The effects of thyroid hormone on the size and performance of the left ventricle (LV) were examined in nine conscious calves with chronically implanted sonomicrometer crystals to record wall thickness, minor diameter, and the motion of a segment in the long axis of the ventricle. A solid-state pressure transducer was placed in the LV cavity, and pacing electrodes were sewn into the left atrium. After stable control measurements were obtained, the tachycardia and mild hypertension of thyrotoxicosis were simulated by pacing at 145 beats/min and infusion of phenylephrine to raise LV systolic pressure to about 146 mmHg. Thyrotoxicosis then was induced by daily intramuscular injection of L-thyroxine (200 μg/kg) for 14 days. LV midwall sarcomere lengths also were measured by electron microscopy in hearts rapidly fixed in diastole from three untreated and three thyrotoxic animals. The results indicate that thyroxine treatment produced significant increases in heart rate, cardiac output, LV dP/dt, end-diastolic diameter, end-systolic diameter, and peak wall stress. LV end-diastolic pressure, percent shortening, and the velocity of circumferential fiber shortening and LV diastolic wall thickness were not changed significantly. LV weight-to-body weight ratios for unoperated and thyrotoxic calves also were unchanged. Mean normalized sarcomere length in euthyroid LV (2.07 ± 0.05 μm) was essentially the same as in thyrotoxic LV (2.06 ± 0.06). By comparison with the dramatic effects of thyroxine, atrial pacing and phenylephrine infusion, alone or in combination, produced insignificant effects on LV performance. Thus thyroxine enhances the contractile performance of each contractile unit in a larger LV circumference by a positive inotropic action that is independent of the effects of tachycardia or the Frank-Starling mechanism. This enables the ventricle to work effectively from a larger end-diastolic volume in meeting the increased blood flow requirements of thyrotoxicosis.
|Original language||English (US)|
|Journal||American Journal of Physiology - Heart and Circulatory Physiology|
|State||Published - Jan 1 1982|
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine
- Physiology (medical)