We investigated the receptor-mediated regulation of nifedipine-insensitive, high voltage-activated Ca2+ currents in guinea-pig terminal mesenteric arterioles (ImVDCC) using the whole-cell clamp technique. Screening of various vasoactive substances revealed that ATP, histamine and substance P exert modulatory effects on ImVDCC. The effects of ATP on ImVDCC after complete P2X receptor desensitization exhibited a complex concentration dependence. With 5 mm Ba2+, ATP potentiated ImVDCC at low concentrations (∼1–100 μm), but inhibited it at higher concentrations (>100 μm). The potentiating effects of ATP were abolished by suramin (100 μm) and PPADS (10 μm) and by intracellular application of GDPβS (500 μm), whereas a substantial part of ImVDCC inhibition by milimolar concentrations of ATP remained unaffected; due probably to its divalent cation chelating actions. In divalent cation-free solution, ImVDCC was enlarged and underwent biphasic effects by ATPγS and ADP, while 2-methylthio ATP (2MeSATP) exerted only inhibition, and pyrimidines such as UTP and UDP were ineffective. ATP-induced ImVDCC potentiation was selectively inhibited by anti-Gαs antibodies or protein kinase A (PKA) inhibitory peptides and mimicked by dibutyryl cAMP. In contrast, ATP-induced inhibition was selectively inhibited by Gαq/11 antibodies or protein kinase C (PKC) inhibitory peptides and mimicked by PDBu. Pretreatment with pertussis toxin was ineffective. The apparent efficacy for ImVDCC potentiation with PKC inhibitors was: ATPγS > ATP≥ADP and for inhibition with PKA inhibitors was: 2MeSATP > ATPγS > ATP > ADP. Neither ImVDCC potentiation nor inhibition showed voltage dependence. These results suggest that ImVDCC is multi-phasically regulated by external ATP via P2Y11-resembling receptor/Gs/PKA pathway, P2Y1-like receptor/Gq/11/PKC pathway, and metal chelation.