Figure 16-3 Schematic representation of different interactions that occur between neuropeptide Y (NPY) and adenosine triphosphate (ATP), and norepinephrine (NE) released from single sympathetic nerve varicosities. A, Diagram shows what occurs in the vas deferens and many blood vessels, where NE and ATP, probably released from small granular vesicles, act synergistically to contract (+) the smooth muscle through α₁ -adrenoceptors and P₂ -purinoceptors, respectively. B and C, Sympathetic neurotransmission in the heart and brain (B) and spleen (C). (From Lincoln J, Burnstock G: Neural-endothelial interactions in control of local blood flow. In Warren J [ed]: The Endothelium: An Introduction to Current Research. New York, Wiley-Liss, 1990, p 21.)