Volume overload hypertrophy elicited by cold and its effects on myocardial capillarity.

Capillarity and fiber cross-sectional areas were measured in the hearts of guinea pigs exposed to cold early during growth. Twelve male guinea pigs were kept at 5 +/- 1 degrees C for 4-18 weeks. Hearts were perfusion fixed via the aorta with a 2.5% glutaraldehyde, 1% formaldehyde-buffered solution, blocks were cut from left (LV) and right (RV) ventricles, post-fixed in OsO4, dehydrated and embedded in Spurr medium. Blocks were cut transversely to fiber orientation, 0.5 micron thick, stained with Toluidine Blue and photographed at 400 x. Heart weights, number and location of capillaries and fiber cross-sectional areas (FCSA) were measured in cold-acclimated animals and in normothermic controls. Growth rates for all guinea pigs were similar. Acclimation to cold caused modest LV and RV hypertrophy. The greater LV weight seemed due to longer fibers of normal FCSA, whereas the greater RV weight was due to larger FCSA. Capillary density, capillary-to-fiber ratio and number of capillaries around the fibers were similar in the two groups of animals. Mean and maximal diffusion distances in cold-acclimated animals were not different from controls. Thus the myocardial hypertrophy induced by chronic volume overload was fully compensated for by increases in capillarity commensurate with increases in fiber girth.