Final answer:
ATP can potentially potentiate the excitatory effect of noradrenaline in post-ganglionic transmission, although this effect is not explicitly listed among the given options.
Step-by-step explanation:
The effect exerted by ATP when combined with noradrenaline in post-ganglionic transmission is not explicitly listed among the options provided. However, given the context of neurotransmission and the role of ATP, the most likely effect ATP could have in this situation is potentiation of the action of noradrenaline, leading to an enhanced excitatory response. ATP historically met with skepticism as a neurotransmitter, has been shown to play a role in extracellular signal transmission. This discovery was backed up by the evidence that a modified form of ATP, which blocks its intracellular functions, would affect extracellular signaling. Noradrenaline acts as a neurotransmitter or a hormone, exerting stimulatory and inhibitory effects depending on the receptor it binds to. In the context of stimulating postganglionic sympathetic neurons, noradrenaline generally has an excitatory effect, which can be further enhanced or potentiated by the presence of ATP.