Reversal of heartbeat was monitored in vivo by noninvasive, multisensor, thermo-cardiographic and pulse-light, opto-cardiographic techniques. The dorsal vessel was sectioned at the beginning, in the middle and near the end of the abdomen. Changes in the heartbeat were simultaneously monitored in both the disconnected anterior and posterior sections of the heart. The results revealed the existence of a caudal regulatory cardiac centre located in the fused A7-A10 abdominal segments. Posterior sections, containing this terminal ampulla of the heart always exhibited a more or less normal heartbeat reversal, including both anterograde and retrograde pulsations. This shows that the forward-oriented as well as the reciprocal, backward-oriented peristaltic waves of the heart are both regulated from the posterior regulatory center, without involvement of the cephalic region. The cardiac pulsations in the anterior sections of the heart were paralysed and seriously impaired by the lesions. During the acute phase after the lesions, anterior sections showed only some convulsion-like, unidirectional, backward-oriented peristaltic pulsations of low frequency. Within one or two days after the lesions, isolated anterior sections of the heart developed a subsidiary heartbeat regulation associated with the oscillating, bi-directional peristaltic waves running alternatively, forward and backward in opposite directions.
After a few days, the previously paralysed anterior sections of the heart were able to develop perfectly coordinated patterns of heartbeat reversal. At this time, the two asynchronous heartbeat patterns ran separately in each of the divided sections of the heart. One or two weeks later, reversal of the heartbeat occasionally occurred synchronously along the entire length of the dorsal vessel. Sectioning of the ventral nerve cord, removal of the cephalic nervous system (brain, frontal ganglion, suboesophageal ganglion and the associated nerves) or removal of the fused terminal abdominal ganglionic mass and adjacent caudal nerves, had no effect on the pattern of heartbeat reversal. These facts indicate that the pupal heart of M. sexta operates purely myogenically, like the human heart. The myogenic pacemakers of the caudal regulatory cardiac centre (analogous to the atrio-ventricular nodes of the human heart) are autonomous, generating inherent rhythmicity without intervention from the nervous system. Development of subsidiary pacemakers regulating rhythmicity in the lesioned myocardium and restitution of the synchronized contracting integrity between the two disconnected sections of the heart are new cardiological features, which merit further investigation.
Larval Manduca prothoracic gland cells in vitro responded to prothoracicotropic hormone (PTTH) from neurosecretory cells of the brain with an increase of intracellular free calcium. This effect is reversible and dose-dependent. Preincubation of the glands with TMB-8 and dantrolene, which inhibit the release of calcium from intracellular stores, did not decrease the PTTH-stimulated increase in calcium, indicating that intracellular calcium stores are not involved in the control of ecdysteroidogenesis. Pharmacological studies of the PTTH effect with calcium channel blockers revealed that the increase in calcium was totally blocked by cadmium, partially inhibited by nickel and lanthanum and by amiloride, an antagonist of T-type calcium channels. All other inhibitors tested were ineffective, suggesting that the increase in cytosolic calcium is induced by opening of calcium channels, presumably of the T-type, in response to PTTH. The action of PTTH on these channels may be mediated by a G-protein as shown by the effect of mastoparan, a G-protein activator, which increased the concentration of cytosolic calcium comparable to that evoked by PTTH., Heiner Birkenbeil, and Lit
Pulsations of dorsal vessel were monitored by the noninvasive techniques of contact thermography on the dorsal cuticle and by strain gauge detection of abdominal elongation movements. Diapausing pupae exhibited periods of forward-oriented, or anterograde pulsations (average duration of each pulsation 5-8 min, frequency of individual systolic strokes 10-15 per min) alternating with somewhat slower, backward-oriented or retrograde cardiac pulsations (average duration of each pulsation 6-10 min, frequency of systolic strokes 7-12 per min). The highest rate of hemolymph flow was associated with the anterograde pulsations. We studied cardiac functions in diapausing pupae because of the almost complete absence of extracardiac hemocoelic pulsations, which are much stronger and could interfere with the recordings of heartbeat in all other developing stages. The movement of abdomen associated with the heartbeat was extremely small, only some 0.14 to 0.9 µm (i.e. from one 428000th to one 66000th of the body length) and thus was not practical for routine recordings of heartbeat.
Simultaneous recordings from multiple thermographic sensors revealed the complete absence of retrograde cardiac pulsations in the head region. There are some indications that the retrograde pulsations were also lacking in the thoracic region of the aorta. The retrograde peristalsis appeared to be used for circulatory functions in the abdomen alone. By contrast, the anterograde cardiac pulsations underwent a profound amplification in the anterior part of the abdomen, entering thoracic aorta with considerable strength before reaching the final destination in the head region. The amplification of anterograde peristalsis was manifested by enhanced hemolymph flow towards the head associated with a two-fold increase in frequency of anterograde heartbeat before reaching the head region. The sensors distributed along the dorsal vessel revealed that the rate of the backward-oriented, retrograde cardiac flow of the hemolymph was also location specific. The rate of flow was lowest at the front of the abdomen, medium in the middle and highest close to the end of the abdomen. The finding of lowest hemolymph circulation at the beginning of the cardiac peristaltic waves suggested that the physiological "raison d' être" for heartbeat reversal was a need for differential enhancement of hemolymph flow towards the extremities of the immobile pupal body. The switchovers from the retrograde to anterograde cardiac pulsations were usually immediate, while the reciprocal, antero- to retro-switchovers were mostly associated with a brief cardiac arrest. Increasing temperature gradients (in 5°C steps) progressively diminished duration of both reciprocal heartbeat periods. The amplitudes of the cardiac systolic strokes also decreased with increasing temperature while the frequencies were substantially elevated.