1. The motility of the distal colon of the rabbit has been examined by the conventional Trendelenburg method and by an isometric, isovolumic modification of this method.2. The colon shows a range of movements, and tetrodotoxin and cold-storage have been used in an attempt to differentiate between myogenic and neurally integrated activities.3. The observable myogenic movements are pendulum movements, ;tone rings' and ;tone waves'; the last of these can be weakly propulsive. The rabbit colon also shows a neurally organized and powerfully propulsive movement which corresponds to the peristaltic wave.4. The implication of a role for nervous structures in the propulsive activity of the rabbit distal colon is at variance with the view of Lee (1960).5. The myenteric reflex arc in the colon has been examined pharmacologically. Cholinergic neural transmission has been implicated at the ganglionic-synaptic and neuroeffector junctions, but additional noncholinergic mechanisms of chemical transmission have not been excluded.
In a previous investigation (Gillespie & Mackenna, 1960) rabbits were given daily intravenous injections of reserpine for several days and the responses of an innervated preparation of the colon from these animals were studied in vitro, the entire parasympathetic and sympathetic outflows being available for separate stimulation. It was expected that reserpine, which is believed to deplete adrenergic nerve endings of their chemical transmitter sympathin, would abolish the inhibitory effect of sympathetic nerve stimulation; in fact, inhibition was consistently replaced by a large contraction. This finding was the starting point of the investigations reported in this paper. In addition it has been found that in preparations from reserpine-treated rabbits the inhibitory effect of sympathetic nerve stimulation can be restored by soaking in adrenaline or noradrenaline; the mechanism of this restoration has been studied. A preliminary account of these results has already been published (Gillespie & Mackenna, 1959).
METHODSRabbits of either sex were killed by a blow on the neck and bled. Two types of innervated intestinal preparations were used. First, the doubly innervated preparation of the rabbit colon in which the extrinsic sympathetic (lumbar colonic) and parasympathetic (pelvic) nerves can be separately stimulated, by means of fluid electrodes. The dissection of this preparation and the details of the electrodes have been described (Garry & Gillespie, 1955). The second type of preparation consisted of a length of about 4 cm of mid ileum together with the mesentery and mesenteric artery supplying the region. The artery, plus some adjacent mesentery, was drawn into a fluid electrode and the periarterial nerves were stimulated. The preparations were suspended in a 200 ml. bath filled with Krebs's saline solution at 360 C and oxygenated by a gas mixture of 95% 02 and 5 % CO2. The composition of the saline solution was (g/l.) NaCl 6-92, KCI 0.35, CaCl2 0-28, KH2PO4 0-16, NaHCO3 21, MgSO4. 7H20 0-29, glucose 2-0. The contractions of the preparations were recorded with a light isotonic gimbal lever exerting a tension of 0*5 g and writing sideways on a smoked drum. The nerves were stimulated with 1 msec rectangular pulses whose voltage was adjusted to give maximal responses. The duration of stimulation was usually 15 sec and the frequency 10/sec for the pelvic nerve and 50/sec for the lumbar colonic nerves. Where other frequencies or durations of stimulation were used this is indicated. 2PHYSIO. CLVI
The response to nicotine of intestinal preparations in vitro is usually a contraction due to stimulation of the parasympathetic cholinergic neurones in Auerbach's plexus. Ambache (1951) and Ambache & Edwards (1951) found that an inhibitory effect of nicotine could be demonstrated if these intrinsic cholinergic motor neurones were first put out of action, either by Clostridium botulinum toxin (rabbit small intestine) or by atropine (kitten small intestine and stomach). This effect could be blocked by nicotine in ganglion-cell-paralysing doses and by hexamethonium. In the ileum of the rabbit an inhibitory effect of nicotine could not be produced in the presence of atropine because the motor effects of nicotine were not abolished by atropine unless high, and probably non-specific, concentrations of this drug were used. In contrast we have found previously, in experiments on preparations of the rabbit colon, that nicotine could produce inhibition in this region of the alimentary canal. By selecting suitable dose levels of nicotine we could obtain either pure inhibitory responses (with low doses), or pure motor responses (with high doses), or mixed inhibition and contraction; cholinergic blocking agents were not required in these experiments.
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