Mean concentration-response curves (n⩾4) for the inhibition of (A) left atrial tension, (B) right atrial rate, (C) ISO-stimulated left atrial tension, (D) ISO-stimulated increase in papillary muscle tension by CPA in the absence (▪) and presence of 8-SPT; 10−6 (•), 3×10−6 M (X), 10−5 M (▴), 10−4 M (▾), 3×10−4 M (⧫), 10−3 M (+). Each point is the mean reduction of tension or rate. Vertical bars represent the s.e.mean, their absence indicating they lie within the symbol.