Beta2‐adrenergic attenuation of capillary pressure autoregulation during hemorrhagic hypo tension, a mechanism promoting transcapillary fluid absorption in skeletal muscle

The aim of this study was to evaluate a possible humoral beta 2-adrenergic effect on the capillary pressure autoregulation capacity in cat skeletal muscle during bleeding. For this purpose capillary pressure autoregulation in response to graded decrease in arterial pressure was studied in sympathectomized muscle in the control state, and during haemorrhagic hypovolaemia in the presence and absence of selective beta 2-adrenoceptor blockade (ICI 118,551). The study was performed with a technique that permits continuous recordings of average capillary pressure in absolute terms and of the regional pre- and postcapillary vascular resistance, from which the pre- to post capillary resistance ratio could be determined. In the pre-haemorrhagic control state, an experimental decrease in arterial pressure from 100 to 50 mmHg caused a fall of capillary pressure from 17.6 by only 1.7 mmHg (delta PA/delta Pc = 29), demonstrating an efficient capillary pressure autoregulation. This autoregulation was accomplished by a decrease in pre- to post capillary resistance ratio in turn being a result of active precapillary dilatation and a passive increase in post capillary vascular resistance. Haemorrhage per se, via a humoral alpha-adrenergic preferentially precapillary vasoconstriction, caused a decrease in capillary pressure to 16.8 mmHg at arterial pressure 100 mmHg. A superimposed decrease in arterial pressure to 50 mmHg resulted in a capillary pressure fall by 3.7 mmHg (delta PA/delta Pc = 14), indicating impaired auto-regulation capacity. This attenuation to a great extent could be ascribed to adrenaline-induced B2-adrenoceptor stimulation, since beta 2-blockade restored the delta arterial pressure/capillary pressure ratio to 20. Low-dose isoprenaline infusion in the control state similarly caused marked impairment of capillary pressure autoregulation. The beta 2-adrenergic attenuation of capillary pressure autoregulation appears to be a beneficial effect in haemorrhagic hypotension, since it lowers capillary pressure passively in relation to the arterial pressure fall, thereby reinforcing the alpha-adrenergic active capillary pressure decrease, leading to more effective transcapillary fluid absorption and, hence, improved replenishment of plasma volume.