CLINICS 2012;67(5):489-496 BASIC RESEARCH DOI:10.6061/clinics/2012(05)14 Tension cost correlates with mechanical and biochemical parameters in different myocardial contractility conditions Cleci M. Moreira,I Eduardo F. Meira,II Luis Vestena,I Ivanita Stefanon,II,III Dalton V. Vassallo,II Alessandra S. PadilhaII I Universidade Federal do Pampa, Uruguaiana/RS, Brasil. II Centro de Cieˆ ncias Fisiolo´ gicas, Programa de Po´ s-Graduac¸a˜ o em Cieˆ ncias Fisiolo´ gicas, Universidade Federal do Espı´rito Santo, Vito´ ria/ES, Brasil. III Escola de Ensino Superior da Santa Casa de Miserico´ rdia de Vito´ ria, EMESCAM, Vito´ ria/ES, Brasil. OBJECTIVES: Tension cost, the ratio of myosin ATPase activity to tension, reflects the economy of tension development in the myocardium. To evaluate the mechanical advantage represented by the tension cost, we studied papillary muscle contractility and the activity of myosin ATPase in the left ventricles in normal and pathophysiological conditions. METHODS: Experimental protocols were performed using rat left ventricles from: (1) streptozotocin-induced diabetic and control Wistar rats; (2) N-nitro-L-arginine methyl ester (L-NAME) hypertensive and untreated Wistar rats; (3) deoxycorticosterone acetate (DOCA) salt-treated, nephrectomized and salt- and DOCA-treated rats; (4) spontaneous hypertensive rats (SHR) and Wistar Kyoto (WKY) rats; (5) rats with myocardial infarction and sham- operated rats. The isometric force, tetanic tension, and the activity of myosin ATPase were measured. RESULTS: The results obtained from infarcted, diabetic, and deoxycorticosterone acetate-salt-treated rats showed reductions in twitch and tetanic tension compared to the control and sham-operated groups. Twitch and tetanic tension increased in the N-nitro-L-arginine methyl ester-treated rats compared with the Wistar rats. Myosin ATPase activity was depressed in the infarcted, diabetic, and deoxycorticosterone acetate salt-treated rats compared with control and sham-operated rats and was increased in N-nitro-L-arginine methyl ester-treated rats. These parameters did not differ between SHR and WKY rats. In the studied conditions (e.g., post-myocardial infarction, deoxycorticosterone acetate salt-induced hypertension, chronic N-nitro-L-arginine methyl ester treatment, and streptozotocin-induced diabetes), a positive correlation between force or plateau tetanic tension and myosin ATPase activity was observed. CONCLUSION: Our results suggest that the myocardium adapts to force generation by increasing or reducing the tension cost to maintain myocardial contractility with a better mechanical advantage. KEYWORDS: Papillary Muscles; Myosin ATPase; Hypertension; Myocardial Infarction; Diabetes. Moreira CM, Meira EF, Vestena L, Stefanon I, Vassallo DV, Padilha AS. Tension cost correlates with mechanical and biochemical parameters in different myocardial contractility conditions. Clinics. 2012;67(5):489-496. Received for publication on November 7, 2011; First review completed on December 12, 2011; Accepted for publication on January 17, 2012 E-mail: daltonv2@terra.com.br Tel.: 55 27 33335-7350 INTRODUCTION Myocardial contraction results from a cyclic interaction between actin and myosin. In the heart, this mechanism adapts to several pathophysiological conditions, including pressure and volume overload, myocardial infarction, and diabetes, to maintain adequate cardiac output. Although a significant amount of data support a positive correlation Copyright 2012 CLINICS – This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non- commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. No potential conflict of interest was reported. between mechanical parameters, muscle-shortening velocity or force generation, and myosin ATPase activity (1-8), controversies still exist. Mechanical activity in different situations, such as myocardial infarction, diabetes, or hypertension with and without hypertrophy, might present different correlations among the variables of force, the myosin isoforms involved and myosin ATPase activity. This indicates that several adaptive mechanisms might act to maintain myocardial contractility under these conditions. Particular changes have already been described for different physiopathological conditions. For example, a previous report described that after 12 weeks of myocardial infarction, papillary muscle force and contraction kinetics were enhanced, but the level of myosin ATPase activity was reduced (6). However, 489