Evidence for the Existence of Endothelial Factors Regulating Contractility in Rat Heart
Force developed by isolated papillary muscles decreases as the cross-sectional area increases. The basis for this decline in force is not clear in as much as theoretical considerations and experimental data have indicated that the rate of diffusion of oxygen into thin bundles should not be limiting. Decline of maximum Ca-activated force with increasing cross-sectional area of detergent skinned papillary muscles can be attributed to the accumulation of inorganic phosphate in the center of the bundle. In both cases, the bundle of intact cells with a possible limitation of diffusion of oxygen into the bundle and of skinned cells with a limitation of diffusion of Pi outward, the lowest level of activity should be in the center of the bundle. We have used quantitative histochemistry for measuring Ca- and actin-activated myosin ATPase activity in cryostatic sections of rapidly frozen isolated trabeculae. The technique is very sensitive and has sufficient spatial resolution to resolve individual myofibrils. At different times after dissection, ventricular trabeculae were quickly frozen, transversely sectioned and Ca- and actin-activated myosin ATPase measured in serial sections both without and with 1 µM cAMP in the assay solution. In none of over 40 trabeculae studied was there an inward gradient of actin-activated ATPase activity of myosin. The most superficial cells had very low enzymatic activity. Cyclic AMP decreased the gradient by raising the enzymatic activity of the less active cells more than the more active cells. Ca-activated myosin ATPase was always uniform across the transverse section. These observations are incompatible with a limitation of diffusion of oxygen inward or of an inhibitory factor produced by muscular activity outward.