Fig. 5. Force/intracellular pCa loops taken before (time=0) and at various times in minutes after a 60 s exposure to 1 µmol l^-1 proctolin. Each loop represents a tetanus produced by a stimulus train (as in Fig. 4). The elevated force during the diastolic period represents the period of contracture. The arrowhead on the zero time trace shows the direction of the changes in force and pCa.