Effects of alpha-adrenergic activation on the shift of electrolytes and fluid after hemorrhage in rats

To assess the effects of alpha-adrenergic activation on hemorrhage-induced shifts of K+, Na+ and fluid between the intravascular and extravascular spaces, we continuously measured changes in plasma concentrations of K+ and Na+ (delta[K+]p, delta[Na+]p) and blood volume (delta BV) over 60 min after hemorrhage in nephrectomized rats. Hemorrhage was conducted over 5 min at the level of 0.5, 1.0 and 1.5% of body weight (H0.5, H1.0, H1.5), and the result was compared with hemorrhage of 1.0% body weight after administration of alpha 1-adrenoreceptor antagonist prazosin (0.2 mg/kg, H1.0P). delta[K+]p increased significantly (p < 0.05) after hemorrhage, and the peak value was proportional to the level of hemorrhage (0.18 +/- 0.08, 0.62 +/- 0.22 and 1.64 +/- 0.19 meq/l in H0.5, H1.0 and H1.5, respectively). delta[Na+]p decreased significantly (p < 0.05) after hemorrhage, and the decrease was sustained until the end of the experiment (-0.8 +/- 0.6, -1.0 +/- 0.5 and -2.2 +/- 0.5 meq/l in H0.5, H1.0 and H1.5, respectively). In H1.0P, the increase in delta[K+]p (0.25 +/- 0.09 meq/l at the peak) and the decrease in delta[Na+]p (-0.2 +/- 0.1 meq/l at the bottom) were suppressed significantly (p < 0.05) compared to H1.0. Although delta BV was greater in H1.0P than H1.0, plasma K+ content was not different between the groups. In H1.0, the calculated concentrations of K+ and Na+ in the fluid which shifted into the intravascular space ([K+]sf, [Na+]sf) in the first 30 min after hemorrhage were higher in [K+]sf (6.25 +/- 0.70 meq/l) and lower in [Na+]sf (128.0 +/- 3.2 meq/l) than the pre-hemorrhage plasma level. With regard to H1.0P, [K+]sf and [Na+]sf were not different from the pre-hemorrhage level of plasma. These results suggest that alpha-adrenergic activation after hemorrhage induces K+ movement into plasma to increase [K+]p, which might be related to the fluid shift from the intracellular to the extracellular space.