CLINICS 2012;67(5):489-496 Tension cost and contractility changes Moreira CM et al. Figure 4 - Relationship between (A) the isometric force (g/mm2) or (B) the plateau tetanic tension and the myosin ATPase activity (nmol/ Pi/min/mg) of the left ventricle in control, myocardial infarction, diabetic, DOCA-salt, L-NAME-treated, SHR, and WKY rats. The results represent the means ¡ SEM. accordance with those showing isometric force reduction reported by Fein et al. (12) and Dhalla et al. (3). Other reports showed no alteration in force alterations (17,34) but differ from our study because myocardial hypertrophy was present in their preparations. Regarding evaluation of the contractile proteins, our results show a reduction in tetanic tension and a decrease in myosin ATPase activity. These findings are in accordance with previous reports (17,35). The DOCA-salt rat exhibited cardiac hypertrophy after 30 days as previously reported (23). However, the reports regarding changes in contractility are controversial (36,37). Our findings show that DOCA-salt-treated papillary mus- cles developed less tension, but other studies show no change or even increased tension (36,37). However, all related studies report cardiac hypertrophy. Tetanic tension and myosin ATPase activity were also reduced, suggesting the involvement of contractile proteins. L-NAME treatment induces severe hypertension but little or no hypertrophy. Our findings show no hypertrophy and are in accordance with previous reports (7,14). Meanwhile, other reports show hypertrophy (38) or hypotrophy (39). The increased isometric force development observed in the present study is in agreement with the results reported by Bartunek et al. (14), which described an increase in contractility indexes. However, we observed no changes in tetanic tension development, although the level of myosin ATPase activity was found to be increased. Therefore, our results, in agreement with previous reports, demonstrate that tension cost is the best way to correlate mechanical and biochemical parameters (15- 17,39). We used this approach to maintain consistency, and our results are in accordance with those reports. Interestingly, as shown in Figure 4, once the sarcoplasmic reticulum activity is reduced, the tension cost (myosin ATPase activity vs. plateau tetanic tension) reveals a clear correlation, suggesting that any existing discrepancies may be due to the interference of sarcoplasmic reticulum adaptation mechanisms. Our results suggest that higher myosin ATPase activity leads to a higher energy cost in association with the development of tension. When myosin ATPase activity in the heart is reduced, there is an accompanying reduction in tension cost. Based on these findings, we suggest that the 495 myocardium increases or decreases tension cost to maintain myocardial contractility with a better mechanical advantage. FUNDING: This work was supported by CNPq and FAPESP/ FUNCITEC. AUTHOR CONTRIBUTIONS Moreira CM performed the experimental protocols, statistical analyses, and wrote and reviewed the manuscript. Meira EF and Vestena L performed the experiments. Stefanon I, Vassallo DV and Padilha AS wrote and reviewed the manuscript. All the authors have read and approved the contents of the manuscript. Each author has made a valuable contribution to the preparation of this manuscript. REFERENCES 1. Barany K. ATPase activity of myosin correlated with speed of muscle shortening. J Gen Physiol. 1967;50(2):197-216, http://dx.doi.org/ 10.1085/jgp.50.6.197. 2. Botinelli R, Canepari M, Cappelli V, Reggiani C. Maximum speed of shortening and ATPase activity in atrial and ventricular myocardia of hyperthyroid rats. Am J Physiol. 1995;269:(3 Pt 1):C785-90. 3. Dhalla NS, Liu X, Panaglia V, Takeda N. Subcellular remodeling and heart dysfunction in chronic diabetes. 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