Effect of Thyroidectomy on the Mechanics of Skeletal and Intestinal Smooth Muscle

Rats made hypothyroid by surgical thyroidectomy were· compared with controls relative to the shortening velocity and maximum shortening ability of the intestinal smooth muscle and the gastrocnemius. Hypothyroidism showed some potentiation of maximum shortening, but no effect on the shortening velocity of the gastrocnemius. Intestinal smooth muscle maximum shortening as well as shortening velocity tend to be depressed. There is some evidence for a differential effect of thyroid hormone lack in the two muscle types. This study has derived out of a desire to ascertain the effect of hypothyroidism on shortening velocity and maximum shortening of two contractile tissues, skeletal muscle and intestinal smooth muscle. It is often stated that thyroxin stimulates motor activity in the intestine and promotes vigorous skeletal muscle contraction ( Gorbman, 1962). Lambert et al. ( 1951) studied the Achilles tendon reflex and have confirmed the impression that contraction is slow in most persons who are definitely hypothyroid. Because of a decreased tetanus fusion frequency, Schwartz and Lein ( 1955) found that in muscle from hypothyroid rats compared with euthyroid and hyperthyroid rats, the steady state tensions tend to be greater when stimulation frequencies are low. MATERIALS AND METHODS Male, Holtzman laboratory rats were used in this study. Seven U1yroidectomized animals were prepared by standard procedur~s (Farris and Griffith, 1949). Six control animals were used. Approximately eight weeks elapsed between thyroidectomy and the measurement of the contractile parameters. Metabolic rates on both control and thyroidectomized animals were obtained by indirect calorimetry just prior to examining the contractile activity using a technique described by Watts and Gourley ( 195j°). One modification of the technique which reduced exploratory activity and movement of the rat involved placing the rat in a light cotton sock prior to inserting the animal into the respirometer. This modification improved the precision of the five serial determinations of 02 consumption which were used to compute the metabolic rate of each animal. Measurement of contractile velocity and maximum shortening of the rat gastrocnemius was determined using standard, student 1 Luther College, Biology Dept.