Factors Influencing Pulmonary Hypertensive Response to 5-Hydroxytryptamine

• Among the various chemical agents known to constrict the pulmonary vessels, 5-hydroxy-tryptamine (5-HT) is the most consistent and powerful vasoconstrictor (see references listed in review1). The vasoconstriction has been demonstrated in two types of preparations: (a) the intact animal in which the rise in pul-monary arterial pressure induced by 5-HT has been shown to be largely due to pulmo-nary vasoconstriction and partly due to in-creased blood flow and (b) the isolated lung preparation perfused with a pump at a con-stant flow of blood in which the rise in pul-monary arterial pressure induced by 5-HT is entirely due to pulmonary vasoconstriction. Between these two extremes of experimental preparations is the heart-lung preparation in which it is possible to control independently each of the following three factors: the venous return and the pulmonary blood flow, pulmo-nary vascular resistance imposed by intra-alveolar pressure, and left atrial pressure as exerted by aortic resistance. It is surprising to note that there are no available studies of 5-HT and other vasoconstrictors on such a classical preparation. The experiments re-ported below are intended to fill this existing gap. Unlike the isolated lung or intact animal preparation, the heart-lung preparation offers an opportunity to assess the importance of right ventricular function in determining the level of pulmonary hypertension induced by 5-HT. By supplementary use of acetyl stro