Figure 1.

Representative tracings showing the response to 5-HT in porcine left atrial pectinate muscles. The upper tracing (a) represents the effect caused by the administration of a single concentration of 5-HT (1 μM). The recording in (b) shows the response following the cumulative administration of 5-HT with half log unit concentration increments, indicated by the arrows. In (c), the response to a cumulative administration of 5-HT in the presence of the nonselective PDE inhibitor IBMX (20 μM) is shown. Experiments were terminated by the administration of a supramaximal concentration of isoprenaline (0.1 mM), still in the presence of previously administered substances.