Pungent chemicals increase cytoplasmic free calcium concentration in mammalian taste receptor cells

Abstract

Pungent compounds are widely used for seasoning. Hot sensation evoked by pungent chemicals including capsaicin, has been known that it is the result of stimulation of free nerve endings in sensory neurons. However, there has not been rigorous study of its direct effects on the taste receptor cells (TRCs). In the present study, we investigated direct effect of two kinds of pungent chemicals, capsaicin and piperine, on the cytoplasmic free calcium concentration ([Ca^2+]_i) in the isolated rat taste receptor cells using micrspectrofluorimetry technique. The effect of bitter taste and sweets on [Ca^2+]_i was also examined for comparision with those of pungent chemicals. Epithelial sheets rich in taste buds and free of muscle tissue were isolated from the circumvallate papilla of the rat tongue by mixed enzyme treatment following microdissection. After exposure to Ca^2+ -free Tyrode solution, we could get taste buds or single TRCs from the epithelial sheets. In the high K^+ bath solution, [Ca^2+]_i transient was observed, suggestion existence of voltage dependent Ca^2+ channels in TRCs. Capsaicin increased [Ca^2+]_i in a dose dependent manner in these cells. 10㎍M capsaicin little affected on [Ca^2+]_i, but 50㎍M capsaicin increased a small amount of [Ca^2+]_i, 100㎍M capsaicin increased a large amount of [Ca^2+]_i, from 1.04±0.05(Mean±S.E.) to 1.40±0.059(n=28). The effect of piperine on [Ca^2+]_i responses was similar to those of capsacin, but poperine was more potent than capsaicin. 1μM piperine little affected on [Ca^2+]_i responses, but 5μM piperine increased a small amount of [Ca^2+]_i. 10μM piperine increased a large amount of [Ca^2+]_i, from 0.99±0.03 (mean ± S.E.) to 1.43±0.05(n=20). The increase of [Ca^2+]_i evoked by both capsaicin and piperine appears to be due to Ca^2+ influx from the extracelllular medium, since the capsaicin induced [Ca^2+]_i increase was not observed in Ca^2+ free bath solution. Furthermore, 10mM ruthenium red also inhibited this capsaicin induced [Ca^2+]_i increase, suggestion it's mediated by vanilloid receptors. We also confirmed that denatonium, one of bitter taste and sucrose, sweet compound, both increase [Ca^2+]_i in these cells. But the Ca^2+ responses to these two substances were quite different to those evoked by pungent chemicals; Ca^2+ response was even observed in the Ca^2+ free bath solution. All our results suggest that pungent chemicals including capsaicin and poperine could increase [Ca^2+]_i responses of TRCs, presumably via vanilloid receptors.