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

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Abstract
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 μm) 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 μm 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 ≈ 170 μm. The results suggest that IP3 can depolarize the taste cell with mediation by intracellular Ca2+.