Photoreleased inositol 1,4,5‐trisphosphate‐induced response in single smooth muscle cells of rat portal vein.

1. The Ca2+ release in response to inositol 1,4,5‐trisphosphate (InsP3) was studied in single patch‐clamped smooth muscle cells of rat portal vein. InsP3 was photochemically produced from a caged InsP3 precursor included in the pipette solution. Changes in internal Ca2+ concentration ([Ca2+]i) were monitored by measuring Ca(2+)‐activated K+ current. 2. Photoreleased InsP3 evoked a transient K+ current which was abolished when 10 mM EGTA or 5 mg ml‐1 heparin was included in the pipette. The amplitude and time course of the K+ current responses depended on the light‐flash intensity. The amplitude increased, and the latency and the time to peak decreased, with increasing flash intensity, suggesting that the amount of released Ca2+ varied as a function of the amount of InsP3 photoreleased. 3. The K+ current response to photolysis of caged InsP3 was abolished in the presence of 10 mM caffeine; conversely, caffeine was inefficient at inducing at K+ current when applied immediately after a light flash of maximal intensity. 4. The time course of the recovery of the K+ response evoked by a light flash of supramaximal intensity was similar to that obtained for the 10 mM caffeine‐induced K+ current. The response recovered to 50% of control with an interval (t1/2) of about 10 s between pulses. The time course of the recovery of submaximal response to photoreleased InsP3 was considerably slower (t1/2 = 1 min), and did not correspond to that obtained for a response of similar amplitude evoked by 2 mM caffeine. 5. Responses to photoreleased InsP3 obtained after the cells were bathed for 3 min in Ca(2+)‐free solution were compared with those obtained in 2 mM Ca2+ solution. Responses to light flashes of submaximal intensity were proportionally more inhibited than those evoked by supramaximal stimulations. 6. In portal vein smooth muscle cells, the InsP3‐sensitive Ca2+ store seems also to be sensitive to caffeine. Our results suggest that the InsP3‐induced Ca2+ release was modulated by regulatory mechanisms.