11752209 Guanosine-5'-O-(3-[35S]thio)triphosphate binding on membranes and cAMP assays in whole cells revealed that only the h5-HT4b isoform coupled to Galphai/o-proteins in addition to its well-documented Galphas coupling. In contrast, the h5-HT4a receptor coupled only to Galphas-proteins, however, was able to trigger an increase in the intracellular calcium concentration ([Ca(2+)]i).