Figure 6.

Schematic of sympathetic co-transmission. ATP and NA released from small granular vesicles (SGV) act on P2X and α₁ receptors on smooth muscle respectively. ATP acting on inotropic P2X receptors evokes excitatory junction potentials (EJPs), increase in intracellular calcium ( [Ca²⁺]_i) and fast contraction, while occupation of metabotropic α₁ adrenoceptors leads to production of inositol triphosphate (InsP₃), increase in [Ca²⁺]_i and slow contraction. Neuropeptide Y (NPY) stored in large granular vesicles (LGV) acts on release both as a prejunctional inhibitory modulator of release of ATP and NA and as a post-junctional modulatory potentiator of the actions of ATP and NA. Nucleotidases are released from nerve varicosities and are also present as ectonucleotidases. [Reproduced from Burnstock and Verkhratsky (2010b) and reproduced with permission from Elsevier.]