Amiloride high- and low-sensitivity, as well as insensitive sites in the blowfly: implications for sugar, water and anion taste reception mechanism

Among chemoreceptor cells of the blowfly (Protophormia terraenovae, the so-called 'sugar' and 'water' cells respond to bovine serum albumin (BSA) and L-alanine (the C-terminal amino acid of the BSA molecule) with the highest spike frequency. In both cell types the response to either BSA or L-alanine was unaffected by 0.1 mM amiloride. However, 0.5 mM amiloride decreased the response of the 'water' cell to both BSA and L-alanine significantly, and similarly inhibited the response of the 'sugar' cell to L-alanine, but in contrast to the 'water' cell, 0.5 mM amiloride did not affect the 'sugar' cell response to BSA. The 'sugar' cell responses to both BSA and L-alanine were unaffected by the putative cell second messengers, cAMP and cGMP. The so-called 'anion' cell unexpectedly gave responses to BSA and L-alanine that were enhanced by amiloride at both concentrations. However, like the 'sugar' cell, the 'anion' cell was also unaffected by cAMP and cGMP.We conclude that the 'sugar' cell must have at least three types of receptor sites: the previously described 'F' (amiloride sensitive) and the 'P' (amiloride insensitive) sites, and a low-sensitivity 'T' site that mediates, at least in part, the response to BSA and L-alanine. The effects of amiloride on the responses of the 'water' and 'anion' cells are more difficult to interpret because fundamental information on their chemoreception mechanisms is still lacking.