Functional and Structural Similarity between Insect and Human

Hearts

Electrocardiography of Insect Hearts for Screening of New Cardioactive Drugs

Karel Sláma

and Radek Aulicky

Institute of Entomology, Czech Academy of Sciences, Drnovská 507, 16100 Praha 6, Czech Republic

Department of Pest Control of Stored Produccts, Crop Research Institute, Drnovská 507, 16100 Praha 6, Czech Republic

Keywords: Peristaltic Myocardial Contractions, Tubular Heart, Heartbeat Reversal, Myogenic Heartbeat, Pacemaker

Nodus, Neuromuscular Paralysis.

Abstract: The primordial formation of insect and human hearts is orchestrated by similar sets of genes. The rhythmi

city of purely myogenic, peristaltic myocardial contractions of insect heart is determined by a posterior

pace-maker nodus, which is analogous to sinoatrial or atrioventricular pacemaker nodi of the human heart.

Insects are very mobile animals; there are vigorous neuromuscular contractions and extracardiac pulsations

in haemocoelic pressure, which may seriously interfere with recordings of the heartbeat. This problem was

solved by using larvae of the waxmoth, whose neuromuscular functions were totally paralysed by

proteinaceous venom of the parasitic braconid wasp. The paralysed larvae survived motionless for 3 to 4

weeks, however, the regular myogenic pulsations of the heart and intestine, regulated by depolarisation

potentials of the myocardial or intestinal cells, remained fully preserved. The paralysed larvae of the

waxmoth are ideal object for cardiological research. By means of a touch-free, optoelectronic method we

found that the larval heart exhibited uninterrupted, forward-oriented (anterograde), peristaltic waves of

systolic myocardial contractions, propagated with a rate similar to that of the human heart (at 37°C).

Extensive screening of various cardioactive drugs revealed that the larval heartbeat, like the human

heartbeat, was sensitive to chronotropic action of digitoxine and the nitrates or cardiomoderating action of

verapamil.

1 INTRODUCTION

The recent availability of electronic recording

techniques (Sláma, 2003; 2010), have revealed

unexpec-ted similarities between the physiological

systems of insects and mammals. For example, the

autonomic, cholinergic neuroendocrine system

regulating insect respiration is structurally and

functionally analogous to the mammalian

parasympathetic nerve system (Sláma, 2008a; 2012).

The mapping of the human and insect (Drosophila)

genome have revealed that the primordial formation

and later functioning of insect and human hearts

were orchestrated by identical sets of genes (review

by Bodmer et al., (2005). And, as a matter of fact,

the pulsations of insect and human hearts are

regulated by similar, purely myogenic mechanisms

based on depolarisation potentials of myocardial

cells. In both cases the rhythmicity depends on a

special regulatory nodi (atrioventricular and

sinoatrial node of the human heart) (Hampton,

2003), or terminal regulatory node in the heart of

insects (Sláma, 2008a); (Sláma and Lukáš, 2011).

When genomic structures were still unknown,

comparative physiological studies were justified

only between the closely related groups of animals.

The mapping of the human and insect genomes,

however, has created a substantially new situation,

which is limited only by a serious lack of physio-

logical data. This is true for electrocardiographic

(ECG) records, which are mostly available only for

the human heart.

The human heart is a compact muscular organ,

which pumps blood into a closed vascular system of

arteries, capillaries and veins, under increased pres-

sure and at a constant temperature of 37°C. The

main function of this mammalian circulatory system

is linked with respiration; the circulating blood is

carrying oxygen from the lungs to distant organs and

prevents respiratory acidaemia by removing the me-

Sláma K. and Aulicky R..

Functional and Structural Similarity between Insect and Human Hearts - Electrocardiography of Insect Hearts for Screening of New Cardioactive Drugs.

DOI: 10.5220/0004615800050012

In Proceedings of the International Congress on Cardiovascular Technologies (CARDIOTECHNIX-2013), pages 5-12

ISBN: 978-989-8565-78-5

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

tabolically produced carbonic acid in the form of

carbon dioxide. Insects do not use a closed cir-

culatory system of arteries, veins and capillaries.

Instead of the lungs, they breathe through a seg-

mentally arranged system of spiracles and air-filled

tracheal tubes, which are ramified all over the body,

thus transporting aerial oxygen directly to tissue and

cells. The insect ‘‘blood’’ (haemolymph) circulates

between the three major body compartments (head,

thorax, abdomen), which are mutually inter-

connected and form an open body cavity, or

haemocoelic cavity (Jones, 1977); (Miller, 1997).

The dorsal vessel of adult insects consists of a

narrow elastic tube called the thoracic aorta and a

larger abdominal portion that is conventionally

called the insect heart in the strict sense. The

myocardium is segmentally prearranged, with

several pairs of usually incoming ostial valves,

perpendicular allatal muscles and pericardial

nutritive cells. In general, the insect heart is a tubular

organ, propagating waves of peristaltic contractions

in the forward direction (larvae), or alternatively in

both forward and backward direc-tions, which is

known as the heartbeat reversal. Recent

investigations show that, in comparison to the

human heart, the dorsal vessel of insects is a

relatively weak circulatory organ which is unable to

pump blood against any large barrier of mechanical

pressure (Sláma, 2000; 2012). Accordingly, the

insect heart is mostly used for mixing haemolymph

between the capital, thoracic and abdominal

compartments of the widely open body cavity. Due

to the limited, though very economic, pumping

ability of their heart, insects have evolved a number

of auxiliary circulatory adaptations such as the

accessory pulsatile organs of the appendages,

peristaltic pulsations of the intestine or strong extra-

cardiac pulsations in haemocoelic pressure (review

Sláma, 2008).

Insects and mammals are phylogenetically very

distant groups of animals (Prostomia and Deutero-

stomia) separated by millions of years of indepen-

dent evolutionary pathways. In spite of this, how-

ever, there exists well substantiated genetic evidence

that both insect (Drosophila) and human hearts share

some common morphogenetic principles (review by

Bodmer et al., (2005). Of particular interest in this

respect is a Tinman (Tin) gene containing a

transcriptional factor for the primordial heart in both

Drosophila and the human body (Ocorr et al.,

2007a); (Zeitouni et al., 2007). Comparisons

between the two phylogenetically distant circulatory

systems have been hampered for a long time by

superficially different anatomical and physiological

structures. Re-cently it has been found, however,

that both insect and human hearts are regulated by

similar, involun-tary and purely myogenic

mechanisms (Sláma and Lukáš, 2011). It also

appears that the rhythmicity of systolic cardiac

contractions in insects depends on a special

pacemaker nodus (Terminal regulatory no-dus)

(Sláma, 2006; 2012), which has a similar phy-

siological role to the atrioventricular, sinoatrial, or

Hiss bundle pacemaker nodi of the human heart

(Hampton, 2003). In addition to segmental, peristal-

tically propagated systolic contractions, certain

insect species have evolved a compact, conical

ventricle in the heart, characterised by a human-like

atrium and synchronic, not peristaltic, mode of

cardiac contractions (Sláma, 2010). Similarities be-

tween insect and human hearts prompted me to

investigate the possibility of similar responses with

respect to medicinal cardioactive drugs. The assays

were facilitated by the development of noninvasive,

touch-free, electrocardiographic methods for insects

(Sláma, 2003; 2006; 2012); (Sláma and Lukáš,

2011).

It appeared that the common cardioactive drugs

(noradrenaline, digitoxine, nitrates, Ca2+ ion

blockers, indeed produced heartbeat responses in

insects similar to those found in the human heart.

Encouraged by the results obtained in Drosophila

(Occor et al., 2007b); (Fink et al., 2009); (Sláma,

2010), I also looked at the hearts of some other flies.

Of particular interest was a family of hoverflies

(Syrphidae) including the presence of several real

champions in sustained flight. One species,

Episyrphus balteatus, revealed a quite uncommon,

anatomically and functionally human-like heart,

with a compact ventricle pumping the insect

‘‘blood’’ into an artery-like aorta (Sláma, 2013).

Here I describe new ECG data for an insect heart

and try to find possible analogies with the known

facts in human cardiology.

2 RESULTS AND DISCUSSION

2.1 Electrocardiography of Pupal

Hearts

The depolarization and repolarization electrical

potentials created by intensive contractions of the

compact human myocardium can be successfuly

recorded by external electrodes located at different

parts of the body. This represents the common prin-

ciple of ECG recordings in medicine (Hampton,

2003). The contracting insect myocardial cells exhi-

CARDIOTECHNIX2013-InternationalCongressonCardiovascularTechnologies

bit similar depolarization and repolarization poten-

tials. However, these potentials are smaller and less

convenient for recording by external electrodes,

because: 1. The myocardium of an insect tubular

heart is divided into metameric segmental compart-

ments, which contract sequentially in the forward

(anterograde heartbeat) or backward (retrograde

heartbeat) directed waves of peristaltic contractions;

2. The rhythmicity of cardiac pulsations is deter-

mined by a pacemaker nodus located in posterior

segments of the heart; 3. Each peristaltic wave of

myocardial contractions can occasionally start

before the previous wave arrived to its final

destination (more peristaltic waves running at the

same time). These data show that, in contrast to the

human heart, the heart of insects creates relatively

small and dispersed depoarization potentials, which

are difficult to be directly recorded.

We are reasonably thinking, however, that due to

recent progress in electronic recording techniques, it

will be soon possible to monitor electrical depolari-

sation potentials of insect myocardial segments, just

like in ECG of the human heart. Recently we have

developed several ECG methods, which enable

accurate monitoring of myocardial contractions in

the insect heart. The methods cannot record the

depolarisation potentials directly, but reveal

precisely the myocardial contractions in one or

multiple segments of the body. So far, we have

obtained the best results with two, previously

described ECG methods suitable for insect heart

(Sláma, 2003); (Sláma and Miller, 2001), i. e.

thermocardiographic and optocardiographic

methods. In principle, the thermocardiographic

method is based on the use of miniature thermistor

sensors positioned externally above the pericardial

sinus of the heart. The sensors are gently warmed to

create a temperature gradient around their bodies.

Figure 1: Miniature thermographic sensors positioned

externally over the heart of a diapausing pupa of Manduca

sexta (from Sláma, 2003).

Figure 2: Heartbeat reversal recorded by two thermographic sensors from the first (1A) and eight (8A) body segments of

diapausing pupa of Manduca sexta. Lower portion shows the detail with expanded time scale (From Sláma, 2003).

FunctionalandStructuralSimilaritybetweenInsectandHumanHearts-ElectrocardiographyofInsectHeartsforScreening

ofNewCardioactiveDrugs

Figure 3: Effect of digitoxine injection (arrow; 20 mg/kg of body mass) on pupal heartbeat in Manduca sexta.

The subintegumental movements of haemolymph

caused by pulsations of the heart disturb the

established temperature gradients around the sen-

sors, which are converted into the corresponding

electrical sisgnals that are finally recorded by the

recording devices (see Figure 1 and 2).

The recording of insect heartbeat is often frus-

trated by movements of somatic muscles and by

special extracardiac pulsations in haemocoelic pres-

sure, used for inspiration and expiration of air

through spiracles. Suitable developmental stages for

recording the heartbeat of insects are immobile

pupae (Figure 1). The records in Figure 2 show that

the peristaltically propagated waves of myocardial

contractions move alternatively forwards and back-

wards. The rhythmicity is determined by a pace-

maker nodus located in posterior region of the heart.

After sectioning in the middle of the heart, posterior

section preserved rhythmicity and periodicity of

heartbeat reversal, whereas anterior section lacking

the pacemaker nodus exhibited only some slow,

indifferent contractions similar to human heart de-

prived from the ventricular nodus (Sláma, 2006).

The forward oriented, anterograde heartbeat, which

pumps abdominal haemolymph into the head,

represents the most important circulatory function.

There are numerous insect larvae which show only

this unidirectional, forward oriented anterograde

cardiac pulsation. In Figure 2, the thermographic

sensors located at the base of pupal abdomen reveal

increased amplitude of haemolymph circulation

during anterograde heartbeat. During the reciprocal,

retrograde heartbeat the relationships are reversed.

Diapausing pupae of Manduca sexta (Figure 1) are

very convenient for recording of insect heartbeat.

They can be stored in refrigerator for several months

before use. Figure 3 shows an example of chrono-

tropic effect of digitoxine injection on pupal heart-

beat of Manduca. Using this model system, we

tested a number of cardio-stimulatory or inhibitory

pharmacological preparations and found similar

structure-activity relationships known from the hu-

man medicine (Sláma, 2008b).

2.2 Optocardiography of Larval and

Pupal Hearts

Thermographic sensors need to be firmly attached to

integumental surface over the pericardial region.

This condition can be satisfied in immobile pupae,

but not in the mobile larvae with smooth and elastic

cuticle. These obstacles were restrained by

development of absolutely touch free, optoelectronic

techniques. The device shown in Figure 4 uses four

independent channels of red pulselight, focused to a

small (0.3 mm

) area over the heart through optic

fibers and lenses from the distance of 7 mm. The

beam of constant, stabilised pulse-light is applied to

the measured epidermal area by one optic fiber.

Changes in optical density, which are caused by

contractions of the heart, are dispatched to photo-

multipliers of the device by incoming optic fibers.

CARDIOTECHNIX2013-InternationalCongressonCardiovascularTechnologies

After multiplication and decoding, the heartbeat

pulsations are converted into output voltage and are

recorded. Figure 4 shows immobile pupa of Zop-

hobas atratus during touch-free recording of heart-

beat. Optocardiographic recordings could be exe-

cuted in all stages with reasonably transparent cuti-

cle and in specimens which do not move for a short

time. The dorsal vessel of insects (insect heart) is

located just under the integumental cover. Many

insects are resting immobile for shorter or longer

periods of time. Their heartbeat can be easily recor-

ded by optocardiographic methods, simply by focu-

sing the pulse-light beam over the pericardial region,

such as in case of M. sexta caterpillar during feding

(see Figure 5).

Figure 4: Four-channel optoelectronic device used for

pulse-light optocardiographic recording of heartbeat.

Figure 5: Caterpillar of M.sexta during the pulse- light,

optoelectronic recording of heartbeat by Sláma (2003).

2.3 Heartbeat during Neuromuscular

Paralysis

Larvae of the greater waxmoth (G. mellonella) are

perhaps the best experimental model of insects.

They are adjusted to live in 37°C of the bee hives. In

the laboratory, they can be reproduced in large

numbers at any time of the year. The larvae exhibit

unidirectional, purely anterograde heartbeat with the

rates comparable to that of the human heart (at

37°C). The problem is that these larvae are extre-

mely mobile and have rather fast developmental

rate. Under these conditions, recording of heartbeat

is very difficult. A great progress in this respect has

been achieved few years ago when Sláma and Lukáš

(2011) found that larvae, which were subjected to

neuromuscular paralysis induced by venom of the

parasitic braconid wasp, exhibited apparently normal

heartbeat. The profound neuromuscular paralysis,

which could last for several weeks, affected all so-

matic muscles innervated through neuromuscular

transmission. The peristaltic contractions of the heart

and intestine, however, which were regulated by de-

polarisation potentials of the myocardium, remained

unaffected and fully functional. The heartbeat

patterns of these motionless, paralysed larvae can be

easily monitored by all types of the recording me-

thods. Electrocardiographic investigations on these

larvae revealed the autonomic (brain independent)

nature of heartbeat regulation. Further advantage of

Galleria depends on the prolonged survival when

deprived of the brain hormone source by ligatures

made behind the head. The ligatured larvae with

arrested development (Figure 6 (A)) can be stored

for many weeks for later use. In combination with

additional paralysis by the braconid venom (Figure 6

(B)), these motionless larvae represent the best

experimental material for routine screenings of va-

rious cardio-active materials on insects. Due to total

absence of the somatic movements, it was possible

to use the pralysed larvae for prolonged optocar-

diographic recordings of the heartbeat. The use of

multiple optocardiographic sensors (see Figure 7),

enabled determination of the propagation velocity of

individual systolic contraction waves.

Earlier recordings (Sláma and Lukáš, 2011)

revealed more or less constant cardiac pulsations in

the paralysed larvae, characterised by 20–25 systolic

contractions per minute. The contractions were peri-

staltically propagated in the forward (anterograde)

direction with a more or less constant speed of 10

mm per second (23–25°C). 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

compartment of the disconnected heart remained

unaffected.

FunctionalandStructuralSimilaritybetweenInsectandHumanHearts-ElectrocardiographyofInsectHeartsforScreening

ofNewCardioactiveDrugs

Figure 6: The fully grown larvae of Galleria, which have

been ligatured behind the head (A), ligatured and injected

with braconid venom (B) or injected without ligaturing.

An example of simultaneous, 4 channel

optocardio-graphic, ECG-like record with the

paralysed larva (as shown in Figure 7), can be found

in Figure 8. The multichannel records of this type

reveal the details of systolic contractions of the

heart. In addition to the rate of heartbeat, these

records reveal the propagation velocity of individual,

systolic contraction waves.

Figure 7: Paralysed larva of Galleria during multiple

optocardiographic recordings.

Figure 8: Example of simultaneous optocardiographic record of heartbeat in paralysed larva of Gallera, obtained by means

of the multichannel setup shown in Figure 7 (from Sláma, 2011).

CARDIOTECHNIX2013-InternationalCongressonCardiovascularTechnologies

Figure 9: Effect of different temperatures on heartbeat rate and velocity of peristaltic myocardial contractions in the

paralysed larva of Galleria (from Sláma, 2012a).

Figure 9 gives detailed acount of the effect of

different temperatures on the rate of heartbeat in the

paralysed larva of Galleria. The larvae are paralysed

by purified extract prepared from the venom glands

of the tiny braconid wasp, Habrobracon hebetor

(10

glands per ml of Ringer, 20 µl injection).The results

show that the rate of heartbeat pulsation is between

80 and 90 pulses per min and the velocity of

peristaltic myocardial contractions is less than one

second, at 35°C.

Figure 10: Compact, human-like heart ventricle seen from

the ventral side of abdomen in an adult hover fly,

Episyrphus balteatus. The ventricle is stretched within the

body cavitly by lateral ligaments (left), with a narrow

posterior heart (right) (From Sláma, 2013).

The compact ventricle found in the hoverfly,

shown in Figure 10, represents a new and enigmatic

feature in comparative animal physiology. The

heartbeat rate shows real animal recod (frequency

over 10 Hz) and, what is most important, the

mechanism is not peristaltic, but synchronic, like in

the human heart (the whole ventricle composed of

several metameric segments contracts unisono). The

compact muscular ventricle and synchronic

contraction mechanism are apparently the

physiological adaptations to increasing demands for

the forward oriented, anterograde pumping of

haemolymph to extensively developed, thoracic

flight musculature.

3 CONCLUSIONS

We are reasonably thinking that due to the described

functional similarities between insect and human

hearts, the new cardioactive or inhibitory substances

can be tested on the hearts of insects. The biassays,

performed by the described, touch-free cardiogical

methods on the paralysed larvae of Galleria, can

potentially serve as a convenient and inexpensive

way how to avoid the use of large animals for

biological testing of cardiologically important

chemicals.

FunctionalandStructuralSimilaritybetweenInsectandHumanHearts-ElectrocardiographyofInsectHeartsforScreening

ofNewCardioactiveDrugs

ACKNOWLEDGEMENTS

We acknowledge with thanks financial support from

NAZV granting agency, Czech republic, for roject

No. QJI310057 to R.A.

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CARDIOTECHNIX2013-InternationalCongressonCardiovascularTechnologies