Fig. 7
Asterotocin causes relaxation of in vitro cardiac stomach and apical muscle preparations from A. rubens. a Representative recording showing that asterotocin causes relaxation of a cardiac stomach preparation. Seawater supplemented with KCl (3 × 10−2 M) was used to induce contraction of the cardiac stomach prior to the application of asterotocin. The relaxing effect of asterotocin is reversed when the preparation is washed with KCl-supplemented seawater. b Graph showing the concentration-dependent relaxing effect of asterotocin on cardiac stomach preparations at concentrations ranging from 3 × 10−11 M to 10−6 M. The responses are expressed as the mean relative percentage (± SEM; n = 16) of the maximal relaxing effect of asterotocin in each preparation. c Representative recording from a cardiac stomach preparation that compares the relaxing effects of asterotocin and the SALMFamide-type neuropeptide S2, both at 10−7 M. As in a, the cardiac stomach preparation was pre-contracted with KCl-supplemented seawater prior to the application of the neuropeptides. Inset compares the effects of asterotocin and S2 on cardiac stomach preparations at a concentration of 10−7 M, expressed as mean percentages (± SEM; n = 5) with the relaxing effect of S2 defined as 100%. The relaxing effect of asterotocin is significantly larger than the effect S2 (Mann-Whitney U test; P = 0.0079; n = 5). d Representative recording showing that asterotocin (10−6 M) causes relaxation of an apical muscle preparation. 10−6 M acetylcholine (ACh) was used to induce contraction of the apical muscle preparation prior to the application of asterotocin. Following washing of the preparation with artificial seawater, it returns to its basal relaxed state. Inset shows the mean percentage (± SEM; n = 4) reversal of 10−6 M ACh-induced contraction of apical muscle preparations caused by 10−6 M asterotocin over a 50-s period after peptide administration