Comparison of the effect of isoproterenol on heart rate (HR) (A), mean arterial blood pressure (MAP) (B), positive first derivative for maximal rates of arterial pressure development (+dP/dt) (C), and left ventricular shortening fraction (LVSF) (D) in young and senescent rats with and without MK571. HR, MAP, and +dP/dt were obtained during invasive catheterization (n = 6 and 4, respectively), and LVSF was obtained during echography (n = 8 in each group). Data are mean ± SD. *P < 0.05 versus group without MK571; †P < 0.05 versus Young group.