What is 'meant by 'antagonistic effect’? Illustrate your answer with an example

It refers to the effect of a hormone that are countered by an antagonistic (opposing) signal, often another hormone. For example, the sympathetic and parasympathetic nervous systems achieve antagonistic effect on heartbeat.
This mechanism involves the use of more than one second messenger. In heart cells cyclic form of adenosine monophosphate (cAMP) serves as a second messenger, speeding up muscle cell contraction in response to adrenaline, while cyclic guanosine monophosphate (cGMP) serves as another second messenger, slowing muscle contraction in response to acetylcholine.