Developmental changes in in vivo cardiac performance in the moth Manduca sexta

While an extensive literature on cardiovascular development exists for inse cts, almost all studies focus on in vitro preparations, and very few report on more than a single developmental stage. The present study examines in v ivo cardiac performance in the intact, unanesthetized larvae, pupae and adu lts of the tobacco hornworm Manduca sexta. For all three stages, electrode pairs of fine steel wire were inserted subcuticularly at two dorsal abdomin al locations. Impedance signals produced by contraction of the dorsal abdom inal vessel (tube heart) were amplified and recorded. In addition to provid ing heart rate, a comparison of the relative timing of the signal from each electrode pair allowed the calculation of the propagation velocity and dir ection of heart contraction. Experimental treatments of intact animals incl uded exposure to hypoxia and hyperoxia (21 %, 15 %, 10 %, 5 %, 0 % and 100 % O-2()), to hypercapnia (0 %, 4 %, 8 %, 16 %, 20 % and 24 % CO2), to tempe rature variation (10, 20 and 30 degrees C) and to 2 min periods of forced a ctivity. The pattern of contraction of the dorsal abdominal vessel of M. serta chang ed substantially with developmental stage. Larvae showed a relatively simpl e, invariably posterior-to-anterior pattern (mean rate 34.8 +/0- 1.16 beats min(-1)). The heart rate pattern in pupal M. serta displayed great variabi lity in rate, amplitude and direction. Periods of regular heart beats (21.5 +/- 1.09 beats min(-1)) were frequently and irregularly interrupted by per iods of cardiac arrests ranging from a few seconds to over 20 min. Adults s howed a highly stereotypic but complex pattern, with periods of 'fast forwa rd' (FF; rate 47.6 +/- 2.6 beats min(-1)), 'slow forward' (SL; 32.8 +/- 3.0 beats min(-1)) and 'reversed' (R; 32.2 +/- 2.4 beats min(-1)) beating. The contraction propagation velocity in larvae and pupae averaged 5.52 +/- 0.3 6 and 2.03 +/- 0.11 cm s(-1), respectively. The SF, R and FF phases of the adults had average propagation velocities of 5.52 +/- 0.51, 5.05 +/- 0.52 a nd 5.43 +/- 0.37 cm s(-1), respectively. Heart rate and contraction propagation velocity were remarkably resistant t o ambient hypoxia and hypercapnia at all developmental stages, decreasing s ignificantly only at 0 % 02 or 24 % CO2. As expected, the heart rates of al l three developmental stages increased significantly with increasing temper ature, with heart rate Q(10) values for larvae, pupae and adults of 2.33, 3 .14 and 1.61, respectively, between 10 and 20 degrees C. Corresponding Q(10 ) values for these stages between 20 and 30 degrees C were 2.22, 2.03 and 2 .29. Larval heart rates showed no significant response to forced activity induce d by prodding. In contrast, adult heart rate increased nearly fivefold from 50.1 beats min(-1) during rest to 223.5 beats min(-1) after 1min of proddi ng. The activity-induced tachycardia in adults ceased within 10-12 min. Patterns of cardiac contraction in larval, pupal and adult M. serta were as dissimilar as their morphological appearances and revealed a gradation fro m simple to complex. These developmentally based distinctive cardiac patter ns are undoubtedly related to developmental differences in both morphology and life-style. Larvae are anatomically 'homogeneous' compared with other s tages, with no distinct head, thorax and abdominal region (or wings) that m ight require selective perfusion or drainage. The far more complex pattern of heart activity seen in pupae probably relates to the dramatic changes in internal morphology during this stage. Simultaneous degradation and synthe sis of tissues throughout the body may expose the heart to numerous peptide s or neurohormones that affect cardiac activity. In adult moths, the comple x and repetitive pattern of cardiac activity is reflected in the previously described complexity of hemolymph movement, together with thermoregulatory capabilities in this species that depend on well-regulated hemolymph movem ents between the thorax, wings and abdomen. Future studies on developmental changes in the control of heart rate in M. serta and other insects should prove of great interest.