Kinetics of Ca²⁺ release evoked by photolysis of caged InsP₃ in rat submandibular cells

Quantitative time-resolved measurements of cytosolic Ca²⁺ release by photolysis of caged InsP₃ have been made in single rat submandibular cells using patch clamp whole-cell recording to measure the Ca²⁺-activated Cl^- and K⁺ currents. Photolytic release of InsP₃ from caged InsP₃, at 100 Joules caused transient inward (V(H) = 60 mV)) and outward (V(H) = 0 mV)) currents, which were nearly symmetric in their time course. The inward current was reduced when pipette Cl^- concentration was decreased, and the outward current was suppressed by K⁺ channel blockers, indicating that they were carried by Cl^- and K⁺, respectively. Intracellular pre-loading of the InsP₃ receptor antagonist heparin or the Ca²⁺ chelator EGTA clearly prevented both inward and outward currents, indicating that activation of Ca²⁺-dependent Cl^- and K⁺ currents underlies the inward and the outward currents. At low flash intensities, InsP₃ caused Ca²⁺ release which normally activated the K⁺ and Cl^- currents in a mono-transient manner. At higher intensities, however, InsP₃ induced an additional delayed outward K⁺ current ((K(delay)). I(K(delay)) was independent of the initial K⁺ current, independent of extracellular Ca²⁺, inhibited by TEA, and gradually prolongated by repeated flashes. The photolytic release of Ca²⁺ from caged Ca²⁺ did not mimic the I(K(delay)). It is suggested that Ca²⁺ releases from the InsP₃-sensitive pools in an InsP₃ concentration-dependent manner. Low concentrations of InsP₃ induce the transient Ca²⁺-dependent Cl^- and K⁺ currents, which reflects the local Ca²⁺ release, whereas high concentrations of InsP₃ induce a delayed Ca²⁺-dependent K⁺ current, which may reflect the Ca²⁺ wave propagation.