Biomedical subjects

R B Robinson

Publications and source records attributed to R B Robinson.

At least 19 recordsLinked to original sources

PubMed2006

Pacemaker current and automatic rhythms: toward a molecular understanding.

The ionic basis of automaticity in the sinoatrial node and His-Purkinje system, the primary and secondary cardiac pacemaking regions, is discussed. Consideration is given to potential targets for pharmacologic or genetic therapies of rhythm disorders. An ideal target would be an ion channel that functions only during diastole, so that action potential repolarization is not affected, and one that exhibits regional differences in expression and/or function so that the primary and secondary pacemakers can be selectively targeted. The so-called pacemaker current, If, generated by the HCN gene family, best fits these criteria. The biophysical and molecular characteristics of this current are reviewed, and progress to date in developing selective pharmacologic agents targeting If and in using gene and cell-based therapies to modulate the current are reviewed.

Action Potentials↗

PubMed2005

Connexin-specific cell-to-cell transfer of short interfering RNA by gap junctions.

The purpose of this study was to determine whether oligonucleotides the size of siRNA are permeable to gap junctions and whether a specific siRNA for DNA polymerase beta (pol beta) can move from one cell to another via gap junctions, thus allowing one cell to inhibit gene expression in another cell directly. To test this hypothesis, fluorescently labelled oligonucleotides (morpholinos) 12, 16 and 24 nucleotides in length were synthesized and introduced into one cell of a pair using a patch pipette. These probes moved from cell to cell through gap junctions composed of connexin 43 (Cx43). Moreover, the rate of transfer declined with increasing length of the oligonucleotide. To test whether siRNA for pol beta was permeable to gap junctions we used three cell lines: (1) NRK cells that endogenously express Cx43; (2) Mbeta16tsA cells, which express Cx32 and Cx26 but not Cx43; and (3) connexin-deficient N2A cells. NRK and Mbeta16tsA cells were each divided into two groups, one of which was stably transfected to express a small hairpin RNA (shRNA), which gives rise to siRNA that targets pol beta. These two pol beta knockdown cell lines (NRK-kcdc and Mbeta16tsA-kcdc) were co-cultured with labelled wild type, NRK-wt or Mbeta16tsA-wt cells or N2A cells. The levels of pol beta mRNA and protein were determined by semiquantitative RT-PCR and immunoblotting. Co-culture of Mbeta16tsA-kcdc cells with Mbeta16tsA-wt, N2A or NRK-wt cells had no effect on pol beta levels in these cells. Similarly, co-culture of NRK-kcdc with N2A cells had no effect on pol beta levels in the N2A cells. In contrast, co-culture of NRK-kcdc with NRK-wt cells resulted in a significant reduction in pol beta in the wt cells. The inability of Mbeta16tsA-kcdc cells to transfer siRNA is consistent with the fact that oligonucleotides of the 12 nucleotide length were not permeable to Cx32/Cx26 channels. This suggested that Cx43 but not Cx32/Cx26 channels allowed the cell-to-cell movement of the siRNA. These results support the novel hypothesis that non-hybridized and possible hybridized forms of siRNA can move between mammalian cells through connexin-specific gap junctions.

Animals↗

PubMed2001

HCN2 overexpression in newborn and adult ventricular myocytes: distinct effects on gating and excitability.

Ventricular pacemaker current (I(f)) shows distinct voltage dependence as a function of age, activating outside the physiological range in normal adult ventricle, but less negatively in neonatal ventricle. However, heterologously expressed HCN2 and HCN4, the putative molecular correlates of ventricular I(f), exhibit only a modest difference in activation voltage. We therefore prepared an adenoviral construct (AdHCN2) of HCN2, the dominant ventricular isoform at either age, and used it to infect neonatal and adult rat ventricular myocytes to investigate the role of maturation on current gating. The expressed current exhibited an 18-mV difference in activation (V(1/2) -95.9+/-1.9 in adult; -77.6+/-1.6 mV in neonate), comparable to the 22-mV difference between native I(f) in adult and neonatal cultures (V(1/2) -98.7 versus -77.0 mV). This did not result from developmental differences in basal cAMP, because saturating cAMP in the pipette caused an equivalent positive shift in both preparations. In the neonate, AdHCN2 caused a significant increase in spontaneous rate compared with control (88+/-5 versus 48+/-4 bpm). In adult, where HCN2 activates more negatively, the effect was evident only during anodal excitation, requiring significantly less stimulus energy than control (2149+/-266 versus 3140+/-279 mV. ms). Thus, ventricular maturational state influences the voltage dependence of expressed HCN2, resulting in distinct physiological impact of expressed channels in neonate and adult myocytes. The full text of this article is available at http://www.circresaha.org.

Animals↗

PubMed2001

MinK-related peptide 1: A beta subunit for the HCN ion channel subunit family enhances expression and speeds activation.

The HCN family of ion channel subunits underlies the currents I(f) in heart and I(h) and I(q) in the nervous system. In the present study, we demonstrate that minK-related peptide 1 (MiRP1) is a beta subunit for the HCN family. As such, it enhances protein and current expression as well as accelerating the kinetics of activation. Because MiRP1 also functions as a beta subunit for the cardiac delayed rectifier I(Kr), these results suggest that this peptide may have the unique role of regulating both the inward and outward channels that underlie cardiac pacemaker activity. The full text of this article is available at http://www.circresaha.org.

Animals↗

PubMed2001

Single-channel properties of the sinoatrial node Na+ current in the newborn rabbit.

We have reported previously that the sinoatrial node (SAN) in the newborn rabbit expresses a Na+ current (INa) with properties similar to the neuronal type-I isoform and that this current contributes to the net inward current flowing during diastolic depolarization. To characterize this current further we conducted cell-attached single-channel experiments in isolated newborn SAN myocytes. The Na+ channel was sensitive to divalent cation block and had a single-channel conductance of 25.6 pS in the absence of divalent cations. Kinetic compatibility between single-channel and previous whole-cell data was confirmed by measuring the time constant of current decay. At pacemaker potentials, time constants were of the order of tens of milliseconds. Additional experiments indicated that this slow inactivation arises because the Na+ channels expressed in the neonatal SAN tend to re-open frequently at potentials in the pacemaker range. We suggest that this is the mechanism by which a small tetrodotoxin (TTX)-sensitive current contributes to the total inward current flowing during slow diastolic depolarization in neonatal (but not adult) pacemaker myocytes.

Animals↗

PubMed2001

L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node.

Although the neonatal sinus node beats at a faster rate than the adult, when a sodium current (I(Na)) present in the newborn is blocked, the spontaneous rate is slower in neonatal myocytes than in adult myocytes. This suggests a possible functional substitution of I(Na) by another current during development. We used ruptured [T-type calcium current (I(Ca,T))] and perforated [L-type calcium current (I(Ca,L))] patch clamps to study developmental changes in calcium currents in sinus node cells from adult and newborn rabbits. I(Ca,T) density did not differ with age, and no significant differences were found in the voltage dependence of activation or inactivation. I(Ca,L) density was lower in the adult than newborn (12.1 +/- 1.4 vs. 17.6 +/- 2.5 pA/pF, P = 0.049). However, activation and inactivation midpoints were shifted in opposite directions, reducing the potential contribution during late diastolic depolarization in the newborn (activation midpoints -17.3 +/- 0.8 and -22.3 +/- 1.4 mV in the newborn and adult, respectively, P = 0.001; inactivation midpoints -33.4 +/- 1.4 and -28.3 +/- 1.7 mV for the newborn and adult, respectively, P = 0.038). Recovery of I(Ca,L) from inactivation was also slower in the newborn. The results suggest that a smaller but more negatively activating and rapidly recovering I(Ca,L) in the adult sinus node may contribute to the enhanced impulse initiation at this age in the absence of I(Na).

Aging↗

PubMed2000

Adrenal ganglioneuroma: a familial case.

A 67-year-old woman was evaluated for asymptomatic microscopic hematuria. Intravenous urography and computed tomography demonstrated an adrenal mass, but the workup for a functional adrenal tumor was negative. The mass was surgically resected, with a histologic diagnosis of adrenal ganglioneuroma. No further treatment was necessary. After the diagnosis was made, the patient reported a family history positive for neuroblastic tumors. Two of her grandchildren had presented at early ages with a ganglioneuroblastoma and a third had presented with a ganglioneuroma. This presentation is unique because ganglioneuroma, especially that of adrenal origin, is rare in the adult population, and familial cases are extremely uncommon.

Adolescent↗

PubMed2000

Sympathetic innervation alters activation of pacemaker current (If) in rat ventricle.

Pacemaker current (If) exists in both neonatal and adult ventricles, but activates at more negative voltages in the adult. This study uses whole-cell patch clamp to investigate the factors that may contribute to the maturational shift of If, comparing neonatal rat ventricular myocytes that were cultured for 4-6 days either alone, in co-culture with sympathetic nerves, or with neurotransmitters chronically present in culture. If recorded from nerve-muscle co-cultures had a significantly more negative and shallower activation-voltage relation than that from control muscle cultures, which was reflected in the midpoint potential (V50) and slope factor (K) of activation. This effect of innervation was prevented by the sustained presence in the culture of the alpha1-adrenergic antagonist prazosin (Pz) at 10(-7) M. In parallel experiments, myocytes treated with noradrenaline (NA) at 10(-7) M or neuropeptide Y (NPY) at 10(-7) M during culture had the same If activation as control cells, but cells treated with NA and NPY together had a significantly more negative and shallower activation curve. Maximum conductance and reversal potential were unchanged. The effect of chronic exposure to NA + NPY was prevented by the sustained presence of either Pz or the NPY Y2 selective antagonist T4-[NPY(33-36)]4 (3.5 x 10(-7) M) in the culture, indicating a requirement for both alpha1-adrenergic and NPY Y2 activation. Substituting NA with the alpha1A-adrenergic selective agonist A61603 (5(-10) x 10(-9) M), in the presence of NPY, did not alter If, suggesting the involvement of alpha1B- rather than alpha1A-adrenoceptors. Further, sequential exposure to NPY followed by NA was effective in reproducing the action of chronic simultaneous exposure to these agonists, but sequential exposure to NA followed by NPY was ineffective. The results are consistent with past studies indicating that NPY affects the functional expression of the alpha1B-adrenergic cascade and suggest that sympathetic innervation induces a negative shift of If in ventricle via a combined action at alpha1B-adrenergic and NPY Y2 receptors. This effect of innervation probably contributes to the developmental maturation of If activation.

Adrenergic alpha-Agonists↗

PubMed2000

Mibefradil, an I(Ca,T) blocker, effectively blocks I(Ca,L) in rabbit sinus node cells.

To test the hypothesis that the Ca(2+) channel blocker mibefradil slows heart rate due to inhibition of T-type Ca(2+) current in pacemaker cells, we studied effects of mibefradil on action potentials and ionic currents of isolated rabbit sinus node cells using the patch clamp technique. Mibefradil (100 nM and 1 microM) reduced spontaneous rate, decreased action potential amplitude and finally stopped impulse initiation. This action was not due to the drug effect on hyperpolarization-activated pacemaker current, but can be explained by attenuation of both T- and L-type Ca(2+) currents, which were inhibited by mibefradil almost equally (55% and 64% inhibition with 1 microM for T- and L-types, respectively).

Action Potentials↗

PubMed2000

Abnormalities in Ca(i)handling in myocytes that survive in the infarcted heart are not just due to alterations in repolarization.

Studies from our laboratory have defined alterations in Ca(i)handling in the non-dialyzed subepicardial cells that have survived in the 5 day infarcted heart (IZs). To determine whether changes in the action potential profile contributed to the observed Ca(i)changes we have used a combined voltage clamp/epifluorescent technique to determine and compare changes in fura 2 ratios in IZs compared to those of epicardial cells from the non-infarcted canine hearts (NZs). We found that Ca(i)changes in voltage clamped IZs persisted. In NZs, Ca(i)transients showed the expected voltage dependence while IZs did not. To determine whether altered NaCa exchanger activity contributed to the observed changes in Ca(i)in IZs, we measured NaCa exchanger Ca(2+)fluxes (reverse and forward mode) and ionic currents in both cell types and under different Na(i)loads (10 and 20 m m). We found that there were no significant differences in resting, peak or magnitude of fura 2 ratio changes or in outward current densities between NZs and IZs even under the different Na(i)loads. Thus, we suggest that chronic up- or downregulation of the NaCa exchanger protein does not underlie observed Ca(i)changes in IZs. Additionally, Ca(2+)released with paced voltage steps represented 79% of that released by caffeine in NZs while, in IZs, caffeine releasable Ca(2+)was equivalent to that released with step depolarization. Thus, abnormalities in Ca(i)handling in IZs appear not to arise secondarily to changes in action potential configuration nor do they appear to be due to disease-induced alteations in NaCa exchanger function.

Animals↗

PubMed2000

Ionic basis for action potential prolongation by phenylephrine in canine epicardial myocytes.

INTRODUCTION: In canine ventricle, alpha-adrenergic agonists prolong action potential duration (APD) without any effect on the action potential notch, suggesting that, in this species, the effect on repolarization might be independent of inhibition of I(to). The present study investigated the action of the alpha-adrenergic agonist phenylephrine on the action potential and the repolarizing currents I(to) and I(K) in isolated canine epicardial myocytes. METHODS AND RESULTS: Isolated cells from canine epicardial tissue, and Purkinje fibers, were studied with the whole cell, voltage clamp method. Phenylephrine 0.1 microM increased APD by 13% +/- 4% at 90% repolarization without affecting the notch or amplitude. Under voltage clamp, concentrations of phenylephrine as high as 10 microM had no effect on I(to) in canine epicardial myocytes. However, I(to) of isolated canine Purkinje myocytes was reduced to 69% +/- 7% of control by 1 microM phenylephrine. Further studies in canine epicardial myocytes revealed an action of phenylephrine to inhibit I(K), and in particular I(Ks). Using a voltage protocol that included a two-step repolarization to separate I(Ks) and I(Kr) tail components, the largely I(Kr) component was not significantly affected by 1 microM phenylephrine, whereas the largely I(Ks) component was reduced to 81% +/- 5% of control value. CONCLUSION: Alpha-adrenergic prolongation of repolarization in canine epicardium does not result from inhibition of I(to). Rather, it appears that reduction of I(Ks) contributes to the action of phenylephrine. The unresponsiveness of epicardial I(to) is not a general characteristic of the canine heart, because Purkinje myocyte I(to) was inhibited, suggesting regional differences in the molecular basis of I(to) and/or alpha-adrenergic signaling in the canine heart.

Action Potentials↗

PubMed2000

Na(+) current contribution to the diastolic depolarization in newborn rabbit SA node cells.

Isolated newborn, but not adult, rabbit sinoatrial node (SAN) cells exhibit spontaneous activity that (unlike adult) are highly sensitive to the Na(+) current (I(Na)) blocker TTX. To investigate this TTX action on automaticity, cells were voltage clamped with ramp depolarizations mimicking the pacemaker phase of spontaneous cells (-60 to -20 mV, 35 mV/s). Ramps elicited a TTX-sensitive current in newborn (peak density 0.89 +/- 0.14 pA/pF, n = 24) but not adult (n = 5) cells. When depolarizing ramps were preceded by steplike depolarizations to mimic action potentials, ramp current decreased 54.6 +/- 8.0% (n = 3) but was not abolished. Additional experiments demonstrated that ramp current amplitude depended on the slope of the ramp and that TTX did not alter steady-state holding current at pacemaker potentials. This excluded a steady-state Na(+) window component and suggested a kinetic basis, which was investigated by measuring TTX-sensitive I(Na) during long step depolarizations. I(Na) exhibited a slow but complete inactivation time course at pacemaker voltages (tau = 33.9 +/- 3.9 ms at -50 mV), consistent with the rate-dependent ramp data. The data indicate that owing to slow inactivation of I(Na) at diastolic potentials, a small TTX-sensitive current flows during the diastolic depolarization in neonatal pacemaker myocytes.

Action Potentials↗

PubMed1999

Distribution and prevalence of hyperpolarization-activated cation channel (HCN) mRNA expression in cardiac tissues.

HCN cation channel mRNA expression was determined in the rabbit heart and neonatal and adult rat ventricle using RNase protection assays. In the rabbit SA node, the dominant HCN transcript is HCN4, representing >81% of the total HCN message. HCN1 is also expressed, representing >18% of the total HCN mRNA. Rabbit Purkinje fibers contained almost equal amounts of HCN1 and HCN4 transcripts with low levels of HCN2, whereas rabbit ventricle contained predominantly HCN2. The SA node contained 25 times the total HCN message of Purkinje fibers and 140 times the total HCN message of ventricle. No reports of hyperpolarization-activated current (If) exist in rabbit Purkinje fibers, and we could not record If in rabbit ventricular myocytes. To investigate the possible role of isoform switching in determining the voltage dependence of If, we determined the prevalence of HCN isoforms in neonatal and adult rat ventricle. We had previously determined the threshold for activation of If to be approximately -70 mV in neonatal rat ventricle and -113 mV in adult rat ventricle. In both neonatal and adult rat ventricle, only HCN2 and HCN4 transcripts are present. The ratio of HCN2 to HCN4 is approximately 5:1 in the neonate and 13:1 in the adult. Taken together, these results suggest that different cardiac regions express different isoforms of the HCN family. The HCN1 and HCN4 isoforms are most closely associated with a depolarized threshold for If activation, whereas the HCN2 isoform is associated with a more negative activation curve.

Animals↗

PubMed1999

Neuropeptide Y contributes to innervation-dependent increase in I(Ca, L) via ventricular Y2 receptors.

The developmental increase in L-type Ca current (I(Ca,L)) density in the rat ventricle is reproduced in vitro by culturing neonatal myocytes with sympathetic neurons. We tested whether this effect of sympathetic innervation results from a chronic or sustained action of neurally released neuropeptide Y (NPY). Ventricular myocytes from newborn rats were cultured in serum-free medium with or without sympathetic neurons, NPY, or NPY analogs. Ca currents were measured in single myocytes at room temperature using the perforated patch clamp. In all cell groups (control, innervated, or NPY treated), the current-voltage relation for I(Ca,L) was represented by a bell-shaped curve with maximal value near 0 mV. The current density at 0 mV normalized to that of corresponding mean control values was 1.63 +/- 0.12 and 1.52 +/- 0.16 for innervated and NPY-treated myocytes, respectively. Both groups differed significantly from control (P < 0.05). NPY analogs exhibited the following rank order of effectiveness: NPY >/= NPY-(13-36) >/= PYY >> [Leu31Pro34]NPY, suggesting that the NPY effect occurs via a Y2-receptor subtype. In confirmation, chronic treatment of innervated cultures with a Y2-selective NPY antagonist prevented the innervation-dependent increase in I(Ca,L). These results indicate that sympathetic innervation contributes to the developmental increase in I(Ca,L) via neurally released NPY acting at Y2 receptors on the ventricular myocytes.

Animals↗

PubMed1998

Characterization of the alpha1-adrenergic chronotropic response in neuropeptide Y-treated cardiomyocytes.

The cardiac alpha1-adrenergic chronotropic response changes from stimulatory to inhibitory post-natally. The mature inhibitory response is mediated by the alpha1B-adrenoceptor and a pertussis toxin sensitive G protein. In vivo and in vitro studies identify sympathetic innervation as critical for the maturation of this inhibitory response. Additional experiments in a culture model indicate the effect of innervation is dependent on neurally released neuropeptide Y. The present study establishes that the individual signaling elements in the neuropeptide Y induced alpha1-adrenergic cascade are the same as those appearing during normal in vivo development. In addition, the data demonstrate that the effect of neuropeptide Y does not result from activation of the putative cardiac Y3 neuropeptide Y receptor subtype, since it is reproduced by the peptide fragment neuropeptide Y-(13-36) but not by [Leu31, Pro34]neuropeptide Y.

Animals↗

PubMed1998

Fas (CD95/Apo-1)-mediated damage to ventricular myocytes induced by cytotoxic T lymphocytes from perforin-deficient mice: a major role for inositol 1,4,5-trisphosphate.

Cytotoxic T lymphocytes (CTLs) that infiltrate the heart are important immune effectors implicated in heart transplant rejection, myocarditis, and other cardiomyopathies. To investigate the mechanism(s) underlying CTL damage to the myocardium through activation of the Fas receptor (Fas/CD95/Apo-1) by the Fas ligand, we explored the interaction between peritoneal exudate CTLs (PELs), derived from perforin gene-knockout (P-/-) mice, and murine ventricular myocytes. Fas expression on isolated ventricular myocytes was demonstrated immunohistochemically. Action potentials, [Ca2+]i transients, and contractions of myocytes conjugated to P-/- PELs or treated with the apoptosis-inducing anti-Fas monoclonal antibody Jo2 were recorded. Action potential characteristics of nonconjugated myocytes and myocytes conjugated with P-/- PELs were, respectively, as follows: Vm, -73.2+/-1.5 and -53.6+/-6.4 mV (mean+/-SEM); action potential amplitude, 117.9+/-3.9 and 74.3+/-21.2 mV; and action potential duration at 80% repolarization, 17+/-6 and 42+/-13 milliseconds (all P<.05). P-/- PELs also induced early and delayed afterdepolarizations as well as arrhythmogenic activity. Diastolic [Ca2+]i increased during the cytocidal interaction with P-/- PELs, from a fluorescence ratio of 0.82+/-0.05 (n=7) to 1.98+/-0.09 (n=13) (P<.05). All of the effects caused by P-/- PELs were reproduced by incubating the myocytes with Jo2. Heparin (50 microg/mL), an antagonist of inositol trisphosphate (IP3)-operated sarcoplasmic reticulum Ca2+ channels, or U-73122 (2 micromol/L), a phospholipase C inhibitor, but not the inactive agonist U-73343, prevented Fas-mediated myocyte dysfunction. Additionally, intracellular application (through the patch pipette) of the active IP3 analogue, inositol 1,4,5-trisphosphate, but not the inactive analogue, inositol 1,3,4-trisphosphate, caused electrophysiological changes resembling those resulting from P-/- PELs and Jo2, suggesting that CTL-induced Fas-based myocyte dysfunction is mediated by IP3. We conclude that a Fas-based perforin-independent mechanism of CTL action can account for the immunopathology seen in the allotransplanted heart, myocarditis, and dilated cardiomyopathy.

Action Potentials↗

PubMed1998

Sympathetic innervation modulates repolarizing K+ currents in rat epicardial myocytes.

During postnatal development, sympathetic innervation of the heart evolves, and repolarization accelerates. Our goal in this study was to test whether sympathetic innervation modulates the ion channels that regulate repolarization. We studied action potentials and repolarizing K+ currents in epicardial myocytes from rats in which sympathetic innervation was accelerated or delayed, respectively, by subcutaneous injection of nerve growth factor (NGF) or NGF antibody (Ab) for the first 15 days of life. A placebo group was included as well. Action potential duration (APD) to 90% repolarization was greater in the Ab (158 +/- 18 ms)-treated than the NGF (106 +/- 10 ms)-treated animals (P < 0.05); the APD at 90% repolarization for the placebo group was intermediate (125 +/- 30 ms). The transient outward (Ito) and inward rectifier (IK1) K+ currents were recorded in freshly dissociated cells using the whole cell patch-clamp technique. Ito was decreased in density at potentials positive to +40 mV in Ab-treated rats when compared with rats treated with NGF (P < 0.05). In addition, the inactivation curve of Ito in Ab-treated rats was shifted 13 mV positive to that of NGF-treated rats. IK1 also decreased in the Ab-treated group compared with the NGF group in the potential ranges of -100 to -90 mV (P < 0.05). However, the channel transcript abundance (RNA) in NGF-, Ab-, or placebo-treated rat hearts did not differ. Our results suggest that sympathetic innervation contributes to the developmental differences in K+ currents and APD postnatally in the rat.

4-Aminopyridine↗

PubMed1998

SIV infection of macaques--modeling the progression to AIDS dementia.

AIDS dementia complex affects 15-20% of HIV-infected adults and a greater percentage of HIV-infected children. Whether or not an HIV-infected individual develops neurological disease and how early in infection the clinical signs appear is most likely the net result of both viral virulence factors and host factors. Important viral factors include cell tropism and sequences that determine neurovirulence. The host factors include the cellular expression of viral co-receptors and maintenance of competent immune responses. The pathogenesis of AIDS dementia complex is difficult to study in the human host because of the difficulty in identifying acutely infected individuals and the inaccessibility of human brain tissue for examination during infection. The SIV/macaque model is excellent for the study of viral virulence factors and host responses to infection. This review outlines how the SIV/macaque model has been used to identify viral factors that are important for the development of neurological disease, to determine when HIV enters the brain, and to characterize the host immune responses affecting virus entry to the CNS and the development of neurological disease.

AIDS Dementia Complex↗