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Relation between left ventricular systolic resistance and contractile rate processes.
S G Shroff1, D Naegelen, W A Clark
1Cardiovascular Institute, Michael Reese Hospital and Medical Center, University of Chicago Pritzker School of Medicine, Illinois 60616.
The American Journal of Physiology
|February 1, 1990
Summary
Thyroid hormone manipulation alters heart muscle composition, influencing left ventricular (LV) systolic resistance. Changes in isomyosin content are key determinants of LV function and resistance in hypertensive rats.
Area of Science:
- Cardiovascular Physiology
- Cardiac Muscle Biology
- Biochemistry
Background:
- Left ventricular (LV) systolic resistance is crucial for cardiac function.
- The role of intrinsic contractile system properties, specifically isomyosin composition, in determining LV systolic resistance is not fully understood.
- Thyroid hormones are known to influence cardiac myosin isoforms.
Purpose of the Study:
- To investigate the hypothesis that left ventricular (LV) systolic resistance is determined by the intrinsic rate processes of the contractile system.
- To examine the effect of thyroid hormone manipulation on isomyosin composition and its subsequent impact on LV systolic resistance and pump performance in spontaneously hypertensive rats (SHR).
Main Methods:
- Studied 40 spontaneously hypertensive male rats (SHR) divided into seven groups: control, propylthiouracil (PTU)-treated (10, 20, 30 days), and thyroxine-treated (5, 10, 15 days).
- Assessed in situ and isolated heart performance, including LV pump performance, normalized LV peak elastance (Emaxn), theoretical maximum flow (Qmax; a measure of LV resistance), and time course of pressure development and relaxation.
- Analyzed the relationship between isomyosin composition (specifically percent slow myosin) and hemodynamic parameters.
Main Results:
- Propylthiouracil (PTU) treatment significantly depressed LV pump performance and increased LV peak elastance (Emaxn) with a high percentage of slow myosin.
- Thyroxine treatment did not alter LV pump performance or Emaxn but increased theoretical maximum flow (Qmax), indicating reduced LV resistance.
- An inverse relationship was observed between Qmax and the percentage of slow myosin (r2 = 0.86), supporting isomyosin composition as a determinant of LV resistance.
- Changes in isomyosin composition toward predominantly slow myosin led to discordant responses in Emaxn and Qmax, potentially preserving overall pump performance.
Conclusions:
- LV systolic resistance is an intrinsic, rate-dependent property of the myocardium, significantly influenced by isomyosin composition.
- Isomyosin composition is a key determinant of LV systolic resistance.
- The study highlights the importance of assessing both LV systolic resistance and elastance for a comprehensive evaluation of left ventricular mechanical pump function, especially when isomyosin composition is altered.