Influence of atrial natriuretic factor on fluid efflux from the splenic circulation of the rat.

1. Atrial natriuretic factor (ANF) causes a reduction in plasma volume that is abolished by splenectomy. Experiments were conceived to investigate whether ANF acts within the spleen to increase efflux of fluid from the intravascular to the extravascular space. 2. ANF, infused into the splenic artery of anaesthetized rats at rates of 1, 5 and 20 ng min -1, caused a dose-dependent increase in the arteriovenous difference in haematocrit as blood flowed through the spleen (basal difference, 0.18 +/- 0.10%; difference after 10 min at 20 ng min -1 ANF, 1.5 +/- 0.18%; n = 6). There was no such change in plasma protein concentration. 3. ANF (20 ng min -1) did not alter splenic arterial blood flow. However, splenic venous blood flow fell so that the arteriovenous difference increased significantly (basal difference, 0.34 +/- 0.19 ml min -1; difference at 60 min, 1.1 +/- 0.20 ml min-1, n = 7). There was no change in mean arterial pressure. 4. These data confirm our hypothesis that ANF acts within the spleen to increase fluid efflux from the intravascular to the extravascular space. Since there is no change in total splenic blood flow, we propose that the effects of ANF are mediated by dilatation of the splenic afferent arterioles and constriction of the efferent venules, thus increasing filtration pressure.