Prostacyclin production in vitro by rabbit aortic endothelium: correction for unstirred diffusional layers

The degree of mixing in fluid layers immediately adjacent to the endothelial surface is a major variable in assessment of prostacyclin (PGI2) production by cultured endothelial cells or intact vessel endothelium in vitro. Lack of adequate mixing should lead to underestimation of true production because PGI2 immediately adjacent to endothelium would be only poorly sampled upon buffer collection. Thoracic aortas from 38 New Zealand white rabbits were therefore excised, opened longitudinally, and mounted endothelial side uppermost in a buffer-filled chamber which excluded cut tissue edges from study. Production of PGI2 under unstirred and magnetically stirred conditions was measured by radioimmunoassay (RIA) for 6-keto-PGF1 alpha. For animals pretreated with the combination of papaverine and heparin (see below), unstimulated and arachidonate-stimulated 6-keto-PGF1 alpha increased with stirring rate toward limits of 2.9 and 28.5 ng/cm2/min, respectively. Unstimulated and stimulated 6-keto PGF1 alpha measured at 650 rpm, for example, were greater than their values at 0 rpm by factors of 3.5 (2P less than .01) and 3.7 (2P less than .001), respectively. The process of vessel excision, however, produces another variable: degree of injury to endothelium caused by such factors as secondary vessel contraction and thrombin generation. Vessel contraction and thrombin generation can be minimized, respectively, by the use of a smooth muscle relaxant and heparin administered prior to killing of the animals. The rabbits were, therefore, grouped according to intravenous (IV) treatment, prior to killing, with saline, papaverine (4 mg/kg), heparin (200 U/kg) or the combination of papaverine and heparin (same doses). As compared with pretreatment with saline, papaverine alone, or heparin alone, pretreatment with the combination of papaverine and saline led to increases in stimulated 6-keto-PGF1 alpha of 1.6- to 2.8-fold. By transmission electron microscopy, endothelium from animals pretreated with saline showed ultrastructural changes, including disruption of cytoplasm, separation without detachment of most endothelial cells from subendothelium, and focal areas of denudation. In contrast, ultrastructural integrity of endothelium was preserved in aortas of animals pretreated with combined papaverine and heparin. These results support the hypothesis that unstirred diffusional layers lead, in vitro, to underestimation of PGI2 production, especially when vessels are protected from excisional injury.(ABSTRACT TRUNCATED AT 400 WORDS)