The hypertensive action of 5-hydroxytryptamine (5-HT) on the pulmonary circulation of the dog has been studied by a number of workers during the past few years. They all agree that pulmonary hypertension of this origin is due to arteriolar vasoconstriction (McCanon and Horvath, 1954;Borst et al., 1956;Vitolo et al., 1957;Bracco and Curti, 1958;Carlier et al., 1958; GomezFerrer, 1958;Kabins et al., 1959;Shepherd et al., 1959;Fowler, 1960). In fact, according to some authors, 5-HT is the most powerful pulmonary vasoconstrictor there is (Ginzel and Kottegoda, 1953;Borst et al., 1957; Attinger, 1957 Attinger, , 1959Gilbert et al., 1957Gilbert et al., , 1958 Rose and Lazaro, 1958).Very little is known about the mechanism whereby pulmonary vasoconstriction is produced. Rose (1957), using a system of partial extracorporeal circulation, injected 5-HT in such a way that it came into contact only with the pulmonary vascular bed; the hypertension thus elicited was attributed to a direct action on the pulmonary vessels. Rudolph and Paul (1957) similarly came to the conclusion that 5-HT acts directly on the pulmonary arteriolar tone. Finally, bilateral cervical vagotomy, alone (Rose and Lazaro, 1958) or combined with section of the spinal cord (Nahas, 1958; Nahas and McDonald, 1959), has been shown to be incapable of modifying the pulmonary pressor response to 5-HT in the dog. However, as far as we know, the classical methods of studying circulatory physiopathology (adrenalectomy, section of the spinal cord, interaction with metabolites, sympatholytics and adrenolytics, etc.) have not so far been applied sufficiently for conclusive results to be obtained. Furthermore these experiments have largely been aimed at discovering the action of 5-HT on the peripheral circulation in various animal species, but we know that the effects of this substance vary to a considerable degree according to the species of animal, the dosage, the means, and the route of administration. These considerations prevent us from accepting the results so far obtained from experiments on other species of animals or in other regions of the canine circulation as valid for the action of 5-HT on the pulmonary circulation of the dog.The lack of studies on the subject, the powerful action of 5-HT on the pulmonary circulation, and the fact that it is the only substance so far known which will act on the pulmonary circulation in a dose insufficient to influence the systemic circulation (Rose, 1957) all led us to continue our studies of the mechanism by which 5-HT produces pulmonary vasoconstriction in the dog.The object of the present study was to discover whether 5-HT produces pulmonary vasoconstriction by increasing the nervous tone through stimulation of the sympathetic nerves or by releasing vaso-active substances originating in the adrenal medulla. To study the first possibility, the changes in pulmonary haemodynamics following injection of 5-HT in the dog in basal conditions were compared with those following ganglionic block of sympathetic impulses wit...
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