P2X receptor stimulation induces vasoconstriction of afferent arterioles through activation L-type calcium channels; however, the role of calcium channels in the afferent arteriolar response to P2Y receptor activation has not been examined. Recent studies have shown that ATP and UTP increase cytosolic calcium concentration in freshly isolated preglomerular smooth muscle cells through distinct voltage-dependent and -independent mechanisms. This study was performed to determine the role of L-type calcium channels on the afferent arteriolar vasoconstrictor response to ATP and UTP. Videomicroscopy experiments were performed in vitro using the blood perfused juxtamedullary nephron technique. Kidneys were perfused at 110mmHg and the responses of afferent arterioles to , ß-methylene ATP, ATP and UTP were determined before and during calcium channel blockade with diltiazem or blockade of calcium influx pathways with cadmium(Cd²⁺). In all arterioles studied, diltiazem treatment increased the diameter of afferent arterioles by 20 ± 1 % from 20.9 ± 0.6 to 25.0 ± 0.6 µm. , ß-methylene ATP (1.0 µM) evoked a sustained decrease in afferent arteriolar diameter of 8 ± 1% under control conditions, and this response was abolished during calcium channel blockade. In contrast, 10 µM UTP reduced afferent arteriolar diameter to similar degree before and during diltiazem treatment. Afferent diameter decreased by 20 ± 4% and 14 ± 4% before and during calcium channel blockade, respectively. Additionally, diltiazem completely prevented the vasoconstriction normally observed with ATP concentrations below 10 µM and attenuated the response obtained with 10 µM ATP. Similar studies were performed using Cd2+ to block calcium influx pathways. Cd²⁺ treatment increased afferent diameter by 37 ± 4 % from 16.2 ± 0.3 to 22.1 ± 0.7 µm. Under these conditions, the sustained vasoconstriction elicited by KCl, , ß-methylene ATP, and ATP were blocked. UTP-mediated vasoconstrictor responses were retained. These data demonstrate that L-type calcium channels play a significant role in the vasoconstrictor influences of , ß-methylene ATP and ATP but not UTP. The data also suggest that other calcium influx pathways may participate in the overall vasoconstrictor response evoked by P2 receptor activation. These observations are consistent with previous findings that UTP-mediated elevation of intracellular calcium concentration in preglomerular vascular smooth muscle cells relies primarily on calcium release from intracellular pools whereas, ATP-mediated responses involve both voltage-dependent calcium influx, through L-type calcium channels, and the release of calcium from intracellular stores. Taken together, these findings support the argument that P2X and P2Y receptors influence the diameter of afferent arterioles in the rat kidney through activation of disparate signal transduction mechanisms.