RELEASE OF CA2-MUSCLE CELLS OF RAT PORTAL-VEIN BY ATP-INDUCED CA2+ ENTRY( FROM INTRACELLULAR STORE IN SMOOTH)

1 The action of adenosine 5'-triphosphate (ATP, 10 mu M) was studied i n single patch-clamped smooth muscle cells of rat portal vein where th e free internal Ca2+ concentration in the cell (Ca-i) was estimated by the emission from the dye indo-1. 2 In the presence of 20 mu M gallop amil (D600), a blocker of voltage-dependent Ca2+ channels, ATP applied to cells held at a holding potential of - 60 mV evoked a transient in ward current and an increase in Ca-i. 3 The rise in Ca-i evoked by ATP was completely suppressed in the absence of external Ca2+ although a transient inward current was still observed. 4 ATP-induced responses w ere not modified by the addition of the inositol 1,4,5-trisphosphate r eceptor antagonist, heparin (1 mM) in the pipette solution. 5 In the p resence of caffeine (5 mM) or ryanodine (100 mu M) in the pipette solu tion, which deplete the intracellular Ca2+ store, the ATP-induced Ca-i rise was greatly reduced. 6 Our results suggest that in single cells from rat portal vein, ATP releases Ca2+ from intracellular stores with out involving InsP(3), but via a Ca2+ release mechanism activated by C a2+ influx through ATP-gated channels.