<Tr> <Td> NPY </Td> <Td> NPY receptor </Td> <Td> Activation of G--> ↓ cAMP--> ↓ PKA activity--> ↓ phosphorylation of MLCK--> ↑ MLCK activity--> ↑ phosphorylation of MLC (calcium - independent) </Td> </Tr> <Tr> <Td> adrenergic agonists e.g., epinephrine, norepinephrine, and dopamine </Td> <Td> ↑ α adrenergic receptor </Td> <Td> Activation of G--> ↑ PLC activity--> ↑ IP and DAG--> activation of IP receptor in SR--> ↑ intracellular Ca </Td> </Tr> <Tr> <Td> thromboxane </Td> <Td> ↑ thromboxane receptor </Td> </Tr> <Tr> <Td> endothelin </Td> <Td> ↑ endothelin receptor ET </Td> </Tr>

Which of the following blood vessels are unable to vasoconstrict