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Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33101
cAMP is a second messenger implicated in
sensory transduction for taste. The identity of adenylyl cyclase (AC)
in taste cells has not been explored. We have employed RT-PCR to
identify the AC isoforms present in taste cells and found that AC 4, 6, and 8 are expressed as mRNAs in taste tissue. These proteins are also expressed in a subset of taste cells as revealed by
immunohistochemistry. Alterations of cAMP concentrations are associated
with transduction of taste stimuli of several classes. The involvement
of particular ACs in this modulation has not been investigated. We
demonstrate that glutamate, which is a potent stimulus eliciting a
taste quality termed umami, causes a decrease in cAMP in
forskolin-treated taste cells. The potentiation of this response by
inosine monophosphate, the lack of response to D-glutamate,
and the lack of response to umami stimuli in nonsensory lingual
epithelium all suggest that the cAMP modulation represents umami taste
transduction. Because cAMP downregulation via ACs can be mediated
through G
i proteins, we examined the colocalization of
the detected ACs with Gi proteins and found that 66% of
AC8 immunopositive taste cells are also positive for gustducin, a
taste-specific Gi protein. Whether AC8 is directly
involved in signal transduction of umami taste remains to be established.
immunohistochemistry; glutamate; umami; taste transduction; gustducin
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