Heartbeat patterns were monitored in the living bodies of decapitated adult flies using several electrocardiographic methods (pulse-light optocardiography, thermocardiography, strain-gauge cardiography). Unlike other insect species, in which there is a peristaltic segmental propagation of cardiac contractions, Drosophila uses extremely efficient synchronic cardiac contractions. The rate of synchronic cardiac pulsation, which is characterized by simultaneous propagation of anterograde systolic contractions along all the segments of the heart, is relatively fast (~ 4 Hz at room temperature). This pulsation is used mainly for the vigorous pumping of haemolymph into the head and thorax through a narrow elastic tube, the aorta (anterograde I heartbeat). In addition, this synchronic pulsation is also used to enhance the circulation of haemolymph throughout the abdominal body cavity (anterograde II heartbeat). The switch between thoracic (anterograde I) and abdominal (anterograde II) haemolymph circulation is regulated by periodically alternating, tetanic contractions and relaxations of the conical heart chamber (ventricle). In the latter there is a pair of slit-like apertures, which are closed or opened by contraction or relaxation of the organ, respectively. During contraction of the conical chamber, the apertures are tightly constricted for several seconds and haemolymph is pumped forwards into the aorta (anterograde I heartbeat). Conversely, during relaxation of the conical chamber, the apertures are wide open for a few seconds, haemolymph leaves the heart and leaks out through open apertures and circulates from the tail to the base of the abdomen. The backward oriented, retrograde heartbeat recorded in other insects, has a lower frequency (1 to 2 Hz), occurs in Drosophila only sporadically and usually in the form of individual or twinned systolic peaks of large amplitude. Unlike the synchronic nature of the anterograde I and II cardiac contractions, propagation of the relatively slow retrograde heartbeat is by peristalsis. The newly discovered, compact ventricle with atrium and synchronic functioning of the insect heart shows structural and functional analogies with the functioning of the human heart.
Heartbeat reversal patterns have been monitored in the body of diapausing pupae of M. sexta 2 h before and 3 h after the injections of [Arg7]-corazonin, using noninvasive thermographic and optocardiographic methods. Large dosages (10-6 M final concentrations of corazonin in the body) caused almost immediate, adrenaline-like enhancement of the anterograde heartbeat. During the relatively short, acute phase of the tachycardia induced by corazonin, the systolic anterograde contractions of the heart increased in average from 10.5 to 24 pulses per min, culminating at 2.5 min after the injections. Duration of the acute period of tachycardia was only 7 to 20 min, which was followed by a period of slightly elevated, residual anterograde heartbeat which persisted occasionally for 1 to 3 h. Smaller dosages of corazonin (10-7M concentrations in the body) occasionally also produced a less intensive cardiotropic effect, while the more diluted samples were completely inactive. In pupae of the beetle T. molitor, injections of corazonin (10-6 M in the body) had no effect on the rate of in vivo heartbeat at all. Pharmacological analysis of the effects of corazonin in M. sexta indicated that the cardiostimulating effects of corazonin did not conform with the expected action of a peptidic neurohormone. A possibility that these effects might be artifacts produced by the low molecular breakdown products of corazonin has been discussed.