Release of Ca2+ from intracellular store in smooth muscle cells of rat portal vein by ATP‐induced Ca2+ entry

Abstract

1 The action of adenosine 5′-triphosphate (ATP, lO μm) was studied in single patch-clamped smooth muscle cells of rat portal vein where the 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 μm gallopamil (D600), a blocker of voltage-dependent Ca²⁺ channels, ATP applied to cells held at a holding potential of − 60 mV evoked a transient inward 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 were not modified by the addition of the inositol 1,4,5-trisphosphate receptor antagonist, heparin (1 μm) in the pipette solution. 5 In the presence of caffeine (5 μm) or ryanodine (100 μm) in the pipette solution, 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 without involving InsP₃, but via a Ca²⁺ release mechanism activated by Ca²⁺ influx through ATP-gated channels.