Bella Felzen

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.

Involvement of the IP3 cascade in the damage to guinea-pig ventricular myocytes induced by cytotoxic T lymphocytes

Abstract We have shown previously that the interaction between cytotoxic T lymphocytes (CTL) and ventricular myocytes, an in vitro model for heart transplant rejection, results in electrophysiological and morphological alterations indicative of overload of the intracellular [Ca2+] ([Ca2+]i). Since these deleterious effects cannot be accounted for by increased L-type Ca2+ current (ICa,L), we hypothesize that [Ca2+]i overload due to Ca2+ release from intracellular stores, e.g. sarcoplasmic reticulum (SR), is initiated by CTL-induced activation of the inositol trisphosphate (IP3) cascade. Patch-clamp and fura-2-fluorescence techniques were utilized to record transmembrane potentials and [Ca2+]i from ventricular myocytes bound to peritoneal exudate CTL (PEL). In ventricular myocyte-PEL conjugates (after 60 min), resting potential was reduced (compared with the nonconjugated state) from –80.9 ± 0.7 to –59.9 ± 2.5 mV, action potential amplitude from 139.5 ± 1.4 to 80.6 ± 1.7 mV and action potential duration to 50% repolarization (APD50) from 797 ± 97 to 52 ± 12 ms. The ratio of fluorescence at 340 and 380 nm (R340/380) increased from a control value (in nonconjugated myocytes) of 0.71 ± 0.02 to 2.07 ± 0.03, 30 min after conjugate formation, and exceeded 4.0 at 60 min, before myocyte destruction. Heparin (50 μg/ml), an antagonist of IP3-induced Ca2+ release from SR channels, or U-73122 (2 μM), a phospholipase C (PLC) inhibitor (drugs were included in the pipette solution), prevented PEL-induced morphological and electrophysiological alterations. Accordingly, heparin attenuated the PEL-induced increase in [Ca2+]i; after 60 min of PEL-myocyte interaction, R340/380 was 1.15 ± 0.09 (compared with approximately 4.0 in the absence of heparin). The results indicate that CTL-mediated damage to ventricular myocytes is, at least partially, mediated by PLC activation and IP3-induced Ca2+ release from intracellular stores. Pharmacological targeting of IP3 in heart transplant rejection is thus suggested.

Electrophysiological effects of thyroid hormones in guinea-pig ventricular muscle: Time course and relationships to blood levels

Abstract Chronic alterations in the thyroid state have been shown to induce marked changes in action potential characteristics, the most pronounced being shortening of action potential duration (APD) by hyperthyroidism and an increase in duration by hypothyroidism. In the present study our major objectives were to investigate the time course of the effect of thyroid hormone on action potential characteristics, to examine the relationships between thyroid hormone levels and these changes, and to test whether the electrophysiological alterations are induced both by thyroxine ( T 4 ) and triiodothyronine ( T 3 ). The major findings were that a single dose of either hormone (100 μg/kg) caused a marked shortening of APD, while resting potential, action potential amplitude and V max were unchanged. APD shortening was observed promptly after ip T 4 or T 3 administration, with maximal effect occurring within 2 to 3 hrs. Having determined thyroid hormone levels in the guinea-pigs used for the electrophysiological experiments, we found a close temporal association and inverse linear relationships (r = −0.82) between total T 4 levels and APD. To determine whether APD shortening is induced due to a direct effect of thyroid hormone on the myocardium, euthyroid papillary muscles were superfused with 10 −6 m T 3 ; within 3 hrs of superfusion with T 3 , APD was shortened from 148.8 ± 4.7 ms to 117.7 ± 6.4 ms ( P

Developmental changes in ventricular action potential properties in guinea-pigs are modulated by age-related changes in the thyroid state

Previous studies have demonstrated that in different cardiac preparations action potential duration (APD) increases with age. As in various species, thyroid hormone levels increase developmentally, and since hyperthyroidism shortens APD while hypothyroidism prolongs it, we hypothesized that developmental changes in APD result from age-related variations in the thyroid state. The hypothesis was tested by analysing ventricular action potentials and total T4 (TT4) levels in guinea-pigs in the age range of 0 days to 3 months (adult), and in hyperthyroid and hypothyroid newborns (0-5 days old). We found that APD50 increased exponentially with age with a time constant of 6.7 days, from 100.6 +/- 3.4 ms in newborns (0-5 days old) to 147.4 +/- 5.2 ms in adults (P less than 0.001). TT4 decreased exponentially with age with a time constant of 4.8 days, from 3.9 +/- 0.4 micrograms/dl in newborns to less than 1.0 microgram/dl in adults. In the age range studied, APD50 and TT4 were linearly correlated: Y = -12.13X + 142, r - 0.865. In contrast to the marked changes in APD, resting potential and action potential amplitude were age-independent, and Vmax only slightly increased with age. Alterations in the thyroid state in newborns affected ventricular action potentials as predicted by the hypothesis. In euthyroid (TT4 = 3.9 +/- 0.4 micrograms/dl), hypothyroid (TT4 = 1.6 +/- 0.4 micrograms/dl) and hyperthyroid (TT4 = 39.8 +/- 10.8 micrograms/dl) newborns, APD50 was: 100.6 +/- 3.4 ms, 117.7 +/- 4.2 ms and 63.7 +/- 7.4 ms, respectively (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms whereby cytotoxic T lymphocytes damage guinea-pig ventricular myocytes in vitro.

We studied possible mechanisms whereby cytotoxic T lymphocytes (CTL) damage the myocardium during the immunological rejection of the transplanted heart, by investigating the in vitro interaction between CTL and cardiac myocytes. We utilized the patch-clamp technique to record membrane currents and action potentials from concanavalin A-treated guinea-pig ventricular myocytes conjugated to mouse peritoneal exudate CTL (PEL). PEL-myocyte interaction reduced action potential duration at 50% repolarization (APD50) from 731.7±57.8 to 195.3±58.0 ms, action potential amplitude from 134.9±1.9 to 104.2±6.2 mV and resting membrane potential (Vm) from −80.9±0.5 to −72.5±1.5 mV. These changes were accompanied by generation of delayed afterdepolarizations, indicative of intracellular [Ca2+] overload. The electrophysiological alterations were associated with myocyte shortening (within 28.9±2.8 min) followed by complete cell destruction (within 43.5±4.3 min). To determine whether intracellular Ca2+ stores were involved in PEL-induced myocyte damage, the protective effects of ryanodine and caffeine were investigated. While ryanodine (10 μM) delayed the electrophysiological and morphological alterations, caffeine (5 mM) provided significant protection, suggesting that Ca2+ release from intracellular stores contributes to PEL-induced damage to the myocytes. Based on our findings, we suggest that the functional derangements seen in myocyte-lymphocyte conjugates can contribute to the overall decline in cardiac function during heart transplant rejection.

Role of Thyroid State in Age-Dependent Cardiac Effects of Ouabain in Guinea Pigs

We tested the hypothesis that the decrease in the thyroid state, with age, contributes to the age-related increase in myocardial responsiveness to cardiac glycosides. Thyroid hormone levels (reflecting the thyroid state): total T4 (microgram/dl) and total T3 (ng/dl) in the 3 groups of guinea pigs were (mean +/- SEM): adults (3 months old): less than 1.0 and 22.6 +/- 1.1; euthyroid newborns (0-5 days old): 3.9 +/- 0.4 and 56.5 +/- 11.9; hypothyroid newborns, (0-5 days old): 1.5 +/- 0.3 and 26.5 +/- 9.8. In euthyroid newborns, T4 and T3 levels were significantly higher than in adults (p less than 0.01 for T4 and p less than 0.05 for T3) and in hypothyroid newborns (p less than 0.05). Isometric twitch was recorded from right ventricular papillary muscles by means of a force transducer. Ouabain 10(-6) M increased isometric twitch tension in adults (tension = 0.66 +/- 0.18 g/mm2) by 123.6 +/- 18.2%, in euthyroid newborns (tension = 0.19 +/- 0.04 g/mm2) by 83.6 +/- 14.5%, and in hypothyroid newborns (tension = 0.12 +/- 0.01 g/mm2) by 170.9 +/- 33.8% (p less than 0.01). Ouabain dose-response curve in the range of 10(-7) M-0.5 x 10(-5) M was significantly different (compared by two-way ANOVA) between euthyroid newborns and hypothyroid newborns (p less than 0.01), and between euthyroid newborns and adults (p less than 0.01). Toxic effects of ouabain reflected by the generation of aftercontractions were also age related and were augmented by hypothyroidism in newborns.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid Hormones and Cardiac Function

Thyroid hormones have profound effects on growth, development, and metabolism of virtually all tissues of higher organisms. Both thyroxine (T4) and triiodothyronine (T3) have been shown to induce major effects on cell replication and differentiation, calorigenesis and O2 consumption, stimulation of enzyme activity, beta-adrenergic responsiveness, and production and secretion of various hormones. As our main interest is in the heart, we will restrict ourselves to issues related to cardiac effects of thyroid hormones. The major changes in cardiac function produced by hyperthyroidism in humans and in experimental animals include marked increases in cardiac output, resting heart rate, and left ventricular ejection fraction, whereas hypothyroid patients have reduced cardiac output, resting heart rate, stroke volume and contractility [1]. These alterations have been attributed both to changes in the peripheral circulation and to direct inotropic and chronotropic effects on the heart. More specifically, the hyperthyroid myocardium is characterized by a decrease in the isometric time to peak tension, an increased shortening velocity and relaxation time, an increased sarcoplasmic reticulum (SR) Ca2+ pumping rate, an elevated actin and Ca2+-myosin ATPase activity, and a high percentage of myosin isoenzyme V1, the fast form of the enzyme.