Fig. 7. 5-HT-induced pH changes in the lumen of the salivary gland as measured with double-barrelled pH-sensitive microelectrodes. (A) Effects of three different 5-HT concentrations (1, 3 and 10 nmol l^–1) on transepithelial potential (upper trace) and luminal pH (lower trace). The slightly positive transepithelial potential in the unstimulated gland became at least transiently negative upon 5-HT stimulation and displayed a large positive overshoot after 5-HT withdrawal as described previously (Berridge and Prince, 1971). In our hands the time courses of the transepithelial potential changes recorded by the reference barrel of the pH-sensitive microelectrodes differed slightly from those recorded by the classical paraffin oil-gap method (e.g. Berridge and Prince, 1971) for unknown reasons. However, similar differences were noted earlier in luminal recordings with double-barrelled K⁺-sensitive microelectrodes by the same group (see e.g. fig. 5 in Gupta et al., 1978). (B) Dose–response relationship of the 5-HT-induced pH changes obtained from recordings similar to those shown in A.