Diaphragm activity during breath holding: factors related to its onset

The electrical activity of the diaphragm during breath holding has been studied. After a period of no or slight tonic activity a marked discharge is recorded, this is followed by others at a progressively higher rate up to the breaking point. The action of the diaphragm, not balanced by antagonist muscles, causes a fall of the intrathoracic pressure. The period of breath holding, in most subjects, may be divided into two parts: the first, characterized by voluntary inhibition of respiratory muscles activity; the second, by involuntary respiratory efforts. The standard error (se) of alveolar Pco2 values determined at the onset of diaphragm activity in groups of three similar tests is <0.5 mm Hg. Arterial Pco2 in the brain stem at the onset of diaphragm activity has been estimated: breathing O2 the mean values ±se in three subjects were, respectively, 46.2 ± 0.3, 46.4 ± 0.2, and 50.2 ± 0.2. The onset of diaphragm activity seems not affected by neurogenic factors related to lung volume or respiratory movements. An alveolar Pco2 – Po2 curve at the onset of diaphragm activity has been determined.

EFFECTS OF CENTRAL DEPRESSANT DRUGS UPON ACETYLCHOLINE RELEASE

Several central depressant and other drugs have been examined for their effects upon acetylcholine release from the stimulated, perfused cat superior cervical ganglion and rat isolated phrenic nerve-diaphragm preparations. The acetylcholine released was assayed biologically. Amylobarbitone sodium, chloral hydrate, trichloroethanol, methylpentynol, methylpentynol carbamate, paraldehyde, procaine hydrochloride and troxidone reduced the presynaptic release of acetylcholine from the ganglion. They also exhibited a postsynaptic blocking action, this component of depressant activity being particularly prominent with paraldehyde and troxidone. Closely analogous findings were obtained at the neuromuscular junction with methylpentynol and its carbamate, paraldehyde, procaine hydrochloride, trichloroethanol and troxidone. At both sites the drug-induced depression, both of transmission and of acetylcholine output, was reversible. Whereas hexamethonium regularly blocked ganglionic transmission with no effect upon acetylcholine release, tetraethylammonium not only completely blocked ganglionic transmission but concomitantly augmented acetylcholine output. These results are discussed in relation to the electrophysiological and metabolic events associated with neuro-effector transmission.

BLOCKAGE OF EPINEPHRINE-INDUCED HYPERGLYCEMIA DURING EXPOSURE TO SIMULATED ALTITUDES

Hyperglycemia elicited by subcutaneous injection of epinephrine (10 µg/rat) was completely abolished during the exposure of the rat to a simulated high altitude (220–250 mm Hg). Addition of 5–7% CO2 to the inspired gas during the altitude stress restored the hyperglycemic activity of epinephrine, suggesting that the alkalosis or hypocapnia provoked by hyperventilation, but not hypoxia itself, would be involved in the prevention of the hyperglycemic action of epinephrine. Glucose tolerance tests and in vitro experiments using isolated rat diaphragm showed that the inhibitory action of epinephrine on peripheral glucose utilization was abolished by shifting the pH of body fluids or incubation media toward higher levels. It was concluded that alkalosis, either directly or indirectly, abolished the hyperglycemic action of epinephrine as a consequence of the release of the inhibitory action of epinephrine on muscle glucose utilization.

BOTULINUM TOXIN, TYPE A: EFFECTS ON CENTRAL NERVOUS SYSTEM

This study has demonstrated that type A botulinum toxin has a depressant effect on the cortical electrical activity of anesthetized and unanesthetized monkeys. Simultaneous recordings of vital signs indicated a relative lack of change in the electrocardiogram, respiration, blood pressure, and heart rate during this time. The change in the electroencephalogram appeared cyclic in nature and independent of dose or time. All animals exhibited signs of respiratory failure characterized by a gradual interference with neuromuscular transmission at the diaphragm, and subsequently died an anoxic death.

THE ACTION OF SODIUM 4-HYDROXYBUTYRATE ON SPINAL REFLEXES

The actions of sodium 4-hydroxybutyrate, gamma-aminobutyric acid and meprobamate have been studied in unanaesthetized animals, in local anaesthetic tests, on isolated organ preparations, on convulsions induced by picrotoxin and strychnine, and on monosynaptic (patellar) and polysynaptic (plantar) reflexes of the spinal cord. Sodium 4-hydroxybutyrate induced a sleep-like state with three unusual features: the righting reflex was remarkably persistent, respiration was good throughout and recovery was abrupt. gamma-Aminobutyric acid was inactive and meprobamate caused flaccid paralysis with loss of the righting reflex. None of the agents affected the responses of the rat diaphragm either to direct stimulation of the muscle or to indirect stimulation through the phrenic nerve. Only meprobamate reduced the responses of theguinea-pig isolated ileum preparation, showed local anaesthetic action and had an anticonvulsant action. All three compounds were capable, after intravenous or topical application, of blocking plantar reflexes in doses which did not affect the patellar reflex. The spinal animal responded in the same way, to the same dose of sodium 4-hydroxybutyrate, as the decerebrate preparation. Topical application to the motor cortex had no effect on spinal reflexes. We conclude that sodium 4-hydroxybutyrate acts preferentially on the internuncial neurones in the spinal cord but differs from meprobamate in its other actions. The similarity between the actions of sodium 4-hydroxybutyrate and of gamma-aminobutyric acid provides furtherevidence in support of the hypothesis that sodium 4-hydroxybutyrate is involved in the gamma-aminobutyric acid metabolic pathways.

SOME EFFECTS OF LONG CHAIN POLYMETHYLENE BISONIUM SALTS ON JUNCTIONAL TRANSMISSION IN THE PERIPHERAL NERVOUS SYSTEM

A survey has been made of the effects on junctional transmission of the complete series of polymethylene bis-trimethylammonium (BTM) and bis-triethylammonium (BTE) salts from the decamethylene compounds (BTM 10 and BTE 10) to those with twenty-one methylene groups in the chain. These were tested for their ability to cause contracture of the isolated chick biventer cervicis preparation, and for their ability to block the twitch responses of this preparation, those of the rat isolated diaphragm preparation, and those of the cat tibialis anterior preparation. They were also tested for their ability to block transmission in the cat superior cervical ganglion, to block the actions of acetylcholine on the guinea-pig isolated ileum, and for ability to inhibit the hydrolysis of acetylcholine by acetylcholinesterase. Their electrical conductivity has been measured in aqueous solution. Ability to cause contracture of the chick biventer cervicis is confined to the compounds BTM 10 to 15; BTE 10, 11 and 12 have some weak activity but the other BTE compounds, and the BTM compounds with more than fifteen methylene groups, have virtually no activity. In the BTE series both neuromuscular blocking and ganglion-blocking activities increase with chain length up to a maximum in the region of BTE 15 to 17 and then decline. In the BTM series ganglion-blocking activity increases with chain length in much the same way as in the BTE series, though the maximum activity is at a slightly longer chain length. At the neuromuscular junction an increase in chain length beyond BTM 10 leads to a decline in activity but this returns to some extent at longer chain lengths, reaching a second maximum at BTM 18, above which it declines further. At the ganglion BTE 16 is only slightly more active than BTM 16 and about five-times as active as hexamethonium; at the neuromuscular junction in the cat BTE 16 is about five-times as active as BTM 16 and about eight-times as active as (+)-tubocurarine. The affinity of the BTE compounds for the postganglionic acetylcholine receptors of the guinea-pig ileum reaches a maximum at BTE 14 but does not decline significantly with further increase in chain length. Anticholinesterase activity, likewise, does not alter significantly between BTM 12 and BTM 21 and the activity of the compounds in the BTE series appears to be similar. This property could conceivably be modifying the actions of some of the intermediate compounds but is not likely to be affecting those of the more active ones. The conductivity experiments indicate that micelle formation could be limiting the actions of the compounds with 20 or 21 methylene groups, but is not likely to be affecting those of the other compounds. The results suggest that there is a regular increase with chain length of the affinity of these compounds for the receptors in the ganglia and at the neuromuscular junction but that efficacy in causing contracture is limited to compounds with three methyl groups in the cationic head and a chain of about ten methylene groups. The connexion between this ability to depolarize and the ability to block transmission by desensitization is discussed.

SOME PHARMACOLOGICAL PROPERTIES OF THIOPROPERAZINE AND THEIR MODIFICATION BY ANTI-PARKINSONIAN DRUGS

The pharmacological properties of a phenothiazine derivative thioproperazine have been compared with those of chlorpromazine, and the modifications by some anti-Parkinsonian drugs of its actions on the central nervous system have been studied. Thioproperazine was less potent than chlorpromazine in lowering blood pressure and antagonizing adrenaline in the cat, in depressing respiratory rate in the rabbit, in producing hypothermia and analgesia and in reducing the minimum anaesthetic dose of hexobarbitone in mice, and in protecting rats from convulsions induced by tryptamine. It was roughly equipotent to chlorpromazine in reducing locomotor activity of mice. Thioproperazine was more potent than chlorpromazine in protecting grouped mice from the acute toxicity of dexamphetamine, in preventing the acute behavioural disturbances produced by dexamphetamine in the rat, in producing a state of experimental catatonia in the rat and in preventing the emetic action of apomorphine in the dog. Hyoscine, benztropine or promethazine greatly reduced the ability of thioproperazine to prevent behavioural changes due to dexamphetamine in the rat and also abolished symptoms of experimental catatonia produced by thioproperazine. In contrast, the antiapomorphine activity of thioproperazine in the dog was not reduced to any extent by hyoscine or benztropine.