Tension cost and contractility changes Moreira CM et al. results from Wang et al. (9) showed a correlation between reduced contractility and a decrease in myosin ATPase activity. Myocardial hypertrophy has been reported to be asso- ciated with reduced contraction speed, but changes in myosin ATPase activity may or may not be present (4,10,11). Diabetes is another condition that affects contractility by reducing force magnitude, increasing contraction duration (3,12,13), and producing a shift from myosin V1 to V3 (12). In L-NAME-treated rats, the force of muscle contraction can be increased without affecting the myosin isoform composi- tion of the cardiac muscle (7,14). Moreover, force generation can either increase or remain unchanged with myosin isoform shifts and changes in the level of myosin ATPase activity. These findings show the multiplicity of the adaptive mechanisms involved in myocardial contractility regulation, in both normal and pathophysiological condi- tions. To minimize the cause of the aforementioned controver- sies, we used a previously suggested approach for the analysis of tension cost: the ratio of myosin ATPase activity to tension (15-17). This ratio reflects the economy of tension development and can be used to predict mechanical changes corresponding to a biochemical change. Therefore, experi- ments were performed in several models (e.g., myocardial infarction, deoxycorticosterone acetate (DOCA)-salt-induced hypertension, L-NAME-induced hypertension, experimental diabetes) and in SHRs under stabilized conditions. The level of myosin ATPase activity was correlated with the force generation capacity and the tension cost for each condition. MATERIALS AND METHODS Male Wistar rats weighing 250 to 350 g, Wistar Kyoto (WKY) rats weighing 230 to 350 g, and spontaneous hypertensive rats (SHR) weighing 200 to 300 g were used in this study. All experiments were conducted in compli- ance with the guidelines for biomedical research as stated by the Brazilian Societies of Experimental Biology. All rats had free access to water and were fed rat chow ad libitum. The rats were divided into 12 groups for the mechanical and biochemical studies (Table 1). For the mechanical studies, the groups included rats with myocardial infarction (n = 21) and sham-operated rats (n = 23); diabetic rats (n = 12) and controls (n = 12); DOCA-salt-treated (n = 10), nephrec- tomized rats (n = 12), salt-treated (n = 12) and DOCA-treated rats (n = 15); L-NAME-treated (n = 7) and untreated (n = 12) rats; and SHR rats (n = 22) and Wistar Kyoto rats (WKY, n = 10) as the controls (Table 1). For the biochemical studies, other groups of rats were used, including infarction (n = 7) and sham-operated (n = 7); diabetes (n = 7) and control Table 1 - Experimental and control groups. The number of samples is indicated in parentheses. Experimental groups Myocardial infarction (21) Spontaneously hypertensive rats (22) Diabetic (12) DOCA-Salt (10) L-NAME (7) Control groups Sham-operated (23) Wistar Kyoto (10) Wistar Control (12) Nephrectomized (12) Salt-treated (12) DOCA-treated (15) Untreated Wistar (12) 490 CLINICS 2012;67(5):489-496 (n = 6); DOCA-salt-treated (n = 6), salt-treated (n = 6), DOCA-treated (n = 6) and nephrectomized (n = 6); L- NAME (n = 9) and control (n = 10); and SHR (n = 8) and WKY (n = 8). Diabetes induction Diabetes was induced with streptozotocin (50 mg/kg body weight diluted in 0.1 M citrate buffer solution, pH 4.5, i.v. single injection). Two main groups of rats were used: control rats and rats treated with streptozotocin for 30 days. Blood glucose was measured using a hemoglucotest (Advantage, Roche) before and after streptozotocin treat- ment. Animals that did not develop hyperglycemia were excluded. The blood pressure of each animal was measured in vivo using a tail cuff (IITC Model 29 Pulse Amplifier, IITC Inc., California). L-NAME-induced hypertension Rats were divided in two main groups. To induce hypertension, one group was treated for 30 days with L- NAME (N-nitro-L-arginine methyl ester). L-NAME (50 mg/ kg/day) was administered in the drinking water. Control rats received untreated water. The day before the experi- ments were conducted, the systolic blood pressure of the rats was measured using a tail cuff (IITC Model 29 Pulse Amplifier, IITC Inc., California). DOCA-salt-induced hypertension To induce hypertension with DOCA-salt, male 45-day-old rats were uninephrectomized under halothane anesthesia, as previously described (18). Four days later, the rats were treated with either deoxycorticosterone acetate (DOCA, 8 mg/kg twice a week) or vehicle (soybean oil, 0.1 mL/ animal). This treatment was repeated twice per week for 30 days. The uninephrectomized rats received drinking water as vehicle, and the DOCA-salt-treated rats received a saline solution (0.9% NaCl) as their drinking water during the treatment period. The day before the experiments were conducted, the systolic blood pressure of all rats was measured using a tail cuff (IITC Model 29 Pulse Amplifier, IITC Inc., California). Animals that did not develop hypertension were excluded. Myocardial infarction model Myocardial infarction was induced by ligation of the left coronary artery as previously described (19). Anesthesia was induced by halothane. After recovery, the rats were kept in collective cages at the animal facility. To avoid stress and because we had previous measurements of systolic blood pressure in a similar group of rats, systolic blood pressure was not measured in the tail. Studies of myocardial contractility The rats received 500 units of heparin i.p., and after 10 min, they were anesthetized with 45 mg/kg of sodium pentobarbital i.p. The hearts were rapidly removed after thoracotomy and perfused with Krebs solution through the aortic stump to permit proper selection and dissection of the left ventricle papillary muscles. The papillary muscles were immersed in a 50-mL water-jacketed bath maintained at 29¡1˚C and gassed with (95% O2 and 5% CO2) Krebs bicarbonate buffer solution. The muscles were stretched to Lmax (muscle length at which active tension is maximal) and