Fig. 6. Force–extension relationships for anterior lateral artery (ALA), sternal artery (SA) and dorsal abdominal artery (DAA) rings. The response to gradual stretch was determined first. Then, the length was reduced from maximum stretch to the test length where PR (proctolin; squares) or glutamic acid (GLU; triangles) was applied. The DAA response to PR is almost buried in the curve. The amount that the drug-induced contraction would have shortened the ring if it were free to move was determined as shown for the SA.