Michael D. Stern

Patrimony and the Evolution of Risk-Taking

The propensity to make risky choices has a genetic component, and recent studies have identified several specific genes that contribute to this trait. Since risk-taking often appears irrational or maladaptive, the question arises how (or if) natural selection favors risk-taking. Here we show, using a stochastic simulation of selection between two hypothetical species, “R” (risk-seeking) and “A” (risk-averse) that, when expected reproductive fitness of the individual is unaffected by the making of the risky choice (winnings balanced by losses) natural selection (taken to the point of extinction) favors the risk-averse species. However, the situation is entirely reversed if offspring are permitted to inherit a small fraction of the parents increased or decreased fitness acquired through risk-taking. This seemingly Lamarckian form of inheritance actually corresponds to the human situation when property or culture are transmitted in families. In the presence of this “cultural inheritance”, the long-shot risk-taking species was overwhelmingly favored, even when 90% of individuals were rendered sterile by a losing choice. Given this strong effect in a minimal model, it is important to consider the co-evolution of genes and culture when interpreting the genetics of risk-taking. This conclusion applies, in principle, to any species where parental resources can directly affect the fecundity of offspring. It might also be relevant to the effects of epigenetic inheritance, if the epigenetic state of zygotes can be affected by parental experiences.

Amplitude distribution of calcium sparks in confocal images: theory and studies with an automatic detection method.

Determination of the calcium spark amplitude distribution is of critical importance for understanding the nature of elementary calcium release events in striated muscle. In the present study we show, on general theoretical grounds, that calcium sparks, as observed in confocal line scan images, should have a nonmodal, monotonic decreasing amplitude distribution, regardless of whether the underlying events are stereotyped. To test this prediction we developed, implemented, and verified an automated computer algorithm for objective detection and measurement of calcium sparks in raw image data. When the sensitivity and reliability of the algorithm were set appropriately, we observed highly left-skewed or monotonic decreasing amplitude distributions in skeletal muscle cells and cardiomyocytes, confirming the theoretical predictions. The previously reported modal or Gaussian distributions of sparks detected by eye must therefore be the result of subjective detection bias against small amplitude events. In addition, we discuss possible situations when a modal distribution might be observed.

High Basal Protein Kinase A–Dependent Phosphorylation Drives Rhythmic Internal Ca2+ Store Oscillations and Spontaneous Beating of Cardiac Pacemaker Cells

Local, rhythmic, subsarcolemmal Ca2+ releases (LCRs) from the sarcoplasmic reticulum (SR) during diastolic depolarization in sinoatrial nodal cells (SANC) occur even in the basal state and activate an inward Na+-Ca2+ exchanger current that affects spontaneous beating. Why SANC can generate spontaneous LCRs under basal conditions, whereas ventricular cells cannot, has not previously been explained. Here we show that a high basal cAMP level of isolated rabbit SANC and its attendant increase in protein kinase A (PKA)-dependent phosphorylation are obligatory for the occurrence of spontaneous, basal LCRs and for spontaneous beating. Gradations in basal PKA activity, indexed by gradations in phospholamban phosphorylation effected by a specific PKA inhibitory peptide were highly correlated with concomitant gradations in LCR spatiotemporal synchronization and phase, as well as beating rate. Higher levels of basal PKA inhibition abolish LCRs and spontaneous beating ceases. Stimulation of β-adrenergic receptors extends the range of PKA-dependent control of LCRs and beating rate beyond that in the basal state. The link between SR Ca2+ cycling and beating rate is also present in vivo, as the regulation of beating rate by local β-adrenergic receptor stimulation of the sinoatrial node in intact dogs is markedly blunted when SR Ca2+ cycling is disrupted by ryanodine. Thus, PKA-dependent phosphorylation of proteins that regulate cell Ca2+ balance and spontaneous SR Ca2+ cycling, ie, phospholamban and L-type Ca2+ channels (and likely others not measured in this study), controls the phase and size of LCRs and the resultant Na+-Ca2+ exchanger current and is crucial for both basal and reserve cardiac pacemaker function.

Sodium channel blockade reduces hypoxic sodium loading and sodium-dependent calcium loading.

Studies have shown that the rise in intracellular ionized calcium, [Ca2+]i, in hypoxic myocardium is driven by an increase in sodium, [Na+]i, but the source of Na+ is not known. Methods and ResultsInhibitors of the voltage-gated Na+ channel were used to investigate the effect of Na+ channel blockade on hypoxic Na+ loading, Na+ dependent Cal2+ loading, and reoxygenation hypercontracture in isolated adult rat cardiac myocytes. Single electrically stimulated (0.2 Hz) cells were loaded with either SBFI (to index [Na+]i) or indo-1 (to index [Ca2+]i) and exposed to glucose-free hypoxia (PO2 < 0.02 mm Hg). Both [Na+]i and [Ca2+]i increased during hypoxia when cells became inexcitable following ATP-depletion contracture. The hypoxic rise in [Na+]i and [Ca2+]i was significantly attenuated by 1 μmol/L R 56865. Tetrodotoxin (60 μmol/L), a selective Na+ -channel blocker, also markedly reduced the rise in [Ca2+]i during hypoxia and reoxygenation. Reoxygenation-induced cellular hypercontracture was re-duced from 83% (45 of 54 cells) under control conditions to 12% (4 of 32) in the presence of R 56865 (P < .05). Lidocaine reduced hypercontracture dose dependently with 13% of cells hypercontracting in 100, μmol/L lidocaine, 42% in 50 μmol/L lidocaine, and 93% in 25 μmol/L lidocaine. The Na+ -H+ exchange blocker, ethylisopropylamiloride (10 μmol/L) was also effective, limiting hypercontracture to 12%. R 56865, lidocaine, and ethylisopropylamiloride were also effective in preventing hypercontracture in normoxic myocytes induced by 75 μmol/L veratridine, an agent that impairs Na+ channel inactivation. Ethylisopropylamiloride prevented the veratri-dine- induced rise in [Ca2+]i without affecting Na+-Ca2+ exchange, suggesting that amiloride derivatives can reduce Ca2+ loading by blocking Na+ entry through Na+ channels, an action that may in part underlie their ability to prevent hypoxic Na+ and Ca2+ loading. ConclusionsNa+ influx through the voltage-gated Na+ channel is an important route of hypoxic Na+ loading, Na+− dependent Ca2+ loading, and reoxygenation hypercontracture in isolated rat cardiac myocytes. Importantly, the Na+ channel appears to serve as a route for hypoxic Na+ influx after myocytes become inexcitable.

A simple numerical model of calcium spark formation and detection in cardiac myocytes.

The elementary events of excitation-contraction coupling in heart muscle are Ca2+ sparks, which arise from one or more ryanodine receptors in the sarcoplasmic reticulum (SR). Here a simple numerical model is constructed to explore Ca2+ spark formation, detection, and interpretation in cardiac myocytes. This model includes Ca2+ release, cytosolic diffusion, resequestration by SR Ca2+-ATPases, and the association and dissociation of Ca2+ with endogenous Ca2+-binding sites and a diffusible indicator dye (fluo-3). Simulations in a homogeneous, isotropic cytosol reproduce the brightness and the time course of a typical cardiac Ca2+ spark, but underestimate its spatial size (approximately 1.1 micron vs. approximately 2.0 micron). Back-calculating [Ca2+]i by assuming equilibrium with indicator fails to provide a good estimate of the free Ca2+ concentration even when using blur-free fluorescence data. A parameter sensitivity study reveals that the mobility, kinetics, and concentration of the indicator are essential determinants of the shape of Ca2+ sparks, whereas the stationary buffers and pumps are less influential. Using a geometrically more complex version of the model, we show that the asymmetric shape of Ca2+ sparks is better explained by anisotropic diffusion of Ca2+ ions and indicator dye rather than by subsarcomeric inhomogeneities of the Ca2+ buffer and transport system. In addition, we examine the contribution of off-center confocal sampling to the variance of spark statistics.

Relation of mitochondrial and cytosolic free calcium to cardiac myocyte recovery after exposure to anoxia.

Mitochondrial calcium overload has been suggested as a marker for irreversible injury in the ischemic heart. A new technique is used to measure dynamic changes in mitochondrial free calcium concentration ([Ca2+]m) in electrically stimulated (0.2 Hz) adult rat cardiac myocytes during exposure to anoxia and reoxygenation. Cells were incubated with indo-1 AM, which distributes in both the cytosol and mitochondria. After Mn2+ quenching of the cytosolic signal, cells were exposed to anoxia, and the residual fluorescence was monitored. [Ca2+]m averaged 94 +/- 3 nM (n = 16) at baseline, less than the baseline diastolic cytosolic free calcium concentration ([Ca2+]c, 124 +/- 4 nM, n = 12), which was measured in cells loaded with the pentapotassium salt of indo-1. [Ca2+]m and [Ca2+]c rose steadily only after the onset of ATP-depletion rigor contracture. At reoxygenation 35 minutes later, [Ca2+]c fell rapidly to preanoxic levels and then often showed a transient further rise. In contrast, [Ca2+]m showed only a slight transient fall and a secondary rise at reoxygenation. At reoxygenation, cells immediately either recovered, demonstrating partial relengthening and retaining their rectangular shape and response to stimulation, or they hypercontracted to rounded dysfunctional forms. Recovery occurred only in cells in which [Ca2+]m or [Ca2+]c remained below 250 nM before reoxygenation. Early during reoxygenation, [Ca2+]m remained higher in cells that hypercontracted (305 +/- 36 nM) than in cells that recovered (138 +/- 9 nM, p less than 0.05), whereas [Ca2+]c did not differ between the two groups (156 +/- 10 versus 128 +/- 10 nM, respectively; p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Direct measurement of SR release flux by tracking ‘Ca2+ spikes’ in rat cardiac myocytes

1 Ca2+ release flux across the sarcoplasmic reticulum (SR) during cardiac excitation‐contraction coupling was investigated using a novel fluorescence method. Under whole‐cell voltage‐clamp conditions, rat ventricular myocytes were dialysed with a high concentration of EGTA (4.0 mm, 150 nm free Ca2+), to minimize the residence time of released Ca2+ in the cytoplasm, and a low‐affinity, fast Ca2+ indicator, Oregon Green 488 BAPTA‐5N (OG‐5N; 1.0 mm, Kd≈ 31 μm), to optimize the detection of localized high [Ca2+] in release site microdomains. Confocal microscopy was employed to resolve intracellular [Ca2+] at high spatial and temporal resolution. 2 Analytical and numerical analyses indicated that, under conditions of high EGTA concentration, the free [Ca2+] change is the sum of two terms: one major term proportional to the SR release flux/Ca2+ influx, and the other reflecting the running integral of the released Ca2+. 3 Indeed, the OG‐5N transients in EGTA‐containing cells consisted of a prominent spike followed by a small pedestal. The OG‐5N spike closely resembled the first derivative (d[Ca2+]/dt) of the conventional Ca2+ transient (with no EGTA), and mimicked the model‐derived SR Ca2+ release function reported previously. In SR Ca2+‐depleted cells, the OG‐5N transient also closely followed the waveform of L‐type Ca2+ current (ICa). Using ICa as a known source of Ca2+ influx, SR flux can be calibrated in vivo by a linear extrapolation of the ICa‐elicited OG‐5N signal. 4 The OG‐5N image signal was localized to discrete release sites at the Z‐line level of sarcomeres, indicating that the local OG‐5N spike arises from ‘Ca2+ spikes’ at transverse (T) tubule‐SR junctions (due to the imbalance between calcium ions entering the cytosol and the buffer molecules). 5 Both peak SR release flux and total amount of released Ca2+ exhibited a bell‐shaped voltage dependence. The temporal pattern of SR release also varied with membrane voltage: Ca2+ release was most synchronized and produced maximal peak release flux (4.2 mm s−1) at 0 mV; in contrast, maximal total release occurred at −20 mV (71 versus 61 μm at 0 mV), but the localized release signals were partially asynchronous. Since the maximal conventional [Ca2+] transient and contraction were elicited at 0 mV, it appears that not only the amount of Ca2+ released, but also the synchronization among release sites affects the whole‐cell Ca2+ transient and the Ca2+‐myofilament interaction.

Rhythmic Ryanodine Receptor Ca2+ Releases During Diastolic Depolarization of Sinoatrial Pacemaker Cells Do Not Require Membrane Depolarization

Abstract— Localized, subsarcolemmal Ca2+ release (LCR) via ryanodine receptors (RyRs) during diastolic depolarization of sinoatrial nodal cells augments the terminal depolarization rate. We determined whether LCRs in rabbit sinoatrial nodal cells require the concurrent membrane depolarization, or are intrinsically rhythmic, and whether rhythmicity is linked to the spontaneous cycle length. Confocal linescan images revealed persistent LCRs both in saponin-permeabilized cells and in spontaneously beating cells acutely voltage-clamped at the maximum diastolic potential. During the initial stage of voltage clamp, the LCR spatiotemporal characteristics did not differ from those in spontaneously beating cells, or in permeabilized cells bathed in 150 nmol/L Ca2+. The period of persistent rhythmic LCRs during voltage clamp was slightly less than the spontaneous cycle length before voltage clamp. In spontaneously beating cells, in both transient and steady states, LCR period was highly correlated with the spontaneous cycle length; and regardless of the cycle length, LCRs occurred predominantly at a constant time, ie, 80% to 90% of the cycle length. Numerical model simulations incorporating LCRs reproduce the experimental results. We conclude that diastolic LCRs reflect rhythmic intracellular Ca2+ cycling that does not require the concomitant membrane depolarization, and that LCR periodicity is closely linked to the spontaneous cycle length. Thus, the biological clock of sinoatrial nodal pacemaker cells, like that of many other rhythmic functions occurring throughout nature, involves an intracellular Ca2+ rhythm.

Dependence of hypoxic cellular calcium loading on Na(+)-Ca2+ exchange.

Na(+)-Ca2+ exchange has been shown to contribute to reperfusion- and reoxygenation-induced cellular Ca2+ loading and damage in the heart. Despite the fact that both [Na+]i and [Ca2+]i have been documented to rise during ischemia and hypoxia, it remains unclear whether the rise in [Ca2+]i occurring during hypoxia is linked to the rise in [Na+]i via Na(+)-Ca2+ exchange before reoxygenation and how this relates to cellular injury. Single electrically stimulated (0.2 Hz) adult rat cardiac myocytes loaded with Na(+)-sensitive benzofuran isophthalate (SBFI), the new fluorescent probe, were exposed to glucose-free hypoxia (PO2 less than 0.02 mm Hg), and SBFI fluorescence was monitored to index changes in [Na+]i. Parallel experiments were performed with indo-1-loaded cells to index [Ca2+]i. The SBFI fluorescence ratio (excitation, 350/380 nm) rose significantly during hypoxia after the onset of ATP-depletion contracture, consistent with a rise in [Na+]i. At reoxygenation, the ratio fell rapidly toward baseline levels. The indo-1 fluorescence ratio (emission, 410/490 nm) also rose only after the onset of rigor contracture and then often showed a secondary rise early after reoxygenation at a time when [Na+]i fell. The increase in both [Na+]i and [Ca2+]i, seen during hypoxia, could be markedly reduced by performing experiments in Na(+)-free buffer. These experiments suggested that hypoxic Ca2+ loading is linked to a rise in Na+i via Na(+)-Ca2+ exchange. To show that Na(+)-Ca2+ exchange activity was not fully inhibited by profound intracellular ATP depletion, cells were exposed to cyanide, and then buffer Na+ was abruptly removed after contracture occurred. The sudden removal of buffer Na+ would be expected to stimulate cell Ca2+ entry via Na(+)-Ca2+ exchange. A large rapid rise in the indo-1 fluorescence ratio ensued, which was consistent with abrupt cell Ca2+ loading via the exchanger. The effect of reducing hypoxic buffer [Na+] on cell morphology after reoxygenation was examined. Ninety-five percent of cells studied in a normal Na(+)-containing buffer (144 mM NaCl, n = 38) and reoxygenated 30 minutes after the onset of hypoxic rigor underwent hypercontracture. Only 12% of cells studied in Na(+)-free buffer (144 mM choline chloride, n = 17) hypercontracted at reoxygenation (p less than 0.05). Myocytes were also exposed to hypoxia in the presence of R 56865, a compound that blocks noninactivating components of the Na+ current. R 56865 blunted the rise in [Na+]i typically seen after the onset of rigor, suggesting that Na+ entry may occur, in part, through voltage-gated Na+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytosolic calcium and myofilaments in single rat cardiac myocytes achieve a dynamic equilibrium during twitch relaxation

1. Single isolated rat cardiac myocytes were loaded with either the pentapotassium salt form or the acetoxymethyl ester (AM) form of the calcium‐sensitive fluorescent probe, Indo‐1. The relationship of the Indo‐1 fluorescence transient, an index of the change in cytosolic calcium [Ca2+]i concentration, to the simultaneously measured cell length during the electrically stimulated twitch originating from slack length at 23 degrees C was evaluated. It was demonstrated that even if the Ca2+ dissociation rate from Indo‐1 was assumed to be as slow as 10 s‐1, the descending limb (‘relaxation phase’) of the Indo‐1 fluorescence transient induced by excitation under these conditions is in equilibrium with the [Ca2+]i transient. Additionally, the extent of Indo‐1 loading employed did not substantially alter the twitch characteristics. 2. A unique relationship between the fluorescence transient and cell length was observed during relaxation of contractions that varied in amplitude. This was manifest as a common trajectory in the cell length vs. [Ca2+]i phase‐plane diagrams beginning at the time of cell relengthening. The common trajectory could also be demonstrated in Indo‐1 AM‐loaded cells. The Indo‐1 fluorescence‐length relation defined by this common trajectory is steeper than that described by the relation of peak contraction amplitude and peak fluorescence during the twitch contractions. 3. The trajectory of the [Ca2+]i‐length relation elicited via an abrupt, rapid, brief (200 ms) pulse of caffeine directly onto the cell surface or by ‘tetanization’ of cells in the presence of ryanodine is identical to the common [Ca2+]i‐length trajectory formed by electrically stimulated contractions of different magnitudes. As the [Ca2+]i and length transients induced by caffeine application or during tetanization in the presence of ryanodine develop with a much slower time course than those elicited by electrical stimulation, the common trajectory is not fortuitous, i.e. it cannot be attributed to equivalent rate‐limiting steps for the decrease of [Ca2+]i and cell relengthening. 4. The [Ca2+]i‐length relation defined by the common trajectory shifts appropriately in response to perturbations that have previously been demonstrated to alter the steady‐state myofilament Ca2+ sensitivity in skinned cardiac fibres. Specifically, the trajectory shifts leftward in response to an acute increase in pH or following the addition of novel myofilament calcium‐sensitizing thiadiazinone derivatives; a rightward shift occurs in response to an acute reduction in pH or following the addition of butanedione monoxime.(ABSTRACT TRUNCATED AT 400 WORDS)