Studies were performed to determine the effect of thapsigargin (THAPS) on ATP-mediated increases in intracellular Ca2+ concentration ([Ca2+]i) in preglomerular MVSMC. Suspensions of freshly isolated preglomerular MVSMC were prepared by enzymatic digestion, loaded with fura 2 and studied using a microscope-based fluorescence spectrophotomcler. ATP mediated cytosolic Ca2+ responses were assessed in single MVSMC in the presence or absence of THAPS. Under control conditions, baseline [Ca2+]i averaged 121 ± 7nM (n=45 cells). Exposure to 100μM ATP stimulated a rapid increase in [Ca2+]i with peak and steady state values averaging 400 ± 133nM (P<0.05 vs baseline; n=5) and 210 ± 53nM (P<0.05 vs baseline); respectively. Pretreatment of the cells with 1μM THAPS in a 1.8mM Ca2+ solution did not significantly alter resting [Ca2+], (120 ± 14nM; n=10 cells) or the response to 100μM ATP. [Ca2+]i averaged 297 ± 37 and 177 ± 17nM during the peak and steady state periods; respectively. Interestingly, when similar experiments were performed in the absence of extracellular Ca2+, ATP treatment still evoked a significant transient increase in [Ca2+), but the plateau phase was abolished. The role of L-type calcium channels in the MVSMC response to ATP was also evaluated. Pretreatment of cells with 10μM diltiazem did not alter resting [Ca2+], (100 ± 9nM; n= 15 cells) compared to control; however, the steady state response to ATP was attenuated by approximately 70%. These data suggest that ATP increases [Ca2+]i, in MVSMC by stimulating calcium release from a THAPS-insensitive intracellular Ca2+ pool as well by stimulating Ca2+ influx via activation of L-type calcium channels.