Experimental hyperthyroidism IV Myocardial muscle mechanics and oxygen consumption in eu-and hyperthyroidism

Summary

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats pretreated with cristalline L-thyroxine (1 mg/kg/day, i.p.) 8–18 days prior to the examination. Isotonic afterloaded and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats.

In isotonic afterloaded contractions the extent of shortening was nearly identical in bot groups. However, maximum rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium. Myocardial oxygen consumption was significantly increased in hyperthyroidism, primarily due to an increased maximum rate of isometric tension development and — to a lesser extent — to increased isotonic contraction velocity.

In isometric contractions maximum tension development (preload near L_max) was similar in both groups. However, maximum rate of isometric tension development was markedly increased in hyperthyroidism. A close and linear relationship was found between maximum rate of isometric tension development (isometric contraction) and myocardial oxygen consumption.

The results demonstrate increases of velocity factors of myocardial performance in experimental hyperthyroidism. Myocardial oxygen consumption is significantly increased. This increase in oxygen consumption quantitatively has its mechanical equivalent in increased isometric contraction velocity and, to a small amount, in increased isotonic contraction velocity.

Zusammenfassung

Herzmechanik und myokardialer Sauerstoffverbrauch wurden an rechtsventrikulären Papillarmuskeln von euthyreoten und hyperthyreoten Katzen untersucht. Die experimentelle Hyperthyreose wurde durch intraperitoneale Injektion von L-Thyroxin (1 mg/kg/Tag 8–18 Tage) erzeugt. Der Sauerrstoffverbrauch wurde polarographisch bestimmt.

In isotonischen, isometrischen und Unterstützungskontraktionen waren die Gesch windigkeitsgrößen der Kontraktionsphase bei der Hyperthyreose erheblic gesteigert. Parallel dazu war eine quantitativ vergleichbare Zunahme des myokardialen Sauerstoffverbrauches nachweisbar. Es bestand eine enge Beziehung zwischen der maximalen isometrischen Spannungsanstiegsgeschwindigkeit und dem myokardialen Sauerstoffverbrauch. Die Befunde lassen den Schluß zu, daß der gesteigerte myokardiale Energiebedarf bei der Hyperthyreose durch eine Veränderung herzmechanischer Größen entsprechend einer Kontraktilitätssteigerung erklärbar ist.