Effects of severe hemorrhagic hypotension on the vasculature of the chicken.

Abstract We have recently reported that hemorrhage to a mean arterial blood pressure (MABP) of 50 mm Hg in the chicken has no effect on total peripheral resistance or skeletal muscle vascular resistance (judged from changes in limb perfusion pressure (Pp) during constant blood flow). In the present study we report the effect of a more severe hemorrhagic hypotension (MABP = 25 mm Hg) on skeletal muscle vascular resistance in the constantly perfused hindlimb of the chicken following severance of the sciatic nerve trunk, bilateral cervical vagotomy, α-adrenergic blockade, or during artificial perfusion of the head with arterial blood. Concentrations of serotonin (SER), dopamine (DA), and norepinephrine (NE) in plasma were determined at different levels of hypotension. While hemorrhage to MABP = 50 mm Hg had no effect on P p, a further hemorrhage to MABP = 25 mm Hg produced a sharp rise in P p which was unaffected by severance of the sciatic nerve truck or bilateral cervical vagotomy. This vasoconstriction could be completely climinated by intraarterial infusion of phentolamine or by pump perfusing the head during the hypotensive interval. Furthermore, concentrations of SER, DA, and NE were significantly elevated only when the rise in P p was evident. We conclude that the vasoconstrictor response to severe hemorrhagic hypotension in the chicken is primarily mediated by an increase in circulating catecholamines due to cerebral ischemia, rather than a baroreflex.