Final answer:
The neurotransmitter released during transmural stimulation in guinea-pig ileum is acetylcholine, and the receptor subtype mediating the contractile response is the muscarinic receptor, which is a G protein-coupled receptor involved in the parasympathetic response in smooth muscle of the digestive tract.
Step-by-step explanation:
The transmitter released by transmural stimulation in an isolated guinea-pig ileum preparation, which causes 'twitch' contractions, is acetylcholine (ACh). The subtype of receptor mediating the contractile response to this transmitter is the muscarinic receptor. Within the cholinergic system, there are two classes of receptors, the nicotinic receptor and the muscarinic receptor. Both bind to ACh and cause changes in the target cell, with the nicotinic receptor being a ligand-gated cation channel and the muscarinic receptor being a G protein-coupled receptor. The muscarinic receptors are the ones that are targeted by postganglionic parasympathetic fibers for their effects on smooth muscle, such as in the digestive tract.