MECHANISM OF BEAT REVERSAL IN SEMIINTACT HEART PREPARATIONS OF THE BLOWFLY PHORMIA-REGINA (MEIGEN)

The blowfly pulsatile organ is a tubular vessel consisting of an abdom inal heart and a thoracocephalic aorta. Its activity consists of the r egular alternance of a fast phase with a slow phase at higher and lowe r beating frequencies, respectively. In adult Phormia blowflies the fa st and slow phases are triggered by separate pacemakers at the abdomin al and cephalic vessel endings, respectively. Owing to the position of the pacemakers, impulses propagate forwards along the vessel during t he fast phase and backward during the slow phase. Accordingly, haemoly mph flows to the head during the fast phase and to the abdomen during the slow phase. Interspike interval and conduction velocity decrease, while spike duration and risetime increase, from the beginning to the end of the fast phase, together with the emptying of the vessel compar tment where it is generated. Prevention of systolic emptying of abdomi nal heart compartments at the beginning of the fast phase abolishes ph ase alternance. Possible stretch sensitivity of the Phormia myocardium accounts for this result.