A comparison in vitro of the vasoconstrictor responses of the mesenteric arterial vasculature from the chicken and the duckling to nervous stimulation and to noradrenaline.

1 The vasoconstrictor responses of isolated mesenteric arterial vasculature of 2 to 5 week old domestic chickens (Gallus domesticus) and domestic ducklings (Anas platyrhynchos) to periarterial nerve stimulation and to intra- and extra-vascular noradrenaline were compared.2 The tissues were perfused at a constant flow rate (2 ml/min) and the change in perfusion pressure produced by the various stimuli was used as a measure of the vasoconstrictor response. In a further study a constant pressure (50 mmHg)-variable flow system was used to corroborate the findings with the constant flow system.3 The mean pressure response produced by nervous stimulation in the duckling mesentery (137 +/- 62 mmHg) was approximately 3 times greater than that produced in the chicken mesentery (46 +/- 29 mmHg; P < 0.001). Cocaine hydrochloride (1 x 10(-5) M) potentiated the responses in the duckling but not in the chicken.4 The mean maximum pressure response evoked by intravascular noradrenaline in the duckling (170 +/- 27 mmHg) was significantly greater than that in the chicken (92 +/- 32 mmHg; P < 0.001). Cocaine produced a similar degree of potentiation in the 2 species.5 The mean maximum pressure response evoked by extravascular noradrenaline in the chicken (70 +/- 23 mmHg) was significantly greater than that in the duckling (36 +/- 25 mmHg; P < 0.001) which was the converse of the effect for intravascular noradrenaline. Cocaine produced a much greater potentiation of the responses to extravascular noradrenaline in the duckling than in the chicken.6 The results from the constant pressure study were similar to the corresponding findings in the constant flow studies. Nervous stimulation arrested flow in the duckling mesentery but not in the chicken. The maximum reduction in flow rate produced by intravascular noradrenaline was significantly greater in the duckling than in the chicken (P < 0.001).7 Quantitative histological studies were performed on transverse sections of arteries prepared with haematoxylin and eosin staining and histochemical fluorescence from 4 chickens and 4 ducklings. The mean wall thickness:lumen diameter ratios of the primary and secondary branches of the duckling mesenteric arterial vasculature were 1.8 and 4.3 times greater than those of the chicken respectively (P < 0.05 and P < 0.001). The mean density of noradrenergic innervation of the main artery and its primary branches in the duckling was 1.7 and 2.4 times greater than that of the chicken respectively (P < 0.05 and P < 0.01).8 The functional differences demonstrated in this study can be explained, at least partially, on the basis of the structural differences observed. During diving in the duck, intense peripheral vasoconstriction is believed to conserve the limited oxygen stores for those tissues most sensitive to oxygen lack. The structural and functional findings in the present study reveal that the duckling mesenteric arterial vasculature is well adapted to produce powerful vasoconstriction and hence play its rôle in oxygen conservation during diving.