These data suggest that increased myocyte length (an intracellular event), instead of myocyte slippage (an extracellular event), is largely responsible for the chamber dilation in ICM. Furthermore, maladaptive remodeling of myocyte shape (e.g., increased myocyte length/width ratio) may contribute to the elevated wall stress (e.g., increased chamber radius/wall thickness) in ICM.

Three independent methods were evaluated in an effort to obtain reliable values for myocyte size in adult Sprague-Dawley rats. Cell volume was determined from isolated myocytes by a Coulter Channelyzer system. Cell volume was also determined from the product of length and cross-sectional area of isolated myocytes. Additionally, myocyte cross-sectional area was measured morphometrically from electron micrographs of whole perfusion-fixed tissue. A major goal was to determine if anatomical methods used to measure cell volume produce values comparable to the more expeditious and objective Coulter Channelyzer method. The results of these experiments showed that myocyte dimensions obtained from all three techniques were similar. The second major objective was to use the above-mentioned techniques to evaluate regional differences in myocyte size. Myocyte cross-sectional area and volume were significantly larger in the endomyocardium than in the epimyocardium of the left ventricle. Right ventricle myocytes had significantly smaller volumes and cross-sectional areas than did left ventricle myocytes. There were no regional differences in cell lengths. We conclude that the Coulter Channelyzer system gives values for isolated myocyte volume that are similar to values obtained with histometric techniques; values for isolated myocyte cross-sectional area were representative of values obtained from myocytes in whole-sectioned tissue; significant regional differences in myocyte size are present in adult rat hearts; and regional variations in myocyte size are due to differences in myocyte cross-sectional area rather than cell length.

The effects of altered neonatal nutrition on cardiac myocyte size and number was examined in 21-day-old and 3-month-old rats. Nutritional differences in growth rate were produced in newborns by adjusting litter size to four (fast-growing), eight (normally growing), or 16 (slow-growing) pups per litter. Isolated myocytes were prepared from animals in each group to evaluate changes in cell size and number. Heart weight (mg +/- S.D.), at 21 days of age, was 398 +/- 51 for "fast-growing" rats, 329 +/- 43 for "normally growing" rats, and 228 +/- 24 for "slow-growing" rats. Body weights showed a comparable decline with reduced nutrition. In adults, treatment-related differences in body and heart weight were present in males but not females. "Slow-growing" rats had 21% fewer myocytes than "fast-growing" rats at 21 days of age, a change that persisted in adults. Values for myocyte number from "normally growing" rats were intermediate between those of "fast and slow-growing" rats at both 21 days and 3 months of age. In each heart region of weanling rats, myocyte length and volume were smallest in 16 per litter rats. Cellular dimensions increased progressively with better nutrition.(ABSTRACT TRUNCATED AT 250 WORDS)

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Skeletal muscle has a great ability to respond to altered functional demands. This response includes, among other adaptations, changes in muscle size, fiber-type composition, and capillary supply, resulting in altered contractile properties and fatigue resistance [1]. The response to the increased functional demand imposed on a muscle by the elimination of agonist muscles involves compensatory hypertrophy. This hypertrophy is accompanied by an increase in the number of type I fibers and capillary proliferation [2][3][4][5][6][7]. Concomitantly, the force generating capacity and fatigue resistance of the hypertrophied muscle increases [3,8].The absence of significant hypertrophy during at least the first week after the functional elimination of synergists [4,9] suggests that the muscle needs time to recover from the early damage [10] before the onset of real growth. Furthermore, the presence of transi- Japanese Journal of Physiology, 53, 181-191, 2003 Key words: capillary, fatigue, specific tension, myosin heavy chain, muscle.Abstract: Capillary proliferation occurs during compensatory hypertrophy. We investigated whether the expression of vascular endothelial growth factor (VEGF) is elevated at the onset of hypertrophy when capillary proliferation is minimal, and whether muscle damage as assessed by muscle force deficits, may occur at the onset of hypertrophy. To investigate this, we induced in 9-month-old rats, under isoflurane anesthesia, hypertrophy of the left plantaris muscle by denervation of the gastrocnemius and soleus muscles. Capillarization was investigated in both the deep (oxidative) and the superficial (glycolytic) regions of the plantaris muscle. After 2 weeks, muscle mass had increased by 16% (pϽ0.01), which was not accompanied by increases in fiber size. The maximal tetanic force (P 0 ) and specific ten-) and twitch characteristics were unaltered, and fatigue resistance of the overloaded muscle was improved (pϽ0.05). However, the myosin heavy chain composition was unaltered. Capillary proliferation was not yet evident, but VEGF mRNA and protein levels were elevated 1.5-and 8-fold, respectively (pϽ0.05). We concluded that the normal specific tension and the elevated VEGF expression after 2 weeks of overload indicate (1) an absence of or minimal muscle damage at this early time point, and (2) that elevated VEGF expression precedes and is involved in capillary proliferation that occurs during the later stages of compensatory hypertrophy.

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The burrowing hermaphroditic anemone, Ceriantheopsis americanus, lives in a mucuslined tube in sandy bottomed grass-flat areas of Tampa Bay. In this hermaphroditic species ova and sperm cysts are interspersed throughout the coupled mesenteries. The gonads occur in every other coupled mesentery beginning with the third couple. A mesentery consists of two layers of flagellated gastrodermis separated by a layer of mesoglea.The mature sperm is arrow-shaped and resembles those of other ceriantids. Ceriantheopsis americanus appears capable of self-fertilization as eggs fertilized by sperm from the same individual have produced viable embryos which have undergone cleavage.

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