Effects of moderate hypothermia on baroreflex and pulmonary chemoreflex heart rate response in decerebrate ferrets.

The effects on the baroreceptor reflexes and pulmonary reflexes of moderate immersion hypothermia (core temperature, 30.2 +/- 0.07 degrees C) have been investigated in the decerebrate ferret. Baroreflex sensitivity was estimated from the relationship between change in cardiac interval and change in systolic blood pressure following i.v. bolus injection of phenylephrine. The responses to phenylephrine in ferrets were best fitted by two linear slopes with an initial shallow slope, a, followed by a steeper slope, b. The slope or sensitivity of b was increased significantly by moderate cooling (n = 6 animals, 36 responses, P < 0.05). Pulmonary J-receptor reflex sensitivity was assessed by i.v. injection of phenylbiguanide (PBG), which evokes a dose-dependent bradycardia. The bradycardic response was unaltered by the hypothermia (n = 6 animals, 88 responses). Electrical stimulation of the right peripheral vagal nerve was employed to assess effects on the efferent components of the reflexes. The bradycardia, in response to stimulation, was significantly increased by moderate cooling at all stimulation frequencies (n = 8 animals, 88 responses, P < 0.001). Thus the results suggested that baroreceptor heart rate reflex sensitivity was enhanced by the moderate hypothermia. At least one component of the enhanced baroreflex response may be the result of changes in the efferent pathway of the reflex response. However, the absence of effect of PBG may indicate a differential afferent and efferent organization of pulmonary J-receptors compared with baroreceptors.

The effect of exposure to increased hydrostatic pressure up to 46 ATA on respiratory sinus arrhythmia in humans.

Respiratory sinus arrhythmia (RSA) has been used as a non-invasive estimate of vagal tone to determine whether the reduction in resting heart rate commonly seen when humans are exposed to increases in the ambient pressure is associated with changes in vagal autonomic control. Two sets of divers (n = 4) were examined during two simulated saturation dives, one to 46 atmospheres absolute (ATA) and another to 37 ATA. A significant reduction in resting heart rate was seen in both dives upon exposure to high pressure compared with controls at 1 ATA. The reduction in heart rate seen at increased pressure was consistent regardless of respiratory rate, indicating that moderate changes in respiratory rate do not affect heart rate under these conditions. No changes in the overall magnitude of RSA were observed over a range of respiratory rates during either dive compared with control values at 1 ATA. During both dives, RSA was significantly larger (P < 0.05) at low respiratory rates compared with higher rates for both 1 ATA controls and at high pressure. The presence of a hyperbaric bradycardia strongly suggests that vagal tone is altered during hyperbaric exposure; however, the lack of change in the magnitude of RSA at high pressure brings into question the viability of using RSA as an accurate indicator of vagal tone and this lack of correspondence deserves further investigation.

The effect of raised hydrostatic pressure on heart rate variability during breath holding in man.

Changes in cardiac interval (difference between two consecutive R waves of the ECG) during an expiratory breath hold (16 s) were examined in four divers during a saturation dive to 450 msw (metres of seawater). At 450 msw, breath hold caused a progressive shortening of cardiac interval that was significantly different from the changes seen at surface. The cause and significance of this shortening is unknown, but it might serve to maintain normal cardiovascular dynamics under hyperbaric conditions.

Possible NMDA antagonist properties of drugs that affect high pressure neurological syndrome.

1. Previous studies have suggested that a series of drugs modelled on part of the strychnine molecule interfere with the development of high pressure neurological syndrome (HPNS) and it was presumed that this effect was via an action on inhibitory glycinergic transmission. We have now used the rat hippocampal slice preparation to examine the possibility that some of these drugs might instead have an action at the strychnine-insensitive (SI) glycine binding site associated with the NMDA receptor. 2. D-2-Amino-5-phosphonovalerate (AP5) and 7-chlorokynurenate (7CK) had no significant effect on the height of the population spike recorded from the CA1 region in 1 mM Mg2+ medium, but both blocked the multiple population spikes recorded in Mg(2+)-free medium. The effect of 7CK, but not AP5, was reversed by 200 microM D-serine which is consistent with the known antagonist action of 7CK at the SI-glycine site. 3. A derivative of benzimidazole, which shows the clearest structural similarities to known SI-glycine site antagonists and ameliorates HPNS, mirrored the effects of 7CK although it was considerably less potent. 4. Gramine, which exacerbates HPNS, significantly increased the number of population spikes evoked in Mg(2+)-free medium. 5. Mephenesin, which is the most potent known drug in ameliorating HPNS, had no significant effect on the response recorded in 1 mM Mg2+ and significantly reduced the number of population spikes recorded in Mg(2+)-free medium, but this effect was only partially reversed by the addition of D-serine. 6. The results are consistent with the benzimidazole derivative, but not gramine, being an antagonist at the SI-glycine receptor. The results with mephenesin are equivocal but leave open the possibility that some of the drugs which are effective against HPNS act via an effect on excitatory NMDA receptor mediated transmission, rather than on inhibitory glycine-mediated transmission.

An investigation of cardiovascular reflexes during a trimix saturation dive to 450 msw (GUSI 17).

The study examines the hypothesis that the carotid sinus heart rate baroreflex responses are changed in human subjects on exposure to 450 msw. Baroreceptor reflex changes in heart rate (expressed as ms/mmHg applied pressure) were evoked by application of negative or positive pressure to a cuff surrounding the neck. At 450 msw using trimix, the mean resting heart rate of divers slowed significantly from 64 +/- 1.3 beats/min at surface to 55 +/- 1.4 beats/min at 450 msw, respiratory rate decreased from 15 +/- 1.4 at surface to 11 +/- 2 at 450 msw, and sinus arrhythmia increased. There was no change in arterial blood pressure. Baroreceptor reflex sensitivity to an increased carotid sinus transmural pressure was reduced from 5.6 +/- 2.9 (mean +/- SEM) at surface to 2.4 +/- 0.8 ms.mmHg-1 at 450 msw; sensitivity to decreased carotid sinus transmural pressure increased from 2.2 +/- 0.4 ms.mmHg-1 at surface to 5.1 +/- 0.2 ms.mmHg-1 at 450 msw. A progressive shortening of cardiac interval during breath hold in expiration was also noted. When this shortening of interval was incorporated into the analysis of baroreceptor reflex sensitivity, no significant change in sensitivity was observed but the overall baroreflex stimulus-response relationship shifted downward.

Role of vagal afferent C fibres in the bradycardiac response to an increase in arterial blood pressure in ferrets.

The effects on baroreceptor reflexes of bilateral application of the C fibre neurotoxin, capsaicin, to the cervical vagi have been investigated in decerebrate ferrets. Baroreflex sensitivity was estimated from the relationship between cardiac interval and systolic blood pressure following I.V. infusion of phenylephrine. Control responses to phenylephrine were fitted by two linear slopes with an initial shallow slope, a, followed by a steeper slope, b. Capsaicin abolished slope b and the resulting relationship was best fitted by a single regression with a slope not significantly different from that of the control response a. This suggests that a significant proportion of the baroreflex response induced by phenylephrine is mediated by cardiopulmonary receptors attached to vagal afferent C fibres.

An evaluation of the structure-activity relationships of a series of analogues of mephenesin and strychnine on the response to pressure in mice.

1. A range of compounds structurally related to the centrally acting muscle relaxant mephenesin and to the chemical convulsant strychnine were synthesized and tested for their ability to alter the threshold pressures for the onset of high pressure convulsions in mice. 2. The ability of both groups of compounds to alter the threshold pressure for convulsions was found to be dependent on the nature of a simple molecular skeleton. Thus, compounds that possessed a negatively polarized group located both in the same plane as and some 4.5 A from an aromatic nucleus increased the thresholds whereas compounds with a positively polarized group at the same location reduced the thresholds. 3. These findings support the suggestion that pressure elicits convulsions via a selection action on a receptor protein complex rather than via some general perturbation of the lipid regions of cellular membranes.

Effect of benzazole-related centrally acting muscle relaxants on HPNS.

A series of benzazole-related, centrally acting muscle relaxants, comprising benzimidazole, chlorzoxazone, and zoxazolamine, were found to give substantial protection against the tremors and convulsions associated with the high pressure neurologic syndrome (HPNS) in the mouse. In this respect they represent a new class of nonanesthetic, anti-HPNS agents. Their anti-HPNS properties, like those previously established for the mephenesin group of centrally acting muscle relaxants, seem to be related to their ability to antagonize the convulsive action of strychnine. These findings are consistent with the suggestion that one of the principal effects of pressure, expressed as HPNS, arises from a perturbation of strychnine-sensitive mechanisms.

Hydrostatic pressure enhances baroreceptor reflexes in the rat.

The effects of increased hydrostatic pressure on baroreceptor reflexes in the decerebrate rat have been investigated. Baroreceptor 'sensitivity' was assessed from the relationship between cardiac interval and systolic blood pressure following infusion of phenylephrine to animals exposed to increases in hydrostatic pressure (1-30 bar) with helium. Resting heart rate and systolic blood pressure were reduced and the 'sensitivity' of the baroreflex heart rate response was significantly increased by exposure to pressure. This enhancement of cardiovascular reflex function, elicited by moderate increases in hydrostatic pressure may, in part, be centrally mediated.

Investigations into the origin of the high pressure neurological syndrome: the interaction between pressure, strychnine and 1,2-propandiols in the mouse.

1. The effects of a variety of structural isomers of the centrally acting muscle relaxant mephenesin on the high pressure neurological syndrome have been investigated. Threshold pressures for the onset of the behavioural signs, tremors and convulsions, were established. The effects of these compounds on the response to pressure were also compared with their ability to antagonize the convulsive action of strychnine. 2. The dose-response relationships for strychnine and picrotoxin were investigated at fixed pressures. Additionally, the dose-response relationship of strychnine, in the presence of mephenesin, at pressure was investigated. 3. All the isomers of mephenesin protected against the effects of both pressure and strychnine. The relative potency was found to be identical with respect to both. Mephenesin was clearly the most effective; it raised the threshold pressure for tremors by 2.5 times, that for convulsions elicited by pressure by 1.5 and the ED50 for strychnine convulsions by 1.6 times. Strychnine was found to be strictly additive with pressure whereas picrotoxin exhibited gross deviations from additivity. Mephenesin ameliorated the combined effects of pressure and strychnine equally. 4. The marked dependence on structure of the anticonvulsant activity of the mephenesin isomers can be interpreted as evidence that pressure acts not by some general perturbation of the membranes of excitable cells but rather via some specific interaction. The finding that strychnine and pressure are strictly additive supports the idea of specificity and also indicates that they may share a common mechanism in the production of convulsions. By analogy with the established mechanism of action of strychnine, it is suggested that the hyperexcitability associated with pressure might arise from an action on glycine-mediated inhibitory processes.

Species variation and the mechanism of pressure-anaesthetic interactions.

The mechanism of general anaesthesia has proved difficult to elucidate (see ref. 1 for a review), although the relative potencies of anaesthetic agents have been used to establish that the site at which anaesthetics act is hydrophobic in nature. One further clue to their mode of action is that the effects of anaesthetics on vertebrates can be eliminated by pressures of approximately 100 atm (refs 3, 4). However, the effects of anaesthetics are not always reversed in model systems, where there is evidence that the pattern of pressure reversal varies significantly. We now find that pressure fails to reverse the effects of anaesthetics on the freshwater shrimp (Gammarus pulex), although the sensitivity of these crustaceans to anaesthetics is comparable with that of higher animals. This is hard to reconcile with traditional bio-physical mechanisms and indicates that anaesthetics may act at a specific protein site rather than having a general effect on cell membranes. The pharmacology of pressure in mammals seems to be more similar to that of strychnine than of any other central stimulant. As glycine, whose action is blocked by strychnine, is absent as a neurotransmitter in the arthropod central nervous system, we believe that this substance may be involved in determining pressure-anaesthetic interactions in vertebrates.

Mechanistic studies on the high pressure neurological syndrome.

The high pressure neurological syndrome (h.p.n.s.) constitutes a major barrier to deep sea exploration by man. Although the signs and symptoms of h.p.n.s. are well documented in both man and experimental animals, the underlying mechanisms remain to be elucidated. Physiological and pharmacological evidence will be presented that confirms that the principal sites of action of pressure are within the central nervous system (c.n.s.). Results from physiological studies not only indicate that there are separate sites of action of pressure within the c.n.s., which mediate the different components of h.p.n.s., but also the response to pressure may be controlled by descending inhibitory pathways. Pharmacological studies support this view and suggest that h.p.n.s. involves a failure of central inhibition.

Competitive antagonism by phentolamine of responses to biogenic amines and the transmitter at a neuroglandular junction.

1. A quantitative study has been made of phentolamine's inhibition of the electrical and secretory responses of the isolated salivary glands of Nauphoeta cinerea Olivier to nerve stimulation and bath applications of agonists. 2. The results suggested that phentolamine is a competitive antagonist having an affinity constant of about 1 micrometer-1 for the receptors for dopamine, noradrenaline, adrenaline and the neurotransmitter. 3. Phentolamine also inhibited responses to 5-hydroxytryptamine in a manner seemingly more complex than competitive antagonism. Attempts to estimate the affinity constant gave values of about 0.08 and 0.015 microgram-1 for inhibition of the secretory and electrical responses respectively. 4. This investigation showed that phentolamine discriminates between two kinds of receptor in this gland, one binding 5-hydroxytryptamine and the other combining with catecholamines and the neurotransmitter.

The salivary glands of the cockroach Nauphoeta cinerea (Olivier). A study of its innervation by light and scanning electron microscopy.

The innervation of the salivary gland of the cockroach Nauphoeta cinerea (Olivier) has been investigated with the use of light and scanning electron microscopy. Light microscopy of methylene blue stained glands reveals the presence of a dual innervation arising from the ventral nerve cord and the stomodeal nervous system; the principal innervation is that from the ventral nerve cord which passes to the gland via the reservoir ducts. Branches of these nerves form a plexus on the acinar surface, the axons of which exhibit swelling at irregular intervals. The presence of this surface plexus and the axonal swellings was confirmed by scanning electron microscopy both in normal glands and in those in which the basal lamina had been removed by means of an HCl-collagenase digestion method. No acinar plexus was seen to be formed by branches of the stomatogastric nerve that were associated with the gland. However, other branches of this nerve were clearly connected with a complex network of multipolar neurones on the surfaces of the anterior regions of both salivary reservoirs.

The actions of some putative neurotransmitters on the cockroach salivary gland.

1. Certain putative transmitters were applied to the innervated cockroach salivary gland and their effects on the resting potential and the neurally evoked secretory potential of the acinar cells were observed. 2. gamma-Aminobutyric acid, glutamate, glycine, aspartate and alanine had no significant effect on the resting potential. However, gamma-aminobutyric acid and glutamate reduced the neurally evoked secretory potential but only at concentrations above 10(-3) M3. Acetylcholine and carbachol appeared to act by modifying transmitter output from the salivary nerves. These substances failed to have any effect on the resting potential. 4. The biogenic amines, adrenaline, dopamine, noradrenaline, 5-hydroxy-tryptamine and octopamine, produced hyperpolarizing responses, graded according to concentration. 5. It is suggested that dopamine, the most potent of the biogenic amines tested, is the transmitter at this junction.