role of cytosolic free Ca2+ concentration, [Cai], in mediating insulin's stimulatory effect on glucose transport was investigated in isolated Ca2+-tolerant rat ventricular cells. Approximately 98% of glucose transport in isolated myocytes was inhibited by phloretin. Insulin-accelerated glucose transport by 50-115% over basal transport rate. Removal of extracellular Ca2+ had no effect on either the basal transport rate or insulin's stimulatory action, indicating that extracellular Ca2+ was not necessary for insulin's effect to be manifest. Addition of A23187 had no effect on glucose transport rate. Under basal conditions, [Cai] was 167 +/- 12 nM as measured by fura-2 fluorescence and 239 +/- 22 nM by null-point titration with arsenazo III. Loading cells with fura-2 did not affect basal glucose transport rates. In addition, the stimulatory effect of insulin on glucose transport was preserved in fura-2 loaded cells. In paired experiments, insulin did not increase [Cai] as measured by fura-2 fluorescence or null-point titration despite acceleration of glucose transport. In contrast, addition of KCl (40 mM) increased [Cai] from 168 +/- 30 to 287 +/- 51 nM and resulted in 50% reduction in glucose transport rate. In other experiments designed to control for the hyperosmolar effects of KCl, NaCl (40 mM) caused no change in [Cai] but also inhibited glucose transport rate by 50%. We conclude that an elevation in [Cai] is unlikely to be the intracellular signal mediating insulin's effect on glucose transport since insulin's stimulatory effect was not reduced by Ca2+ -free media, insulin had no detectable effect on [Cai], and elevation of [Cai] by KCl did not result in stimulation of glucose transport.

Effects of procaine and caffeine on the contractility of enzymatically isolated myocytes and intact cardiac tissue

1. Procaine and caffeine exerted opposite effects on the contractility of rat cardiac tissue. 2. Procaine enhanced and caffeine depressed isometric contractile force of right ventricular strips. 3. In enzymatically isolated cells, caffeine increased the frequency and decreased the amplitude of spontaneous contractions, while procaine retarded the beating rate but concomitantly raised the shortening amplitude of the myocyte. 4. Both drugs depressed the propagation velocity of the contractile waves. 5. The results are suggested to be due to the opposite effects of the two drugs on the Ca-release process of the sarcoplasmic reticulum.

Isolation and characterization of single myocardial cells from the quail, Coturnix coturnix japonica

1. The enzymatic cell isolation technique was applied to the bird heart resulting in myocytes of which 10-50% maintained their spindle-shaped morphology, excluded the vital dye, Evans blue and tolerated physiological concentration of Ca2+ ions. 2. The length of spindle-shaped myocytes was on average 289 +/- 7 microns, and the maximum width was 10.2 +/- 0.3 microns. The mean length of the sarcomeres was 2.18 +/- 0.03 microns. 3. In electron micrographs the fine structure of the spindle-shaped myocytes looked normal--regular sarcomeric organization with clear A and I bands, mitochondria with tightly located cristae and well-developed sarcoplasmic reticulum (SR). 4. Most (80%) of the spindle-shaped myocytes were quiescent in physiological calcium concentration and practically all of them could be induced to twitch by electric field stimulation. Some beat spontaneously showing mostly slowly-propagating (135 +/- 6 microns/sec at 20 degrees C) contraction waves, so-called phasic contractions. Sometimes spontaneous twitch-type contractions could also be seen.

Modified gating behaviour of aconitine treated single sodium channels from adult cardiac myocytes

Currents through single Na channels of ventricular cells from adult mouse and guinea pig hearts were studied using the patch clamp technique. Under control conditions the majority of openings is brief and a clustering at the beginning of the depolarizing pulse can be observed. Only between 1 and 8% of the sweeps show long lasting bursts. The bursting may account for a second slow phase of decay of macroscopic currents. In the presence of aconitine in the pipette up to 80% of the sweeps showed bursts underlying the slow inactivation of aconitine modified macroscopic currents. The mean open times are unchanged but the number of openings per sweep is dramatically increased due to aconitine. It is discussed that Na channels may function in different "modes". One mode is characterized by a fast transition into an absorbing state the other one by frequent reopenings ("bursts"). aconitine favours the "bursting" mode.

Analysis of intracellular oxygenation of isolated adult cardiac myocytes

The influence of cellular shape, cellular O2 consumption rate, and intracellular diffusion coefficient for O2 on the magnitude of intracellular O2 gradients during hypoxia was analyzed with the model of Boag (Curr. Top. Radiat. Res. 5: 141-195, 1969) to determine whether these parameters could account for the experimentally measured O2 dependence curves for myoglobin (Mb) oxygenation and cytochrome a + a3 oxidation in heart cells. The analysis shows that the intracellular diffusion coefficient for O2 must be below 4 X 10(-6) cm2/s for a substantial intracellular gradient to occur. The intracellular diffusion coefficient was calculated from the difference in half-maximal oxidation (P50) values for isolated Mb and intracellular Mb and was found to be 1.76 X 10(-6) cm2/s. Use of this value and appropriate geometric models satisfactorily described the O2 dependence of Mb oxygenation and cytochrome a + a3 oxidation in cells over an eightfold range of O2 consumption rates. However, the analysis does not account for the correspondence of intracellular P50 values of Mb oxygenation and cytochrome a + a3 oxidation. This implies that there exists an intracellular heterogeneity of either Mb distribution, mitochondrial distribution, or mitochondrial respiratory characteristics. Such heterogeneity would further contribute to diffusion limitation of O2 supply during hypoxia and could be a major factor underlying the cardiac myocyte structure-function relationship.

Myoglobin-dependent oxidative metabolism in the hypoxic rat heart

The role of myoglobin in facilitating O2 diffusion for oxidative energy production was investigated at high (0.9 mM) and low (0.1 mM) O2 tensions in the Langendorff-perfused rat heart. 31P nuclear magnetic resonance was used to monitor the intracellular pH and concentrations of high energy phosphates. NaNO2 or phenylhydrazine was used to inactivate greater than 85% of intracellular myoglobin. During hypoxia, ATP and phosphocreatine were depleted significantly more rapidly in hearts with reduced concentrations of functional myoglobin than in control hearts. However, at 0.9 mM O2, myoglobin inactivation did not limit oxidative energy metabolism. It is concluded that facilitation of O2 diffusion by cardiac myoglobin plays a significant role in O2 delivery to the mitochondria at low O2 tensions.

Oxygen dependence of mitochondrial function in isolated rat cardiac myocytes

The O2 dependence of respiratory functions was studied in suspensions of isolated rat cardiac myocytes. Direct optical spectroscopy of the oxidation of cytochromes and oxygenation of Mb showed that cytochrome alpha 3 oxidation measured at 445-460 nm parallels Mb oxygenation; half-maximal values were at 8.0 and 8.5 microM, respectively. Thus there appears to be a close functional relationship between these components in the cells. The values are very high relative to comparable values for cytochrome alpha 3 oxidation in isolated rat heart mitochondria under state 3 conditions (0.43 microM) and isolated rat heart Mb (2.8 microM). Moreover, the measured values for half-maximal oxidation of cytochromes and oxygenation of Mb in the cells are sensitive to factors that alter the O2 consumption rate of the cells. These results indicate that mitochondrial respiration results in establishment of a gradient of O2 concentration from the suspending medium to the mitochondrial inner membrane. A portion of this gradient is between the suspending medium and the region occupied by Mb, and the remainder is between the region occupied by Mb and the inner mitochondrial membrane. The intracellular O2 gradient is an important factor in determining the O2 dependence of mitochondria in cells and hence may contribute to the O2 dependence of cardiac myocyte function in vivo.

The control of sugar uptake by metabolic demand in isolated adult rat heart cells

To investigate the control of sugar uptake by metabolic demand, we used isolated quiescent adult rat heart cells in suspension, under conditions similar to those found during anoxia. Metabolic demand was varied by exposing cells to rotenone plus various levels of p-trifluoromethoxyphenylhydrazone. Without glucose, the time taken for half of the cells to undergo contracture was inversely proportional to the metabolic demand as measured by the rate of lactate production. For any metabolic demand, the onset of contracture was preceded by a sudden drop in adenosine triphosphate. The permeability of contracted cells to glucose was investigated using 3-O-methylglucose. The rate of 3-O-methylglucose uptake by such cells was strongly dependent on the time taken for half the cells to undergo contracture, with low rates at low times to half contracture, and insulin-like rates at high times to half contracture. This suggests that the full induction of glucose transport by metabolic demand can be prematurely curtailed by the loss of adenosine triphosphate. This phenomenon appeared to limit glucose utilization in cells with a high metabolic demand when glucose was present: such cells underwent contracture unless insulin was also present, the rate of glucose uptake as measured with 2-deoxyglucose was inhibited, and the rate of lactate production was inhibited. Isoproterenol depressed glucose transport by two mechanisms. First, by stimulating the basal metabolic demand of the cell it reduced the time taken for half the cells to undergo contracture and, hence, the level of induced sugar transport. Second, it significantly delayed the onset of sugar permeability with respect to the contracture event.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of single myocardial cells from the perch, Perca fluviatilis

In applying the enzymatic cell isolation technique to the fish heart about 40% of the dispersed myocytes maintained their spindle-shaped morphology, and about half of them tolerated physiological concentration of Ca2+ and excluded the vital dye, Evans blue. The length of spindle-shaped myocytes was on average 133 +/- 3 micron and the maximum width was 4.2 +/- 0.1 micron. The mean length of the sarcomeres was 2.1 +/- 0.1 micron. The sizes of the myocytes did not vary significantly with the weights of the fish. Electron microscopic examinations showed typical fish myocardial cell structure; absence of transverse tubule system, a sparse network of sarcoplasmic reticulum and from a few up to eight or more myofibrils. The cells were mononuclear. Most of the Ca2+-tolerant myocytes were quiescent, but the contraction in them could be induced by electric field stimulation. Both the spontaneous and electrically triggered contractions were of twitch type. The slowly propagating contraction waves, so-called phasic contractions common in isolated mammalian cardiac myocytes, could not be seen at all.

Effects of insulin on glucose metabolism in isolated heart myocytes from adult rats

The study examined the effect of insulin on glucose metabolism in freshly isolated calcium-tolerant heart myocytes from adult rats. The uptake of 2-deoxyglucose demonstrated an initial lag in response to insulin and the maximal insulin effect was not attained until after 3 min preincubation with the hormone. A dose-response study of 14CO2 production from [14C]glucose revealed that the maximum insulin stimulation of glucose utilization occurred with 5 mU/ml. Both the uptake and the oxidation of glucose proceeded at a linear rate in the absence and presence of insulin. However, insulin exerted a greater effect on the uptake (42-54%) than on the oxidation (17-22%) of exogenous glucose. Incorporation of glucose into glycogen was markedly increased by insulin and resulted in the myocyte glycogen concentration returning to in vivo levels. In the absence of insulin, glucose incorporation plateaued within 10 min of incubation and the glycogen concentration was not altered. Our findings also indicate that at equilibrium, insulin-treated cells exhibited a higher glycogen turnover rate. It thus appears that insulin exerts a differential effect on the different pathways in glucose metabolism in the isolated cardiac cells. This may be related in part to their quiescent state and lower energy demand.

A novel type of cardiac calcium channel in ventricular cells

Calcium influx is vital for several aspects of cardiac activity, so it is important to ask if heart cells possess a single or multiple types of Ca channel. Only one Ca channel type has been identified in patch-clamp studies of unitary current, despite suggestions to the contrary from whole-cell recordings in heart cells and unitary recordings from other cells. Here we describe a novel type of cardiac Ca channel with several properties that distinguish it from the hitherto-identified Ca channel in heart cells. Its conductance in isotonic Ba is small (8 pS), and is no larger in Ba than in Ca. It activates and inactivates at relatively negative potentials and remains functional long after patch excision. It is insensitive to dihydropyridines such as nimodipine and the Ca agonist Bay K 8644, and is more resistant to block by external Cd than the previously described type of cardiac Ca channel.

The effect of insulin and glucagon on systolic properties of the normal and septic isolated rat heart

Controversy exists in the literature concerning the effects of insulin and glucagon on cardiac muscle contractility, in particular during anoxia, ischemia or sepsis. The purpose of the present study was to determine the effects of insulin and glucagon on the systolic function of the normal and the dysfunctioning septic rat myocardium in the Langendorff preparation. In the normal isolated rat heart, neither insulin nor glucagon exhibited any lasting inotropic effect on systolic function or coronary flow. Sepsis (cecal ligation and puncture) resulted in a dramatic reduction of systolic function to 44% of control animals. All insulin-containing formulations tested improved systolic function in septic hearts by a mean of 85% compared to Krebs and glucose only. However, this improvement did not reach statistical significance compared to the use of Krebs and glucose only. Glucagon at 100 micrograms/l was doing as well as Krebs and glucose alone while at 1 mg/l glucagon was only able to maintain pre-perfusion contractility. Our results suggest that neither insulin nor glucagon seem to possess special inotropic properties for the isolated perfused normal or septic rat heart.

Modulation of uncoupler-induced sugar uptake in isolated adult rat heart cells by isoproterenol

When cells (2.4 mg/ml) in the presence of glucose were exposed to 0.15 microM p-trifluoromethoxyphenylhydrazone (FCCP), the time until 50% of the rod-shaped cells had undergone contracture was more than twice as long for cells without isoproterenol as for cells with isoproterenol. The cause of this large effect was revealed in experiments without glucose where 3-O-methylglucose entry, ATP levels, and cellular configuration were measured simultaneously. It was found that the onset of contracture was almost coincident with the decline in total measured ATP, suggesting that, in any cell, contracture was accompanied by a sudden and total ATP loss. In control cells, FCCP stimulated 3-O-methylglucose entry at or before the time this ATP catastrophe occurred. In cells exposed to isoproterenol, however, the stimulation of 3-O-methylglucose entry by FCCP did not occur until after the ATP catastrophe, and the extent of stimulation was reduced. This suggests that, when glucose was present, the FCCP-induced glucose influx was sufficient to significantly delay the onset of contracture in control cells but not in cells treated with isoproterenol. This conclusion was borne out by the observation that the effect of isoproterenol on contracture could be overcome with insulin.

Fate of intercellular junctions in isolated adult rat cardiac cells

Freeze fracture and thin section techniques have revealed morphological changes in gap junctions and intercalated discs of adult rat myocytes following enzymatic dissociation. Cell separation leaves behind small vesicular remnants of formerly adjacent cells connected to the intact cell by gap junctions; in contrast, desmosomes cleave at the region of intercellular contact. Apparently, the next step in gap junction breakdown is internalization of the remnants. In thin section, lanthanum penetration reveals that the cleft of some apparently internalized gap junctions is in contact with the sarcolemma, while that of others is not. In freeze fracture replicas, cytoplasmic gap junctions frequently possess hexagonally packed domains of E-face pits separated by smooth regions that may correspond to separations of membranes of internalized junctions found in thin section. Study of material maintained overnight at 37 degrees C showed no surface junctional remnants; topologies of cytoplasmic gap junctions were generally complex, and concentric membrane vesicles were common. These observations suggest that enzymatic dissociation initiates a progressive, defined sequence of junctional internalization that begins with attached cell remnants and may have as the last determinable step the separation into single membranes inside the intact cell.

Measurement of cytosolic free calcium concentration in isolated rat ventricular myocytes with quin 2

Cytosolic free calcium concentration was determined in isolated ventricular myocytes from adult rats with the calcium-sensitive indicator, quin 2. The fluorescence signal from resting cells indicated that cytosolic free calcium concentration was 181 +/- 18 nM (mean +/- SEM, n = 18). Inhibition of the sodium-potassium pump with strophanthidin (0.1 mM) resulted in an increase of cytosolic free calcium concentration from 186 +/- 17 to 736 +/- 129 nM (n = 6). The results indicate that it is possible to measure cytosolic free calcium concentration in cardiac muscle cells that have been isolated enzymatically. Moreover, they confirm the observation that inhibition of the sodium-potassium pump increases cytosolic free calcium concentration, presumably via the sodium-calcium exchange mechanism.

A new calcium current underlying the plateau of the cardiac action potential

A small and very slow inward calcium current has been identified in isolated single ventricular cells using TTX and Cd2+ to block the sodium and fast calcium currents. Activation requires about 300 ms at the threshold potential of -60 mV, decreasing to 80 ms at the peak current voltage of -30 mV. Inactivation is five to ten times longer. Half steady-state activation and inactivation are at -50 and -45 mV respectively. The current is distinctively different in both its kinetics and pharmacology from the conventional calcium current described in single heart cells. It is proposed that it contributes significant current to help maintain a major portion of the long ventricular action potential.

Different modes of Ca channel gating behaviour favoured by dihydropyridine Ca agonists and antagonists

Single cardiac transmembranous Ca channels have three modes of gating behaviour in the absence of drugs, expressed as current records with brief openings (mode 1), with no openings because of channel unavailability (mode 0 or null mode) and with long-lasting openings and very brief closings that appear only rarely (mode 2). The dihydropyridine Ca agonist Bay K 8644 enhances Ca channel current by promoting mode 2, while the Ca antagonists nitrendipine and nimodipine inhibit the current by favouring mode 0.

High selectivity of calcium channels in single dialysed heart cells of the guinea-pig

Membrane currents and action potentials were recorded in single ventricular cells obtained from guinea-pig hearts by enzymatic dissociation. Ca2+ channel currents carried by Ba2+ or Ca2+ were recorded with a suction pipette (5-10 microns diameter) for voltage clamp and internal dialysis. Currents through Na+, K+ and non-selective monovalent cation channels were suppressed by suitable holding potentials and external and internal solutions. The dialysis method allowed exchange within minutes of alkali metal cations (e.g. Cs+) and small molecules (e.g. quaternary derivatives of lidocaine and verapamil). Nevertheless, Ca2+ channels remained functional for considerable periods, typically 20 min and sometimes more than 1 h. With Ba2+ outside and Cs+ inside, current flow through Ca2+ channels changed from inward to outward at strongly positive levels beyond a clear-cut reversal potential Erev. Several methods for defining Erev were in close agreement: (1) zero-crossing of leak-subtracted peak current, (2) inversion of time-dependent current changes during channel activation or inactivation, (3) inversion of drug-sensitive current as defined by channel blockers such as Cd2+ or D-600. Erev varied with external Ba2+ or internal Cs+. Erev increased by 29 mV per 10-fold increase in Ba2+. Interpreted with constant-field theory, Erev values correspond to PBa/PCs of approximately 1360. With 5 mM-Ca2+ outside and 151 mM-Cs+ inside, Ca2+ channel current reversed near + 75 mV, corresponding to PCa/PCs approximately 6000. Earlier measurements of Erev (Lee & Tsien, 1982) suggest that PCa/PK greater than 1000. At strongly positive membrane potentials where channel activation is maximal, the Ca2+ channel current-voltage relationship is strongly non-linear, with conductance increasing on either side of an inflexion point near Erev. Activation of inward or outward currents through Ca2+ channels follows a sigmoid time course, as expected if activation were a multi-step process.

Effects of amiloride on calcium uptake by myocytes isolated from adult rat hearts

Amiloride at high concentrations inhibits the uptake of Ca by rat heart myocytes containing elevated levels of intracellular Na and retards the development of Ca-dependent hypercontracture in these cells. In contrast, amiloride enhances the net uptake of Ca in Ca-tolerant myocytes containing normal levels of Na. The results suggest that amiloride may inhibit Na-Ca exchange across the sarcolemma of cardiac myocytes.