Mechanism of beat reversal in semi-intact heart preparations of the blowfly Phormia regina (Meigen)

The blowfly pulsatile organ is a tubular vessel consisting of an abdominal heart and a thoracocephalic aorta. Its activity consists of the regular alternance of a fast phase with a slow phase at higher and lower beating frequencies, respectively. In adult Phormia blowflies the fast and slow phases are triggered by separate pacemakers at the abdominal and cephalic vessel endings, respectively. Owing to the position of the pacemakers, impulses propagate forwards along the vessel during the fast phase and backward during the slow phase. Accordingly, haemolymph 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 compartment where it is generated. Prevention of systolic emptying of abdominal heart compartments at the beginning of the fast phase abolishes phase alternance. Possible stretch sensitivity of the Phormia myocardium accounts for this result.Abbreviations CRO cathode-ray oscilloscope - ECG electrocardiogram - UV ultraviolet