Biomedical subjects

L Protas

Publications and source records attributed to L Protas.

5 recordsLinked to original sources

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

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

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↗

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

Carbachol increases contractions and intracellular Ca++ transients in guinea pig ventricular myocytes.

We tested hypotheses concerning the muscarinic receptor subtype and the involvement of L-type Ca current (ICa) in the stimulation of contractions by carbachol (CCh) in single guinea pig ventricular myocytes. When superfused with Tyrode's solution (36 degrees C, 5.4 mM [Ca++]o) and stimulated at 0.2 Hz, CCh (EC50 approximately 18 microM) increased the early component of isotonic contractions by acting at muscarinic receptors indistinguishable from the M2 subtype because AF-DX 116 (M2-selective) was more potent than pirenzepine (M1-selective) as an antagonist of the CCh effect. Action potential duration decreased slightly and ICa was not increased when CCh increased contractions. Carbachol increased intracellular Ca++ transients and contractions reversibly, which indicated an effect via sarcoplasmic reticulum (SR) Ca stores. Ryanodine (1-10 microM) blocked the early contraction component increased by CCh, another indication that CCh action depends on SR Ca stores. We previously found that CCh increased a background Na+ current by occupancy of M2 receptors. We now report that the increased contractions by CCh can also originate at M2 receptors and that SR Ca stores are involved in the CCh effect. Because CCh did not significantly increase ICa, the initial increase of intracellular Na+ by CCh may eventually act through Na-Ca exchange to enhance excitation-contraction coupling.

Action Potentials↗