Biomedical subjects

B Fermini

Publications and source records attributed to B Fermini.

27 records · Page 2Linked to original sources

PubMed1992

Differences in rate dependence of transient outward current in rabbit and human atrium.

Both human and rabbit atrial cells possess a large 4-aminopyridine-sensitive transient outward current (I(to1)). However, the slow reactivation of this current in rabbits suggests that its role may be limited to very slow heart rates. We used whole cell voltage-clamp recordings to evaluate the rate dependency of I(to1) in rabbit and human atrial myocytes. Our results show that at physiological temperatures in human atrium, I(to1) is rate independent at rates between 0.1 and 4.0 Hz. Peak I(to1) at 4.0 Hz in rabbit was 3.4 +/- 1.4% (mean +/- SE) of current at 0.1 Hz (P < 0.001, n = 8), whereas in humans, peak I(to1) at 4.0 Hz averaged 88.8 +/- 6.1% of the current at 0.1 Hz (P > 0.05, n = 7). These differences were due to marked discrepancies in reactivation time course, which was biexponential with time constants that averaged 650 +/- 159 ms and 8.4 +/- 1.1 s in rabbit (n = 8) compared with a single exponential time constant of 33.6 +/- 6.8 ms (n = 8) in human atrium (both at 30 degrees C). These findings suggest that I(to1) can contribute importantly to atrial repolarization at all physiological heart rates in humans. Furthermore, these results emphasize that there are important interspecies variations in the rate dependence of I(to1), which need to be considered in understanding the physiological and pharmacological regulation of atrial repolarization.

4-Aminopyridine↗

PubMed1992

Sustained outward current observed after I(to1) inactivation in rabbit atrial myocytes is a novel Cl- current.

In rabbit atrial myocytes, depolarization of the membrane results in a rapidly activating transient outward current (I(to)) that then decays to a sustained level. The sustained current (Isus) remains constant for at least 5 s during continued depolarization. The present study was designed to identify the ionic mechanism underlying Isus with the use of whole cell voltage-clamp techniques. After exposure to 2 mM 4-aminopyridine (4-AP), the 4-AP-sensitive transient outward current (I(to1)) was abolished, but Isus was unaffected. Isus was not blocked by the K+ channel blockers tetraethylammonium chloride and Ba2+, was not changed by increasing superfusate K+ concentration, and was still present when K+ was replaced by Cs+ in both the superfusate and the pipette. Isus was significantly reduced by the Cl- transport blockers 4-acetamido-4'-isothiocyanatostilbene-2.2'-disulfonic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The current-voltage relations of Isus showed outward rectification, and the reversal potential of Isus shifted with changes in the transmembrane Cl- gradient in the fashion expected for a Cl- current. We conclude that Isus in rabbit atrium is due to a noninactivating Cl- current which, unlike previously described cardiac Cl- currents, is manifest in the absence of exogenous stimulators of adenosine 3',5'-cyclic monophosphate formation, cytosolic Ca2+ transients, or cell swelling.

Animals↗

PubMed1992

Utilization of three-dimensional culture for early morphometric and electrophysiological analyses of solitary cerebellar neurons.

When cells from the cerebellum of late-gestation rats were grown at low density (50-100 cells/mm2) in a three-dimensional culture system, they rapidly expressed morphological and electrophysiological properties of neurons. Growth and differentiation of the population of solitary neurons as a whole was statistically assessed at 6, 12, 18, 24, 48 and 72 h using data obtained from image analysis. Mean length of axons and dendrites increased 17- and 220-fold, respectively, from 6 to 72 h. Average number of branch points rose 35-fold. Other indices of complexity increased 2- to 3-fold. Whole-cell voltage clamp revealed that as early as 24-30 h the cells displayed a tetrodotoxin-sensitive Na+ current (INa), a nifedipine-sensitive high-threshold Ca2+ current, a Ni(2+)-sensitive low-threshold Ca2+ current, and two voltage-dependent K+ outward currents consisting of a 4-aminopyridine-sensitive fast transient outward current and a CsCl-sensitive slow delayed component. These observations correlate closely with voltage-activated currents previously recorded in neonatal or young rat cerebellum, and demonstrate that the culture model is useful for analyzing the early rapid growth, differentiation and intrinsic ionic currents of these neurons as single cells.

Action Potentials↗

PubMed1991

Determinants of action potential duration in neonatal rat ventricle cells.

STUDY OBJECTIVE: The aim was to study the currents that determine the action potential duration in ventricular cells from neonatal rats. DESIGN: Microelectrode measurements of action potentials from ventricle strips were compared with action potentials obtained from isolated myocytes with the whole cell patch clamp method in current clamp mode. Ionic currents were studied in myocytes in voltage clamp mode using recognised modulators of channel activity. EXPERIMENTAL MATERIAL: Neonatal rats (2 d old) were decapitated and myocytes were prepared from the apical third of collagenase treated hearts. MEASUREMENTS AND MAIN RESULTS: Modification of the action potential by 1.8-5.0 mM Ca, 2.0 mM Co, 8 mM 4-aminopyridine, 1.8 mM Sr, and 20 mM tetraethylammonium suggested the presence of the slowly inactivating Ca current ICa,L, an early outward current Ieo, and at least one other K current. Action potentials from myocytes and ventricular strips were comparable. Voltage clamp experiments were confirmatory and revealed currents with the following properties: (1) ICa,L: a Ca current with a current density of 21.7 microA.cm-2, activated between -30 and -20 mV, saturated at 1.8 Cao, inactivated faster at 5 than at 1.8 mM Cao, more permeable to Ba and Sr than to Ca, and with Sr as charge carrier blocked by Ca; (2) Ieo: the peak current had a linear I/V relation between 0 and 70 mV and was abolished by 4 mM 4-aminopyridine; (3) IK1: the current was an inward rectifier that showed a relaxation at potentials negative to -90 mV. CONCLUSIONS: Action potentials obtained from neonatal rat ventricle with microelectrodes are comparable with those measured in myocytes in current clamp mode. The action potential duration is mainly determined by ICa,L, Ieo, and IK1, and there is no evidence for the presence of a delayed rectifier.

Action Potentials↗

PubMed1991

Removal of sialic acid alters both T- and L-type calcium currents in cardiac myocytes.

The whole cell configuration of the patch-clamp technique was used to test the hypothesis that the presence of sialic acid residues influences both T- and L-type Ca2+ currents (ICa,T and ICa,L) in cultured pacemaker cells isolated from the rabbit sinoatrial node. Removal of these anionic sugar moieties by neuraminidase (1.0 U/ml for 5-20 min) increased ICa,T in five of nine cells (by a factor of 2.2-5.1) and ICa,L in three of six cells (by a factor of 1.2-1.6). In cells that did not exhibit such an increase, the enzyme reduced ICa,T but had no significant effect on ICa,L. In cells that exhibited an increase in ICa,T, exposure to neuraminidase also shifted the activation curve to more negative potentials and increased the slope of the inactivation curve. The enzyme did not influence the gating of ICa,L or the rates of inactivation of either ICa,T or ICa,L. The enhancement of ICa,T and ICa,L could not be mimicked by including neuraminidase in the patch pipette or by adding a contaminant of the enzyme preparation, phospholipase C, to the bath. When external Ca2+ was replaced by Ba2+, neither ICa,T nor ICa,L was increased significantly by neuraminidase. It is proposed that by removing sialic acid residues neuraminidase might directly alter the gating of T-type Ca2+ channels. On the other hand, the increased amplitudes of ICa,T and ICa,L might be due to a rise in intracellular Ca2+.

Animals↗

PubMed1991

Repolarization differences between guinea pig atrial endocardium and epicardium: evidence for a role of Ito.

It has long been known that ventricular epicardial action potential duration (APD) is shorter than endocardial, and recent evidence suggests that a larger transient outward current (Ito) in epicardium is responsible for the difference. To evaluate possible corresponding regional variations in atrial tissue, we studied guinea pig atrial epicardial and endocardial action potentials using standard microelectrode techniques. Epicardial APD was consistently shorter than endocardial, but the difference was greatly diminished by rapid pacing or early premature activation, situations in which Ito availability should be limited. 4-Aminopyridine (4-AP), at concentrations (0.5 mM) producing specific Ito blockade, increased APD significantly in atrial epicardium without affecting endocardium. The effect of 4-AP on APD was most marked at slow rates, at which Ito would be greatest, and was negligible at rapid rates or during premature activation, during which Ito would be largely inactivated. At larger concentrations (5 mM) 4-AP caused an equalization of epicardial and endocardial APD. The equimolar substitution of strontium for calcium did not affect APD at slow rates and increased APD (particularly in endocardium) at rapid rates, suggesting that the Ito underlying endocardial-epicardial differences was unlikely to be calcium dependent. We conclude that epicardial-endocardial differences in APD, well documented in ventricular tissue, can also occur in atrial tissue and that the underlying ionic mechanisms appear to be similar.

4-Aminopyridine↗

PubMed1990

Sialic acid and the surface charge associated with hyperpolarization-activated, inward rectifying channels.

The whole-cell configuration of the patch-clamp technique was used with cultured pacemaker cells from the rabbit sinoatrial node to test the hypothesis that sialic acid residues (NANA) constitute much of the negative surface charge associated with hyperpolarization-activated, inward rectifying channels. Activation-voltage relationships (between -70 and -140 mV) were determined for hyperpolarization-activated (inward rectifying) current (i(f)). Addition of 10 mM Ca2+ shifted the half-activation potential (V 1/2) from -89.5 +/- 0.9 mV to -77.9 +/- 2.6 mV (P less than 0.01), confirming the presence of negative fixed charges on the myocytes after 3 to 5 days in culture. Addition of 20 mM dimethonium, an organic divalent cation that "screens" but does not bind to negative surface charge, shifted V 1/2 from -86.8 +/- 1.4 mV to -75.0 +/- 1.7 mV (P less than 0.001) without affecting the amplitude of the current. In contrast, 10 mM Ca2+ reduced the amplitude of i(f) significantly. Incubation of cells with a highly purified preparation of neuraminidase (0.1-2.0 U/ml, 1 hr, 37 degrees C), an enzyme that selectively removes NANA from glycoproteins and glycolipids, failed to alter V 1/2 or the amplitude of i(f) significantly. Pretreatment of cells with neuraminidase (1.0 U/ml, 1 hr, 37 degrees C) failed to alter the positive shift of V 1/2 produced by dimethonium. The results suggest that NANA does not constitute the negative surface charge associated with hyperpolarization-activated, inward rectifying channels.

Animals↗

PubMed1990

Membrane resistance increases when automaticity develops in explanted rat heart cells.

We compared the passive electrical properties of isolated ventricular myocytes (resting potential -65 mV, fast action potentials, and no spontaneous activity) with those of 2- to 7-day-old cultured ventricle cells from neonatal rats (resting potential -50 mV, slow action potentials, and presence of spontaneous activity). In myocytes the specific membrane capacity was 0.99 microF/cm2, and the specific membrane resistance increased from 2.46 k omega.cm2 at -65 mV to 7.30 k omega.cm2 at -30 mV. In clusters, the current-voltage relationships measured under current-clamp conditions showed anomalous rectification and the input resistance decreased from 1.05 to 0.48 M omega when external K+ concentration was increased from 6 to 100 mM. Using the model of a finite disk we determined the specific membrane resistance (12.9 k omega.cm2), the effective membrane capacity (17.8 microF/cm2), and the lumped resistivity of the disk interior (1,964 omega.cm). We conclude that 1) the voltage dependence of the specific membrane resistance cannot completely explain the membrane resistance increase that accompanies the appearance of spontaneous activity; 2) a decrease of the inwardly rectifying conductance (gk1) is mainly responsible for the increase in the specific membrane resistance and depolarization; and 3) approximately 41% of the inward-rectifying channels are electrically silent when spontaneous activity develops in explanted ventricle cells.

Animals↗

PubMed1985

Influence of explantation procedure on the electrical and morphological properties of cultured neonatal rat ventricle cells.

The ultrastructure and electrophysiological properties of ventricle cells from newborn rats were studied before and after explantation. The cultured cells were dissociated either with trypsin or with collagenase, the latter enzyme being used with and without stirring with a magnetic bar. The explanted cells were studied 10 hr and 48 hr or more after explantation. At 10 hr after explantation, the cells exhibited fast-rising action potentials, but their myofibrils were disorganized, except for stirred collagenase-dispersed cells, which were also depolarized and inexcitable. At 2 days and later after explantation, all preparations had well-defined sarcomeres and myofibrils oriented in parallel similar to the ventricle before explantation, but the cells showed slow-response action potentials together with spontaneous activity. These findings suggest that the disorganization of myofibrils does not reflect damage to the surface membrane. Moreover, collagenase seems more damaging to the cells than trypsin under similar conditions (comparable periods of mechanical stirring), especially 10 hr after explantation.

Animals↗