Inositol 1,4,5-trisphosphate activates non-selective cation conductance via intracellular Ca2+ increase in isolated frog taste cells

The effect of intracellular Ca2+ increase was analysed in isolated frog taste cells under the whole-cell patch clamp. External application of a Ca2+-ionophore, ionomycin (3 microM) induced the sustained inward current of -200+/-17 pA (mean +/- SE, n = 23) at -50 mV in taste cells. The ionomycin-induced response was observed in most of the cells exposed in the drug, but not when 10 mM BAPTA (1,2-bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) was included in the pipette (eight cells). Steady-state I-V relationships of ionomycin-induced currents were almost linear and reversed at -8+/-1 mV (n = 23). The simultaneous removal of Na+ and Ca2+ from the external solution eliminated the response completely (three cells). Intracellular dialysis with 1 mM Ca2+ or 50 microM inositol 1,4,5-trisphosphate (IP3) in K+-internal solution also induced an inward current in the taste cells. The Ca2+-induced and IP3-induced responses were observed in 82% and 36% of the cells dialysed with the drugs, respectively. The Ca2+-induced and IP3-induced currents were inhibited by external Cd2+ (1-2 mM). The reversal potentials of the inward currents were -15+/-3 mV (n = 9) in Ca2+ dialysis and -11+/-3 mV (n = 13) in IP3 dialysis. The half-maximal Ca2+ concentration in the pipette to induce the inward current was approximately 170 microM. The results suggest that IP3 can depolarize the taste cell with mediation by intracellular Ca2+.