In isolated cats' lungs, perfused at steady inflows with a mixture of plasma and dextran, we measured the pulmonary arterial pressure, the left atrial pressure, the tracheal pressure and the inflow of perfusate. We were able to obtain directly or to calculate relations between any pair of these four variables with the remaining pair at constant values. The results are used to analyze how tracheal pressures affect the pulmonary vascular bed and to consider some consequences of the changes in pressure which occur during cardiac and respiratory cycles in the whole animal.INTRODUCTION IN this paper we analyze the way in which the pressure in the air spaces of the lung acts upon the pulmonary blood vessels. The analysis is based upon a study of the relations between the four variables flow, arterial pressure,

MacIntosh (1938) was the first to relate the time of failure of conduction through a denervated sympathetic ganglion with a diminution in its acetylcholine content. From the work of Brown & Feldberg (1936) on perfused ganglia, and of Feldberg (1943) on incubated extracts of ganglia, it appeared that both events might be attributed to a failure of the preganglionic endings to synthesize acetylcholine. However, Feldberg's methods (1943) were relatively insensitive (Nachmansohn, John & Berman, 1946) and did not provide unequivocal evidence that the reduced acetylcholine synthesis obtained in denervated ganglia was due to loss of the enzyme concerned (choline acetylase) rather than to loss of some other constituent of the system such as the activator fraction. We therefore decided to repeat Feldberg's work using the more refined methods he later developed (Feldberg & Mann, 1945. These methods provide a means of aetermining the choline acetylase activity of a tissue independently of other variables, and have been adapted in our experiments so that synthesis measurements could be made on individual sympathetic ganglia of the cat. METHODSOperative and experimental procedures. Twenty-nine cats and three kittens were used for the tests. Of these, nine animals with no previous operation were treated as controls; in seventeen the superior cervical ganglion on one side was denervated (aseptically) by the removal of 1-2 cm. of the cervical sympathetic trunk from a site low in the neck. In three others the vagus nerve central to the nodose ganglion and the post-ganglionic trunk of the superior cervical ganglion were both cut on the one side. The operations were performed 1-29 days before the acute experiments. For the latter, anaesthesia was obtained either by Pentobarbitone sodium, 40 mg./kg., or by 'Dial' (Ciba), 0 7 ml./kg., injected intraperitoneally or intravenously. After tracheotomy, the two cervical sympathetic nerves were prepared for stimulation. Voltages ranging from 0*5 to 20 V., with pulse durations of 0-1 or 1 msec. were delivered from a 'Ritchie-Sneath' Square-Wave Stimulator. The nerve was stimulated for 5 sec. at approximately fifty shocks per sec. and contractions ofthe nictitating membrane were recorded to provide evidence for ganglionic transmission. In the nodose ganglion experiments, the peripheral end of the vagus was tested for stretch afferent

RESPIRATORY responses to the intratracheal inhalation of lung irritants have been described by many workers since the time of Paul Bert. Laqueur and Magnus [1921], and more recently Newton [1941], Fegler [1942], Smythe [1943], and Whitteridge [1948], have described some of the more immediate effects of phosgene inhalation seen in the cat. In the experiments described in the first part of this paper we have studied the action of phosgene on anwesthetised cats and dogs and compared it with that of ammonia, which is a typical respiratory irritant capable of initiating prompt and characteristic changes in the breathing. After analysis of these experiments it seemed desirable to determine whether there was any direct action on the lungs by these gases which, apart from the stimulation of afferent systems of the animal, might influence the total response. Experiments specially designed for this purpose were carried out on isolated blood-perfused dog lungs and are described in the second part of this paper. METHODS.AnCesthetics and Operative Procedure8.Mixtures of chloralose 005 g. and urethane 05 g./kg. given subcutaneously were used for ancesthesia in the phosgene experiments. In the ammonia series dogs were given either the chloralose and urethane mixture, or nembutal (0032 g./kg.) or sodium barbital; cats were given nembutal 0-032-0-06 g./kg.; and rabbits 0032 g./kg. The barbiturate anesthetics were given either intraperitoneally (dogs and cats) or intravenously (rabbits). So far as we could judge the type of aneesthetic was not a factor which decisively modified the results.All animals were tracheotomised. One group of five cats had their lungs chronically sympathectomised (bilateral removal of the stellate and T 1-4 ganglia) in a two-stage operation four and three weeks before

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