Exercise training initiated after the onset of diabetes preserves myocardial function: effects on expression of beta-adrenoceptors

Abstract

The present study was undertaken to assess cardiac function and characterize beta-adrenoceptor subtypes in hearts of diabetic rats that underwent exercise training (ExT) after the onset of diabetes. Type 1 diabetes was induced in male Sprague-Dawley rats using streptozotocin. Four weeks after induction, rats were randomly divided into two groups. One group was exercised trained for 3 wk while the other group remained sedentary. At the end of the protocol, cardiac parameters were assessed using M-mode echocardiography. A Millar catheter was also used to assess left ventricular hemodynamics with and without isoproterenol stimulation. beta-Adrenoceptors were assessed using Western blots and [(3)H]dihydroalprenolol binding. After 7 wk of diabetes, heart rate decreased by 21%, fractional shortening by 20%, ejection fraction by 9%, and basal and isoproterenol-induced dP/dt by 35%. beta(1)- and beta(2)-adrenoceptor proteins were reduced by 60% and 40%, respectively, while beta(3)-adrenoceptor protein increased by 125%. Ventricular homogenates from diabetic rats bound 52% less [(3)H]dihydroalprenolol, consistent with reductions in beta(1)- and beta(2)-adrenoceptors. Three weeks of ExT initiated 4 wk after the onset of diabetes minimized cardiac function loss. ExT also blunted loss of beta(1)-adrenoceptor expression. Interestingly, ExT did not prevent diabetes-induced reduction in beta(2)-adrenoceptor or the increase of beta(3)-adrenoceptor expression. ExT also increased [(3)H]dihydroalprenolol binding, consistent with increased beta(1)-adrenoceptor expression. These findings demonstrate for the first time that ExT initiated after the onset of diabetes blunts primarily beta(1)-adrenoceptor expression loss, providing mechanistic insights for exercise-induced improvements in cardiac function.

Fig. 1.

Representative echocardiograms (top) and cardiac parameters (table at bottom) of sedentary control, sedentary streptozotocin (STZ)-diabetic, exercise-trained (ExT) control, and ExT STZ-diabetic rats. For this, animals were lightly anesthetized with a cocktail containing ketamine (100 mg/ml) and acepromazine (10 mg/ml). Three loops of M-mode were captured for each animal, and data were averaged for 7–8 animals/group. Values shown are means ± SE. LVEDD, left ventricular end-diastolic diameter; LVESD, left ventricular end-systolic diameter; bpm, beats/min. *Significantly different from sedentary control (P < 0.05). **Significantly different from sedentary STZ-diabetic group (P < 0.05).

Fig. 2.

A: representative left ventricular (in vivo) pressure recordings of hearts from sedentary control, sedentary STZ-diabetic, ExT control, and ExT STZ-diabetic rats. Animals (7–8 animals/group) were lightly anesthetized with Inactin (20 mg/kg ip), and an F-2 micromanometer-tipped catheter (Millar Instruments, Houston, TX) was inserted via the right carotid artery into the left ventricle. B: mean values of left ventricular (LV) pressure. *Significantly different from sedentary control (P < 0.05). **Significantly different from sedentary STZ-diabetic (P < 0.05).

Fig. 3.

A: representative recordings of rates of left ventricular pressure changes (±dP/dt) in hearts from sedentary control, sedentary STZ-diabetic, ExT control, and ExT STZ-diabetic rats. Animals (7–8 animals per group) were lightly anesthetized with Inactin (20 mg/kg ip), and an F-2 micromanometer-tipped catheter (Millar Instruments) was inserted via the left carotid artery into the left ventricle. Data were obtained using Powerlab data-acquisition system (ADInstuments, Colorado Springs, CO). B: mean values for ±dP/dt. Iso, isoproterenol. *Significantly different from sedentary controls (P < 0.05). **Significantly different from sedentary STZ-diabetic (P < 0.05).

Fig. 4.

Representative Western blots showing steady state levels of β₁- (A), β₂- (B), and β₃-adrenoceptors (AR) (C) and corresponding β-actin in left ventricular tissues from sedentary control, sedentary STZ-diabetic, ExT control, and ExT STZ-diabetic rat hearts. For this, 30 or 50 μg of membranes was used. Values n graphs represent the average data obtained from >5 separate preparations from each group analyzed in duplicate. *Significantly different from sedentary control (P < 0.05). **Significantly different from sedentary diabetic (P < 0.05).

Fig. 5.

A: shows the ability of membranes prepared from sedentary control, sedentary STZ-diabetic, ExT control, and ExT STZ-diabetic rat hearts to bind [³H]dihydroalprenolol. Values in graphs represent the average data from >5 separate preparations from each group analyzed in duplicate. *Significantly different from sedentary control (P < 0.05). **Significantly different from sedentary diabetic (P < 0.05). B: displacement [³H]dihydroalprenolol binding assays were used to determine relative levels of β₁- and β₂-adrenoceptors in rat ventricular tissues using the β₁-adrenoceptor selective agonist CGP-20712. Graph represents the average of >5 separate experiments done in duplicate (membranes were prepared from sedentary control ventricular tissues).