Effect of thyroid hormone on in vivo contractility of the canine diaphragm

This study was designed to examine the effects of long-term (4 wk) administration of thyroid hormone on the in vivo contractility of the canine diaphragm. We implanted a pair of piezoelectric crystals chronically in the left crural and costal parts of the diaphragm by a midline laparotomy. Contractility was assessed by changes in the shortening of muscle fibers after twitch stimulation of both the crural and the costal parts of the diaphragm and in the transdiaphragmatic pressure (Pdi) after tetanic stimulation (10 to 100 Hz). As a reference, we also studied the response of the quadriceps femoris. Pretreatment measurements were taken 2 wk after surgery. Then, dogs assigned to the hyperthyroid group were given thyroid powder, 0.6 g/kg/day, orally for 4 wk. The control group was fed a diet without thyroid powder for 4 wk. Serum free-T4 level (RIA) in the hyperthyroid group (n = 9) increased from 0.68 +/- 0.07 to 5.72 +/- 0.95 ng/dl (mean +/- SE) (p less than 0.01). Pdi decreased 30 to 40% at all frequencies (p less than 0.05) except 10 Hz. Twitch shortening of the crural and costal parts, compared with pretreatment state, decreased significantly by 47.7 +/- 13.1 and 48.1 +/- 15.0%, respectively (p less than 0.05). The maximal rate of relaxation became significantly faster, by 63.5% (p less than 0.05), in the crural part, whereas that of the costal part tended to become faster but not to a significant extent. In the quadriceps femoris, although twitch force showed no change, both the maximal rate of contraction and maximal rate of relaxation became faster, and tetanic force decreased. Histologic examination of hyperthyroid dogs showed vacuolization and loss of fiber area of the diaphragm. These observations suggest that thyroid hormone impairs contractility of both the crural and costal parts of the diaphragm similarly, and that the decrease in contractility may be due to a loss of muscle mass and summation impairment of twitch contraction, which differs from that in other skeletal muscles.