William V. Whitehorn

Properties of Myocardium in Cardiomegaly

The adult rats ventricular myocardium is able to increase its mass markedly while maintaining its unit quality. It does so by maintaining constant the design of the sarcomeres: an increase in length is accomplished by the addition of sarcomeres in series; an increase in tension production is accomplished by the addition of more cross-sectional area of a uniform quality. This was shown by the almost constant concentration of the contractile protein, actomyosin, as well as by the histologic evidence of the constancy in the sarcomere lengths. Functional support was obtained by the finding of an identity in the parameters of the length-tension curves; the curves differed only in the absolute magnitude of the tensions, a difference that completely disappeared when suitable corrections were made for the size of the muscle. The electrical parameters also indicated a lack of change in the quality of the excitatory membrane phenomena. However, some data were presented that suggest that the myocardium may show altered properties dependent on the age of the animal. No evidence was found in support of the concept of detrimental consequences at least with this degree of cardiomegaly. It is rather concluded that this degree of cardiomegaly is accomplished without change in the basic architecture, properties, or concentration of the contractile mechanism.

Properties of Hyperthyroid Rat Myocardium

Electric and mechanical properties of right and left ventricular muscle of hyperthyroid rats have been studied in vitro. Excitability was not modified but refractory period was lengthened in left ventricular muscle. Developed tension was significantly reduced in both ventricles, the greater reduction being on the right. The results suggest direct and disproportionate effects of thyroid hormone on right and left ventricular myocardium.

A Muscle Column Preparation from the Rat's Left Ventricle

A muscle preparation from the left ventricle of the rat heart suitable for in vitro studies is described. It consists of one of the trabeculae carneae of the posterior ventricular wall extending from the apex of the ventricle to the auriculoventricular junction. Studies on the excitability, extensibility, tension development, and oxygen consumption of the preparation are reported. The preparation offers economic and technical advantages over other types of isolated heart muscle preparations currently in use.

Hypophyseal Influence on Function and Composition of Cardiac Muscle

The effects of hypophysectomy and of growth hormone (GH) on the properties and composition of rat myocardium were studied. GH, in dosage sufficient to maintain a normal rate of growth, did not correct the microcardia of hypophysectomy. Tension production of surviving left ventricular muscle columns was not modified by hypophysectomy or GH administration. Ventricular actomyosin concentration was slightly but significantly elevated in hypophysectomized animals, with or without GH. However, “ATP sensitivity” of the extracted actomyosin was markedly depressed by hypophysectomy and partially restored by GH. Differential hormonal regulation of cardiac actin and myosin is suggested.

Summation and tetanus in cardiac muscle; effects of temperature, epinephrine and digitoxin.

Summary 1. Summation of contractions and incomplete tetanus may be consistently produced in frog ventricle strips by elevation of bath temperature. In these experiments a temperature of about 30° C regularly induced the summated response. 2. Digitoxin in cone, of 0.3 μg/ml potentiated the summated response and caused its appearance at lower temperatures. 3. Epinephrine in cone, of l0-6 prevented summated response to digitoxin or high temperature. 4. The summated response to digitoxin or high temp. is associated with reduction in duration of refractory period and the inhibitory effect of cooling or epinephrine with increase in duration of refractoriness. 5. Rat ventricle strips may show summation, incomplete tetanus and post-tetanic potentiation at temp. of 38° C and in the absence of treatment with drugs. The mechanisms of these phenomena are not yet delineated.