Saccharin activates cation conductance via inositol 1,4,5-trisphosphate production in a subset of isolated rod taste cells in the frog.
The transduction mechanism of the conductance activated by saccharin was analysed in isolated bullfrog taste cells under whole-cell voltage-clamp. Bath application of 30 mM saccharin induced an inward current of -34 +/- 12 pA (mean +/- SEM, n = 10) at a membrane potential of -50 mV in 10 (23%) of 44 rod cells. The concentration-response relationship for the saccharin-gated current was consistent with that of the gustatory neural response. The saccharin-induced current was accompanied with a conductance increase under internal low Cl- condition (E(Cl) = -56 mV), suggesting that saccharin activated a cation conductance. The reversal potential of the saccharin-induced current was -17 +/- 2 mV (n = 10). Intracellular dialysis of 0.5 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) completely blocked the saccharin-induced response, suggesting the involvement of a G protein in the transduction. The dialysis of heparin (1 mg/mL) also inhibited the response almost completely, but the dialysis of 1 mM 8-Br-cAMP did not affect the response significantly. Intracellular 50 microM inositol 1,4,5-trisphosphate (1,4,5 InsP(3)) also induced the inward current in five (38%) of 13 rod cells, but intracellular Pasteurella multocida toxin (5 microg/mL, G alpha q-coupled PLC activator) did not elicit any response in the cells. The results suggest that saccharin mainly activates a cation conductance in frog taste cells through the mediation of IP3 production.
Okada
Y
Y
Department of Physiology, Nagasaki University School of Dentistry, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan. okada@net.nagasaki-u.ac.jp
Fujiyama
R
R
Miyamoto
T
T
Sato
T
T
D000818:Animals; D000925:Anticoagulants / Q000494:pharmacology; D002412:Cations / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005930:Glossopharyngeal Nerve / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000096:biosynthesis* / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D011892:Rana catesbeiana; D012439:Saccharin / Q000494:pharmacology*; D013549:Sweetening Agents / Q000494:pharmacology*; D013649:Taste / Q000502:physiology; D013650:Taste Buds / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D013873:Thionucleotides / Q000494:pharmacology
D000925:Anticoagulants; D002412:Cations; D013549:Sweetening Agents; D013873:Thionucleotides; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C023427:guanosine 5'-O-(2-thiodiphosphate); D015544:Inositol 1,4,5-Trisphosphate; D006493:Heparin; D012439:Saccharin