Myogenic nature of insect heartbeat and intestinal peristalsis, revealed by neuromuscular paralysis caused by the sting of a braconid wasp

Larvae of the greater waxmoth (Galleria mellonella) become paralysed by the venom of the braconid wasp (Habrobracon hebetor) a few minutes after intoxication. The profound neuromuscular paralysis, which may last for several weeks, includes all somatic muscles that are innervated through neuromuscular transmission. The peristaltic contractions of the heart and intestine, which are regulated by the depolarisation potentials of the myocardium or intestinal epithelial muscles, remain unaffected and fully functional. Heartbeat patterns and intestinal pulsations were monitored in the motionless, paralysed larvae by means of advanced electrocardiographic recording methods (contact thermography, pulse-light optocardiography). The records revealed more or less constant cardiac pulsations characterised by 20–25 systolic contractions per minute. The contractions were peristaltically propagated in the forward (anterograde) direction, with a more or less constant speed of 10 mm per second (23–25 °C). Additional electrocardiographic investigations on larvae immobilised by decapitation revealed the autonomic (brain independent) nature of heartbeat regulation. Sectioning performed in the middle of the heart (4th abdominal segment) seriously impaired the pacemaker rhythmicity and slowed down the rate of heartbeat in the anterior sections. By contrast, the functions of the posterior compartments of the disconnected heart remained unaffected. These results confirmed our previous conclusions about the existence of an autonomic, myogenic, pacemaker nodus in the terminal part of an insect heart. They show an analogy to the similar myogenic, sinoatrial or atrioventricular nodi regulating rhythmicity of the human heart. Peristaltic contractions of the intestine also represent a purely myogenic system, which is fully functional in larvae with complete neuromuscular paralysis. Unlike the constant anterograde direction of the heartbeat, intestinal peristaltic waves periodically reversed anterograde and retrograde directions. A possibility that the functional similarity between insect and human hearts may open new avenues in the field of comparative cardiology has been discussed.

Larvae paralysed by Habrobracon venom remain motionless for several weeks due to general inhibition of neuromuscular transmissions. Pulsations of dorsal vessel are myogenic and the heartbeat of the paralysed larvae can be monitored in vivo without intervention of the neurogenic muscular systems. One picture shows paralysed larva during a touch-free, optocardiographic recording of heartbeat, which resulted in electrocardiographic record exemplified by the second illustration. Using this method, a motionless insect larva can be used for monitoring the effects of cardioactive pharmacological preparations.Figure optionsDownload as PowerPoint slideResearch highlights▶ Our results demonstrate the purely myogenic nature of insect heartbeat, which is exactly similar to myogenic regulation of human heart. ▶ We describe the regulatory, myogenic pacemaker nodus, similar to sinoatrial nodus of human heart, residing in the terminal ampulla of an insect heart. ▶ New electrocardiographic methods have been invented for in vivo motinoring of insect heartbeat, which may pave the way for inexpensive testing of cardioactive pharmacological preparations on insects.