Which of these statements depicts vasomotor reversal of Dale?
(A) Propranolol followed by adrenaline (B) Propranolol followed by noradrenaline (C) Decrease in hea rate on adrenaline administration after phentolamine has been given (D) Noradrenaline followed by propranolol
(C) Ans: C. Decrease in hea rate on adrenaline administration after phentolamine has been given (Ref: KDT 7/e e p131, 140).This was first demonstrated by Sir HH Dale (1913) and is called vasomotor reversal of Dale.Blockade of vasoconstrictor alpha-1 (also alpha-2) receptors reduces peripheral resistance and causes pooling of blood in capacitance vessels a venous return and cardiac output are reduced a Fall in BP.Postural reflex is interfered with a marked hypotension occurs on standing a dizziness and syncope. Hypovolemia accentuates the hypotension.Decrease in hea rate on adrenaline administration after phentolamine has been given depicts vasomotor reversal of Dale.The alpha-blocker abolishes the pressor action of adrenaline (injected IV in animals), which then produces only fall in BP due to beta-2 mediated vasodilatation.