Neuropeptide Y (NPY) and ATP are cotransmitters of norepinephrine (NE). Modulation of ATP-mediated purinergic neurotransmission by NPY was investigated in rat perfused kidney. Beta,gamma-Methylene-L-ATP (beta,gamma-mATP; 1.0 to 1.5 microM, n = 8), NE (0.1 microM, n = 8), and NPY (0.1 microM, n = 14) increased perfusion pressure by maximally 12 +/- 1, 17 +/- 2, and 9 +/- 1 mmHg, respectively. In the presence of NPY, responses to ATP and NE were dramatically enhanced. Renal nerve stimulation in the presence of the alpha-adrenoceptor antagonist phentolamine (1 microM) induced pressor responses of 54 +/- 5 mmHg (n = 6). Alpha-blockade-resistant responses were abolished by the P2-purinoceptor blocker suramin (300 microM) and thus mediated by ATP. Purinergic responses were also reduced significantly (50%) by the NPY-Y1 receptor blocker BIBP 3226 (1 microM). NPY (0.1 microM) potentiated purinergic pressor responses and enhanced ATP release from 0.7 +/- 0.2 to 4.1 +/- 0.9 pmol (n = 4) associated with a significant increase of soluble ATPase activity. All NPY effects were prevented by BIBP 3226. Pressor responses to renal nerve stimulation delivered at short time intervals, mimicking enhanced sympathetic drive to the kidney, were not constant but showed a progressive rise, which was prevented by BIBP 3226. In this study, it is suggested that purinergic vasoconstriction in rat kidney depends on concomitantly released NPY. NPY by itself is only a weak vasoconstrictor but acts as a modulator of renal vascular resistance by enhancing the effects of its sympathetic cotransmitters, especially during sympathetic overactivity.