Which of the following effects of adrenaline would be blocked by phentolamine but NOT by propanolol?: (A) Cardiac stimulation, (B) Relaxation of bronchial smooth muscle, (C) Relaxation of the uterus, (D) Contraction of the radial smooth muscle in the iris
Answer is D. Phentolamine is an a blocker whereas propranolol is a b blocker. So, phentolamine will block those actions which are mediated by a receptors .i.e. contraction of radial smooth muscle in the iris.