Evanescent action of acetylcholine on Dog BP

 The word evanescent means 'fast disappearing'.

Acetylcholine is an ubiquitous neurotransmitter acting in many neuronal pathways in both the central and peripheral nervous system.

The action of acetylcholine at the post-synaptic receptors is rapidly terminated  because of fast degradation of acetylcholine by acetylcholine-esterase in the synaptic cleft.

The human body also contains an enzyme in the plasma analogous to acetyl-choline esterase. The enzyme is known as butyrlcholineesterase or pseudocholineesterase which can degrade any exogenously administered cholineester including acetyl choline. 

The enzyme is also present in plasma of dogs. 

So, when acetycholine is administered by intravenous route in a dog, there is fall in mean blood pressure (due to peripheral vasodilatation, M3 receptors) and fall in heart rate ( M2 receptors) but the effect is rapidly terminated due to fast degradation of acetylcholine by pseudocholineesterase. Therefore, the effect of injected acetylcholine on dog BP is evanescent.