Myocardial Hypertrophy: The Distribution of Cell Growth within the Layers of the Myocardium Tomas A. Salerno, F.R.C.S.(C), Harry M. Shizgal, M.D., and Anthony R. C. Dobell, M.D.

ABSTRACT The spatial distribution of radioac- tively labeled microspheres was studied in normal and in experimentally hypertrophied hearts of adult pigs. The hearts had been "tagged" with micro- spheres when the pigs were young. When the adult animal was killed, the spatial distribution of the microspheres was identical in the normal and in the hypertrophied hearts. This demonstrates that hypertrophic growth had occurred evenly through- out all layers of the myocardium.

Left ventricular hypertrophy is experimentally produced in animals by banding the ascending aorta [lo, 14, 16,191. Does hypertrophic growth occur evenly in all layers of the stressed myo- cardium? The experiments described here were designed to answer this question.

Materials and Methods Twenty-one Poland-China pigs 4 k 0.3 weeks old and weighing 9.0 f 0.8 kg were selected for the experiments. The 7 control animals were anesthetized with Pentothal (sodium thiopen- tal), intubated, and connected to a positive- pressure respirator. Under sterile conditions, a small left anterolateral thoracotomy was per- formed. The pericardium was opened, and 10 pc of strontium 85 carbonized microspheres* measuring 15 f 5 p was injected into the left atrium. These microspheres lodge in the pre- capillary arterioles of the myocardium acting as "markers" within the myocardium and are dis- tributed as a percent of the cardiac output [8,9, 201. The animals were allowed to live until they were 18 weeks old (44 f 6.0 kg). Then they

From the Departments of Surgery, Royal Victoria Hospital and McGill University, Montreal, Que, Canada. Accepted for publication May 12, 1978. Address reprint requests to Dr. Salemo, Department of Surgery, Queen's University, Kingston, Ont, Canada K7L 2V7. 4 M Company, Nuclear Products, St. Paul, MN.

underwent a second thoracotomy during which 12 pc of niobium 95 microspheres was injected into the left atrium. One hour later each animal was killed. The free wall of the left ventricle was divided into subepicardial, midmyocardial, and subendocardial layers of equal thickness. Light microscopy was performed in each layer of the myocardium. Each layer was then minced, dried for 48 hours, weighed, and counted in a Packard Tricarb Liquid Scintilla- tion Counter. The activity of each layer per gram of dry tissue was calculated using a com- puter program to solve simultaneous equations and ratios of epicardium to endocardium and of endocardium to midmyocardium. In addition, the percent of cardiac output to each of the three layers was calculated since the total num- ber of counts was determined.

The 14 experimental animals were subjected to similar procedures. Strontium 85 micro- spheres were injected into the left atrium, as in the control animals, but half an hour after injec- tion the ascending aorta was banded above the aortic valve until the left ventricle-as- cending aorta gradient was 8 to 12 mm Hg. By studying the spatial distribution of these mi- crospheres in normal and hypertrophied hearts it was possible to determine where cell growth had taken place within the myocardium. Before death each animal received a second injection of niobium 95 microspheres. Pressure gradients across the aortic stenosis measured 40 to 90 mm Hg (mean, 68 mm Hg). Tissue handling was as in the control anin;als.

Results The mean heart weightlbody weight ratio was 4.90 k 0.88 (f standard error of the mean) for the control animals and 8.41 f 2.01 for the ex- perimental animals (Figure). Seven pigs in the experimental group died (subacute bacterial

141 0003-4975179/020141-03$01.25 @ 1978 by Tomas A. Salemo

142 The Annals of Thoracic Surgery Vol 27 No 2 February 1979

T

Banded Normal

Degree of myocardial hypertrophy in normal and banded adult hearts of pigs. (I = standard error of the mean.)

endocarditis in 2, severe congestive heart fail- ure in 1, sudden death in 2, and erosion of the band into the aorta in 2).

By light microscopy alone, differential cell growth in any one particular layer could not be distinguished, although the banded hearts had uniform myocardial cell hypertrophy. The spa- tial distribution of the strontium 85 micro- spheres at 18 weeks was similar in both groups of animals (Table 1). These microspheres were injected into the left atrium when the pigs were 4 weeks old and therefore were lodged in the precapillary arterioles while the myocardium was undergoing hypertrophy in the banded animals. Differential growth of the inner layer would have resulted in a less dense concentra- tion of microspheres in the inner third of the hypertrophied myocardium, which would have led to a decrease in the ratio of endocardium to midmyocardium and an increase in the ratio of epicardium to endocardium. However, these results did not occur. The endocardiumlmid- myocardium ratio of counts per minute was 0.84 f 0.03 in the hypertrophied group and 0.83 f 0.04 in the control group. This difference was not statistically significant. In addition, the mean ratios of epicardium to endocardium, of epicardium to midmyocardium, and of endo- cardium to midmyocardium were not signif- icantly different (see Table 1).

Coronary blood flow, expressed as a percent

Table 1 . Distribution of Microspheres (Strontium 85) Injected into the Hearts of 4-Week-Old Pigs and Counted 14 Weeks Later

Control Experimental Ratios (f SEM) (Banded) v Valuea

EpilEndo 0.84 f 0.05 0.83 f 0.06 NS EpilMid 0.81 k 0.06 0.80 f 0.04 NS EndolMid 0.83 f 0.04 0.84 f 0.03 NS

aStudent t test. SEM = standard error of the mean; Epi = epicardium; Endo = endocardium; NS = not statistically significant; Mid = midmyocardium.

Table 2 . Myocardial Blood Flow as a Percentage of the Cardiac Output

Age Experimental (Banded) p Valuea (wk) Control

4 6.80 f 1.10 7.10 5 1.40 NS 18 8.60 f 1.30 10.70 f 2.1 NS

5tudent t test. NS = not statistically significant.

of cardiac output (Table 2) was slightly greater in the hypertrophied group both at 4 and 18 weeks. These differences were not statistically significant.

Comment In both the development and regression of left ventricular hypertrophy, the number of cardiac cells remains unchanged [2, 12, 17, 211. Their size alone is altered [12]. The number of capil- laries per surface area in the hypertrophied myocardium is thought to be either decreased [151 or unchanged [181 compared with normal hearts, whereas the ratio of capillaries to the cross-sectional area of the muscle fibers is rela- tively constant during hypertrophic growth [15]. Collagen content increases while the heart is enlarging during constriction of the aorta [6].

The subendocardium is particularly vulner- able to ischemia. Although factors such as in- creased tissue pressure from surface to deeper areas of the myocardium [3-51, and decreased oxygen tension from epicardium to endocar- dium [ll] with a concurrent decrease in blood flow to the deeper layers [l, 7, 131 have been

143 Salemo, Shizgal, and Dobell: Myocardial Hypertrophy

postulated as causative agents, the possibility of the deeper layers of the myocardium hyper- trophying more than the surface areas has not, to our knowledge, been clearly demonstrated. The present experiments indicate that each layer of the myocardium hypertrophies equally and to the same degree during constriction of the ascending aorta. The stimulus for hyper- trophy, therefore, must act in myocardial cells of all levels of the heart. Ischemia of the deeper layers cannot be explained by hypertrophy alone because it appears that all myocardial cells are equally hypertrophied.

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