Influences on central hemodynamics in hemorrhage of β₂‐adrenergic vascular control mechanisms

Abstract

Central hemodynamic responses evoked by standardized hemorrhage (exsanguination of 20 ml x kg bwt-1) were followed during 2 h in cats with intact and blocked vascular beta 2-adrenoceptors using the 'selective' beta 2-blocker, ICI 118, 551. In the first 10 min after bleeding blood pressure and cardiac output (CO) decreased and total peripheral resistance (TPR) increased by the same amount in the 'intact' and beta 2-blocked animals. Whereas blood pressure later on reached approximately the same hypotension level in both groups, other hemodynamic variables were distinctly different. In the 'intact' animals there was a gradual, partial recovery of stroke volume (SV) and CO in the face of a restoration to control of TPR. In the beta 2-blocked animals TPR continued to increase in the face of a maintained low CO and declining SV. The lower SV in the latter group was ascribed to abolition of beta 2-adrenergic restoration of plasma volume via absorption of tissue fluid into the circulation. The gradual decline of TPR in the 'intact' animals was attributed to beta 2-adrenergic dilator interaction with constrictor influences on the resistance vessels. It is concluded that beta-adrenergic vascular control mechanisms help to improve nutritional tissue blood flow during hemorrhage by increasing plasma volume, and hence venous return and CO, and by decreasing TPR. These reflex, beta 2-adrenergic circulatory events are similar to those aimed at in current shock therapy by transfusion and vasodilator treatment.

β2‐adrenergic control of plasma volume in hemorrhage

“…It seems evident that such large increase in intravascular volume can significantly increase venous return to the heart and thereby stroke volume and cardiac output, a conclusion strongly supported by expts. in which central hemodynamics during bleeding were studied in cats with intact and blocked beta*-adrenoceptors (Gustafsson et al 1982). Cardiac performance may be further improved by increase in the amount of 'circulating' red cells as a result of constriction of the spleen, which in the cat has been reported to discharge as much as 15-30 ml of blood with high hematocrit in response to bleeding (Barcroft et al 1925, Barcroft & Pool 1927.…”

“…The recently described highly 'selective' &blocking agent ICI 118, 551 (Bilski et al 1980) was used thereby avoiding undesirable blockade of the piadrenoceptors of the heart (cf. Gustafsson, Hillman & Lundvall 1982). Preliminary reports of this study have been presented elsewhere (Lundvall 1981 They uere tracheotomized and breathed spontaneouslq .…”

“…The absorption process, which is mainly caused by reflex decrease of the capillary hydrostatic pressure (see Mellander 1960, Oberg 1964, has recently been shown to be causally linked to a /3-adrenergic dilator influence interacting with the concomitant a-adrenergic constrictor response (Lundvall & Hillman 1978). The experimental evidence indicated that activation of the /3-adrenoceptors, in the presence of a concomitant a-adrenergic constrictor influence, leads to decrease of capillary pressure as a result of a relatively more pronounced dilator interaction in post-than in pre-capillary resistance vessels and that the absorption process is further facilitated by /3-adrenergic relaxation of the precapillary 'sphincter' vessels leading to an increase in capillary surface area available for fluid exchange (Lundvall & Hillman 1978, Lundvall, Hillman & Gustafsson (1982. These microvascular effects in skeletal muscle evoked by activation of /32-adrenoceptors (Hillman & Lundvall 1981 a), are to some extent mediated by the sympathetic vasomotor fibres but mainly by catecholamines released from the adrenal medulla (Hillman & Lundvall 1981 b) and could be shown to be responsible for about 70% of the cumulated fluid absorption from skeletal muscle to blood (Hillman 1981).…”

“…The gamma-radiation from the arterial blood passing the carotid shunt circuit was measured continuously during 50 min after each injection of the tracer using an external scintillation detector connected to a scalear and linear rate meter (Tri-carb Scintillation Spectrometer, model 3022, Packard) operatinga Rikadenki B-341 recorder (cf. Gustafsson et al 1982). Pulses corresponding to gamma-energies between 20 and I 0 0 keV were counted and the tracer was injected in amounts producing a 3-5 times increase in count rate above the background level.…”

Volume control during periodic changes of blood volume in the alert rat

“…This increase of venous compliance would decrease venous return (because heart rate and hence cardiac output is even slightly depressed in our experiments), a reduced sensitivity of arterial baroreceptors (3) playing a permissive role. On the other hand, periodic hypovolemia may be just as effective as chronic hypovolemia in stimulating 132 dilatory reflexes, decreasing mean total peripheral resistance (7). Both depression of heart rate and fall of precapillary vascular resistance during the hypervolemic phases may be the expression of a reflex of the Bainbridge type (5).…”