The effects of single-dose fexofenadine, diphenhydramine, and placebo on cognitive performance in flight personnel

BACKGROUND

Sedation and functional impairments are side effects associated with the use of first-generation antihistamines that preclude their use in aviation. Selected second-generation antihistamines do not have such side effects and have been proposed for use in aircrew.

METHODS

Forty-two healthy naval aviation personnel served as subjects in this double-blind, randomized, placebo-controlled crossover study. Subjective drowsiness, cognitive performance, and vigilance were measured under three conditions: 180 mg fexofenadine (F), 50 mg diphenhydramine (D) as a positive control, or placebo (P).

RESULTS

Subjects receiving F vs. D tended to have a faster mean hit reaction time (adjusted mean difference +/- SE, -10.5 +/- 6.8 ms, p = 0.127). Subjects performed faster and better with F vs. D on measures of omission errors and commission errors (p < 0.05). Variable symbol digit coding delayed recall accuracy was better for F vs. D (p = 0.023), and approached significance for shifting attention and divided attention tasks (p = 0.062 and p = 0.057, respectively). Subjects reported significantly more drowsiness (p < 0.005) with D than F.

CONCLUSIONS

Diphenhydramine administration resulted in significant psychomotor decrements compared with fexofenadine, while the effects of fexofenadine were similar to placebo. These results provide additional support for the safe use of fexofenadine by aviation personnel.

Effects of lovastatin and pravastatin on cognitive function in military aircrew

BACKGROUND

As evidence has accumulated for the role of HMG-CoA reductase inhibitors in the primary prevention of coronary artery disease, younger individuals with no other co-morbid conditions will be increasingly exposed to these agents. Some HMG-CoA reductase inhibitors have been reported to cause impairment of daytime cognitive processes that have the potential to directly impact the ability of pilots and other aircrew to perform. These studies suggested that there might be cognitive effects of these medications that would argue against their routine use in populations whose activities required high, sustained levels of cognitive performance. The objective of this study is to establish the effects of pravastatin and lovastatin on aircrew daytime cognitive function using tests that are correlated with actual cockpit tasks and inflight performance.

METHODS

Military aircrew with hypercholesterolemia were enrolled in the study and assigned to lovastatin, pravastatin or placebo groups. Baseline cognitive and vigilance testing was performed with computerized test instruments. Following a 4-wk treatment period, subjects were retested on both cognitive and vigilance tasks.

RESULTS

Laboratory studies confirmed that both medications were effective in lowering cholesterol. No major treatment-related side effects were encountered. Cognitive performance was not affected by either active treatment, and was not different from that of the placebo group.

CONCLUSIONS

The tested medications did not have significant effects on performance as measured by two computerized performance tests. The data suggest that neither medication has significant effects on flight-related performance.

The reduced oxygen breathing paradigm for hypoxia training: physiological, cognitive, and subjective effects

The current training program for hypoxia familiarization requires a low-pressure chamber that places aviator trainees at risk for decompression sickness. A cost-effective reduced oxygen-breathing (ROB) paradigm that decreases oxygen (O2) concentration leading to normobaric hypoxia was assessed as an alternative to the hypobaric chamber.

PURPOSE

To help establish the validity of the ROB paradigm, this report documents cognitive performance, cardiopulmonary and subjective changes during ROB exposure.

METHODS

Performance on a two-dimensional tracking task, as well as BP, heart rate, end-tidal carbon dioxide (ETCO2), O2 saturation, and subjective reports of hypoxia symptoms were observed in 12 U.S. Navy divers during exposure to normoxic air followed by one of four experimental gas mixtures per session. All participants received all gas conditions that differed in their relative concentrations of O2 and nitrogen (6.20/93.80, 7.00/93.00, 7.85/92.15, and 20.85/79.15% O2/N2).

RESULTS

ROB caused increases in tracking task error (p < 0.0001). ROB also increased heart rate (p < 0.001) and systolic BP (p = 0.004), and decreased ETCO2 and O2 saturation (p < 0.0001). Finally, subjects responded to ROB-induced hypoxia with higher subjective symptom ratings (p < 0.0001).

CONCLUSIONS

These data are consistent with those expected from hypoxic states and support the validity of the ROB paradigm for hypoxia training. Future validation studies comparing a ROB device with hypobaric chambers are needed.

Hemodynamic effects of basal and stimulated release of endogenous nitric oxide in isolated human lungs

Background-We compared the hemodynamic responses to inhibition or stimulation of endothelial nitric oxide (NO) release of isolated explanted lungs from transplantation recipients with pulmonary hypertension and in normotensive unallocated donor lungs. Methods and Results-Lungs from 10 patients with severe pulmonary hypertension (SPH) and from 16 patients with severe chronic obstructive lung disease (COLD) were studied. Fourteen normotensive lungs were studied as controls. The lungs were perfused at a constant flow. In protocol 1 N(G)-nitro-L-arginine methyl ester caused a similar rise in baseline pulmonary artery pressure (PAP) that was similar in SPH (+17.1+/-4.2 mm Hg; n=5), COLD (+15.5+/-4.8 mm Hg; n=8), and control lungs (+14.5+/-1.5 mm Hg; n=7). Arterial occlusion demonstrated that most of the changes with N(G)-nitro-L-arginine methyl ester were precapillary. The response to sodium nitroprusside (10(-8) to 10(-4) mol/L) was similar in all groups. In protocol 2, the lungs were preconstricted, and acetylcholine (10(-9) to 10(-5) mol/L) caused a lesser fall in PAP in both COLD and SPH lungs compared with control (-41.9+/-8.6%, -55. 7+/-7.6%, and -73.2+/-2.5%, respectively; P<0.05), whereas sodium nitroprusside (10(-5) mol/L) decreased PAP to initial levels in all lungs. Conclusions-Stimulated release of NO is impaired in arteries of lungs with plexogenic or hypoxemic pulmonary hypertension. In contrast, basal release of NO appears to be maintained.

An abscess that wasn't: renal calyceal diverticulum in a student Naval aviator

The authors report a case of a 24-yr-old female Navy student pilot who presented with pyelonephritis, and on workup, was found to have two diverticula arising from calyces in her left kidney. Her infection resolved after several weeks of antibiotic therapy. She was granted a waiver to return to flight status based on the judgment that her risk of recurrent infection, sudden incapacitation, future need of surgery, and compromise of mission effectiveness were acceptably low. She has remained asymptomatic after more than 48 mo of follow-up. Renal calyceal diverticulum has not previously been discussed in the literature of aerospace medicine. The definition, anatomy, possible etiologies, epidemiology, presentation, treatment options, and aeromedical disposition, including human-machine interface issues, for calyceal diverticula are discussed. Aviators with calyceal diverticula are at increased risk of urinary tract infection and other complications; aeromedical disposition should rest on evaluation of such risk on a case-by-case basis.

Nature and site of action of endogenous nitric oxide in vasculature of isolated pig lungs

The site of action of endogenous and exogenous nitric oxide (NO) in isolated pig lungs was investigated by using arterial, double, and venous occlusions, which allowed precapillary, postcapillary, and venous segments to be partitioned into arterial, precapillary, postcapillary, and venous segments. NG-nitro-L-arginine (L-NNA; 10(-5) M) increased resistance in the arterial (35 +/- 6.6%. P = 0.003), precapillary (39.3 +/- 5.1%, P = 0.001), and venous (18.3 +/- 4.8%, P = 0.01) segments, respectively. Sodium nitroprusside (10(-5) M) NO (80 parts/million) reversed the effects of L-NNA. Total pulmonary vascular resistance fell with increasing flow, due to a fall in precapillary resistance and dynamic resistance, and was significantly lower than mean total resistance. L-NNA increased the resistances but did not alter the pattern of the pressure-flow relationships. It is concluded that, in isolated pig lungs, the effect of endogenous NO seems to be dependent on flow in the arterial segment and independent of flow in the precapillary segment, but variation of its release does not appear to be fundamental to accommodation to changes in steady flow.

A case of a naval aviator on quinidine for lone atrial fibrillation

While neither atrial fibrillation (AF) nor the therapeutic use of quinidine represent medical contraindications for civil aviation in the United States, they have both been indications for permanent grounding in the U.S. armed forces. We report a case of a naval aviator who was found to be in asymptomatic AF while undergoing a routine annual flight physical. The patient was pharmacologically converted and maintained in normal sinus rhythm with quinidine. Following review of his case by a Special Board of Flight Surgeons, he was returned to limited flight status for dual controlled aircraft only while maintained prophylactically on quinidine to prevent the recurrence of AF.

Exhaled nitric oxide in isolated pig lungs

Endothelium-derived nitric oxide (NO) is an important regulator of vascular resistance. Low concentrations of NO have been recorded in the exhaled breath of spontaneously breathing animals and humans. To determine whether NO synthesis in the lung contributes to the NO measured in the breath, we measured the concentration of NO in the exhaled air of isolated perfused and ventilated porcine lungs by using a chemiluminescence method. With NO-free normoxic ventilation (21% O2-5% CO2-74% N2) of eight porcine lungs perfused with a Krebs-dextran and albumin perfusate, baseline exhaled NO was 5.8 +/- 1.8 parts per billion (ppb) and pulmonary vascular resistance (PVR) was 8.9 +/- 1.8 mmHg.l-1.min. Hypoxic ventilation (5% O2-5% CO2-90% N2) caused a fall in NO to 3.6 +/- 1.8 ppb and a rise in PVR to 13.6 +/- 3.6 mmHg.l-1.min. Vasoconstriction with the thromboxane analogue U-46619 (10(-9) M) raised PVR to 31.7 +/- 6.8 mmHg.l-1.min but did not decrease NO levels from baseline. Subsequent addition of acetylcholine (10(-6)M) lowered PVR to 22.1 +/- 4.5 mmHg.l-1.min and increased exhaled NO to 7.0 +/- 2.0 ppb. Addition of a NO synthase inhibitor, NG-nitro-L-arginine methyl ester (10(-5) M), to four lungs caused a rise in PVR to 43.0 +/- 7.0 mmHg.l-1.min and a decrease in NO to 1.5 +/- 1.0 ppb. Addition of autologous blood to the perfusate of four lungs caused no change in PVR from baseline but decreased exhaled NO to 2.7 +/- 0.5 ppb.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of inhibitors of nitric oxide release and action on vascular tone in isolated lungs of pig, sheep, dog and man

1. The actions of inhibitors of the release or action of nitric oxide (NO) on pulmonary vascular resistance (PVR) were investigated in lungs isolated from pig, sheep, dog and man. 2. In pig, sheep and human lungs perfused with Krebs-dextran solution, both N omega-nitro-L-arginine methyl ester (L-NAME; 10(-5) M) and Methylene Blue (10(-4) M) increased basal PVR. This increase was reversed by sodium nitroprusside (10(-5) M). In pig lungs N omega-monomethyl-L-arginine (10(-4) M) increased PVR by 154%. This increase was partially reversed by L-arginine (10(-3) M). L-NAME had no effect in dog lungs. 3. Pulmonary artery pressure-flow (PPA/Q) relationships were studied over a wide range of flows. In pigs, sheep and human lungs perfused with Krebs-dextran solution, L-NAME increased the PPA/Q slope. This increase was reversed by sodium nitroprusside. In dog lungs L-NAME had no effect. 4. In blood-perfused lungs, the respective responses to L-NAME were similar to those observed with saline. Acute hypoxia in pig and dog lungs increased intercept pressure. Addition of L-NAME during hypoxia increased the PPA/Q slope in both species. 5. In the human, there was no difference in the absolute increase of PVR or PPA/Q slope elicited by L-NAME between hypertensive and control lungs. 6. We conclude that NO is continuously released in the pulmonary vascular bed of pig, sheep and humans under normoxic conditions. In dog lungs inhibition of NO synthesis increases PVR only under hypoxic conditions. In human lungs with pulmonary hypertension, NO is still released under basal conditions.

Wide complex tachycardia in an asymptomatic aviator

The case of a healthy aviator incidentally found to have an asymptomatic wide complex tachycardia on routine exercise treadmill testing is discussed. Electrophysiologic studies provided the diagnosis of AV nodal reentrant tachycardia (AVNRT) with aberrant conduction. Radio frequency modification of the AV node was performed with good results. Following the procedure, the reentrant tachycardia could not be induced either by repeat electrophysiologic testing or by an exercise treadmill test. The individual was returned to flight status in a restricted capacity.

The effects of N omega-nitro-L-arginine methyl ester, sodium nitroprusside and noradrenaline on venous return in the anaesthetized cat

1. The vascular actions of N omega-nitro-L-arginine methyl ester (L-NAME), sodium nitroprusside and noradrenaline were investigated in cats under chloralose anaesthesia with controlled vascular tone and ventilation. Cardiac output, heart rate, vascular pressures and mean circulatory filling pressure (MCFP) were measured. Total peripheral resistance (TPR) and resistance to venous return (Rvr) were calculated from steady-state readings. 2. L-NAME (37 mumol kg-1, i.v.) administered to ten cats receiving noradrenaline (6 nmol kg-1 min-1, i.v.) increased aortic pressure by 47.5 +/- 7.1 mmHg from 106 mmHg, and MCFP by 1.56 +/- 0.36 mmHg from 10.0 mmHg (means +/- s.e. means). Mean changes in portal venous pressure, RAP and heart rate were not significant. Cardiac output fell by 29.7 +/- 3.3% from 130 ml min-1 kg-1. TPR rose by 108 +/- 7.2% from 796 mmHg l-1 min kg and Rvr by 58.4 +/- 4.5% from 64 mmHg l-1 min kg. 3. Infusion of sodium nitroprusside into cats receiving noradrenaline evoked dose-related falls in aortic pressure, MCFP, TPR and Rvr. Changes in portal venous pressure, RAP and heart rate were not significant and cardiac output fell slightly. After L-NAME, sensitivity to nitroprusside was increased by 139 +/- 34% for MCFP, 176 +/- 19% for TPR and 351 +/- 39% for Rvr, and cardiac output rose slightly. The nitroprusside infusion required to restore TPR after L-NAME was estimated to be 5.8 x 10(+/- 0.41) nmol kg-1 min-1, which was approximately three times more than that required to restore MCFP. 4. Infusion of noradrenaline evoked dose-related increases in aortic and portal venous pressures, heart rate, cardiac output, MCFP, TPR and Rvr. After L-NAME and nitroprusside (4.4 nmol kg-1 min-1, i.v.),TPR and Rvr were not significantly different, but MCFP was reduced by 1.76 +/- 0.24 mmHg, and cardiac output by 22 +/- 1.9%. After subsequent expansion of the circulating blood volume (5-7.5 ml kg-1 dextran-saline), mean values for all parameters were restored to their previous levels. Sensitivity to noradrenaline was not significantly altered for heart rate, TPR and Rvr but was reduced by 31.8 +/- 12%for MCFP and by 66.5 +/- 18% for cardiac output.5. The depression of cardiac output by L-NAME is attributed to the increase in Rvr, partly compensated by the rise in MCFP. For a given rise in MCFP, the increase in R, was seven times greater after L-NAME than after noradrenaline, and the difference in the relative actions of the two drugs on resistance and capacitance vessels largely accounts for their contrasting effects on venous return. A procedure is suggested for replacement of vascular nitric oxide by nitroprusside infusion and blood volume expansion.

Heart rate changes evoked by hypoxia in the anaesthetized, artificially ventilated cat

Reflex changes in heart rate evoked by hypoxia were investigated in cats anesthetized with chloralose and ventilated by positive pressure during administration of vecuronium or gallamine. In five cats receiving vecuronium and with aortic pressure stabilized, systemic hypoxia (arterial O2 pressure (Pa, O2) 34.9 mmHg) reduced heart rate by 55.8 +/- 7.5 beats min-1 (mean +/- S.E.M.). After administration of atropine, hypoxia (Pa, O2 32.1 mmHg) increased heart rate by 28.2 +/- 3.4 beats min-1. After subsequent bilateral ablation of carotid sinus and vagus nerves, hypoxia (Pa, O2 31.9 mmHg) increased heart rate by 7.1 +/- 1.8 beats min-1. The cardiac accelerator response to hypoxia was further examined in groups of cats treated with gallamine and atropine. In four vagotomized cats, local perfusion of both carotid sinuses with hypoxic blood (Pa, O2 37.7 mmHg) increased heart rate by 15.5 +/- 2.3 beats min-1. In the same cats, systemic hypoxia (Pa, O2 38.3 mmHg) increased heart rate by 16.4 +/- 2.3 beats min-1. The heart rate increment in cats which had undergone either bilateral adrenalectomy or cardiac sympathectomy was similar to the increment in unoperated cats. The increment was significantly less in cats which had both adrenal glands and cardiac sympathetic nerves ablated. It is concluded that stimulation of the carotid bodies in the cat excites both parasympathetic and sympathetic cardiac nerves simultaneously.

Mean circulatory filling pressure during splanchnic nerve stimulation and whole-body hypoxia in the anaesthetized cat

1. Mean circulatory filling pressure (MCFP) was measured in cats under chloralose anaesthesia by obstruction of blood flow in the pulmonary artery. Pressures in the aorta, hepatic portal vein and right atrium were recorded, and MCFP was estimated from the value at which all three pressures became equal when blood was pumped from aorta to vena cava during circulatory arrest. Simultaneous equality was not attained at MCFP values below 5 mmHg. 2. In cats ventilated by positive pressure after administration of gallamine, MCFP was 9.7 +/- 0.3 mmHg (n = 14). The values of MCFP measured in six cats before and after administration of gallamine did not differ significantly. Change of blood volume altered MCFP linearly over the range 5-21 mmHg. Noradrenaline (7.5 micrograms kg-1 min-1) increased MCFP from 9.3 +/- 0.9 to 16.5 +/- 0.6 mmHg (n = 4), and phentolamine (2 mg kg-1) reduced it to 5.6 +/- 0.3 mmHg (n = 5). 3. Changes in MCFP were evoked at different circulating blood volumes by stimulation of the splanchnic sympathetic nerves and by whole-body hypoxia. Ablation of all splanchnic nerves reduced MCFP from 9.4 +/- 0.5 to 7.1 +/- 0.3 mmHg (n = 5) and stimulation of their distal ends at 10 Hz increased it by 4.1 +/- 0.4 mmHg (n = 4); similar increments were obtained at different blood volumes and initial values of MCFP. 4. Hypoxia increased MCFP by 0.23 mmHg per 1 mmHg fall in arterial oxygen tension below Pa,O2 56 mmHg (r = -0.86; n = 24). Similar increments were obtained at different blood volumes and initial values of MCFP. Ablation of all splanchnic nerves reduced the increments by 60%, and administration of phentolamine abolished them.

Oncogene transcription in normal human IMR-90 fibroblasts: induction by serum and tetradecanoyl phorbol acetate

We report studies of oncogene transcription induced by the addition of serum to quiescent cultures of human IMR-90 fibroblasts. Oncogene messenger RNAs for c-myc, c-erbB and c-ras were increased in a specific temporal sequence after the addition of serum. Compounds that are proposed to exert their actions by the stimulation of cell growth were tested for their effect on oncogene transcription in IMR-90 fibroblasts. The tumor promoter tetradecanoyl phorbol acetate (TPA) was found to selectively induce the transcription of c-myc without observable effect on the transcription of the other oncogenes studied, and without inducing cell division. The inactive analog, phorbol didecanoate (PDD), and two complete carcinogens dimethylbenzanthracene (DMBA) and 4-nitro quinoline-1-oxide (4NQO) were without effect on the transcription of the genes studied. These results suggest that the complete ordered sequence of gene transcription is necessary to achieve the physiologic response of cell division, and that classical promoters and complete carcinogens achieve their effects through different pathways.

Characterization of human IMR-90 fibroblasts as a model system for the study of chemical carcinogenesis in vitro

The effects of chemical carcinogens on various aspects of cellular metabolism were measured in human IMR-90 fibroblasts. These studies were undertaken to assess the ability of IMR-90 cells to be used for the study of chemical carcinogenesis in vitro. It was found that chemical carcinogens inhibited DNA synthesis in synchronous cultures of IMR-90 fibroblasts; the effective concentrations of carcinogens showing effects in this system were lower than for other published in vitro cell assay systems; compounds that require metabolic activation to a carcinogenic form demonstrated the ability to inhibit DNA synthesis in IMR-90 fibroblasts; a fluorometric determination of aryl hydrocarbon hydroxylase (AHH) activity in IMR-90 cells revealed a low level of inducible AHH; IMR-90 cells can metabolize 7,12-dimethylbenzanthracene (DMBA) to compounds that are identified as metabolites of DMBA in vivo.

Haemodynamic responses to stimulation of the splanchnic and cardiac sympathetic nerves in the anaesthetized cat

The changes in cardiac output and mean right atrial pressure (R.A.P.) evoked at different circulating blood volumes by stimulation of the splanchnic sympathetic nerves were investigated in adrenalectomized cats under chloralose anaesthesia, with unopened chests and spontaneous respiration and with active vascular reflexes. The cardiac autonomic nerves were cut or blocked pharmacologically. Stimulation of the distal ends of the splanchnic nerves at 4 Hz caused aortic pressure and R.A.P. to rise to maximum values at 2 min before declining slowly. Cardiac output rose more slowly to a steady state at 3 min; at higher circulating volumes it fell initially. Although the output increments were slower in development they were better sustained than those in total peripheral resistance. The proportionate output increments were largest and the R.A.P. increments least at low circulating volumes whereas at high volumes the R.A.P. increments were large but the output changes were small or negative; the pattern of changes resembled that resulting from infusion of blood. Stimulation of the cardiac sympathetic nerves evoked a rise in output and a fall in R.A.P. related in magnitude to the initial value of R.A.P. On simultaneous stimulation of the splanchnic and cardiac sympathetic nerves the changes in output combined whereas the R.A.P. changes cancelled, to give output increments of 25-50% with little change in R.A.P. at all circulating volumes. At high circulating volumes infusion of blood did not usually alter output or aortic pressure, but splanchnic nerve stimulation increased peripheral resistance and aortic pressure and commonly evoked a rise in left ventricular stroke work which could not be accounted for by known adrenergic mechanisms or by elevation of left ventricular end-diastolic pressure. Portal venous pressure was consistently elevated by splanchnic nerve stimulation; it rose more slowly than did aortic pressure or R.A.P. and was independent of a changing central venous pressure provided this did not exceed +5 mmHg. The cardiac output increments were not related to changes in the ratio between the input and output resistances of the portal vein and it is concluded that displacement blood from the peripheral to the central vasculature was induced by contraction capacitance vessels.

Haemodynamic responses to stimulation of the cardiac autonomic nerves in the anaesthetized cat with closed chest

1. The changes in cardiac output and mean right atrial pressure (R.A.P.) evoked by stimulation of the cardiac autonomic nerves were investigated in cats under chloralose anaesthesia, with unopened chests and spontaneous respiration, and with active vascular reflexes. Cardiac output was measured by thermal dilution; the technique used was calibrated against the direct Fick method.2. The initial values of R.A.P. and output were varied by infusion of dextran-saline solution followed by withdrawal of blood. At positive values of R.A.P. withdrawal of blood caused a fall in R.A.P. with no change in cardiac output. At negative R.A.P. blood withdrawal caused a fall in output with little change in R.A.P.: the linear regression coefficient for output on R.A.P. was 48.2 ml./min.kg.mmHg (S.E. 2.06, n = 63, nine cats).3. Stimulation of the right cardiac sympathetic nerve increased heart rate by 69.2 beats/min (S.E. 4.0) from the resting rate of 158 beats/min (S.E. 6.3, ten cats). The acceleration was accompanied in most instances by a rise in cardiac output and a fall in R.A.P. and the magnitude of the rise in output was related to that of the fall in R.A.P.4. In no experiment could R.A.P. be reduced below -2.5 mmHg either by withdrawal of blood or by sympathetic stimulation. At negative values of R.A.P. the fall in R.A.P. and rise in output evoked by sympathetic stimulation were small; substantial changes could be obtained only from positive initial values of R.A.P. The proportional increase in output evoked by a given proportional increase in heart rate during near-maximal sympathetic stimulation had a linear relationship to the initial value of R.A.P. over the range -2 to +8 mmHg. The output increment was less than proportional to the rate increment at all values of R.A.P. below +3 mmHg.5. In five experiments stimulation of the left cardiac sympathetic nerve evoked a greater increase in output for a given increase in heart rate than did stimulation of the right nerve; on the other hand both nerves gave similar increments of output for a given fall in R.A.P.6. Stimulation of the distal end of the right vagus nerve slowed the heart and caused a fall in cardiac output and a rise in R.A.P. The change in output associated with a given change in R.A.P. was significantly greater (P = 0.05) during sympathetic than during vagal stimulation in 14 out of 18 tests; the difference increased as circulating volume was reduced.7. It is concluded that the relationship between cardiac output and R.A.P. during sympathetic and vagal stimulation is consistent with the hypothesis that neurally evoked changes in cardiac performance vary output mainly, but not exclusively, by inducing changes in R.A.P. which alter the pressure gradient for the return of blood to the right atrium from the periphery.

The activity of post-ganglionic sympathetic nerves to the uterus of the rabbit

1. The spontaneous activity recorded from the central ends of the uterine nerves of the rabbit is described. The discharges resembled those reported from other post-ganglionic sympathetic nerves; the individual units appeared to discharge remarkably regularly at frequencies between 4/sec and 1 per 3 sec.2. The resting discharge in late pregnant rabbits was usually more intense than in non-pregnant rabbits; otherwise no clear relation existed between variations in the discharges and in the sexual state.3. The discharge was reduced by stimulation of the central end of the depressor nerve and increased by asphyxia.4. Stimulation of either the preganglionic or the post-ganglionic nerve by a single shock evoked a long after-discharge in the post-ganglionic nerve. It is shown that this after-discharge is a property of some of the post-ganglionic neurones supplying the rabbit's uterus which does not depend on the integrity of the preganglionic nerves or of the spinal cord, or on transmission through the ganglia, and that it is not a general property of the sympathetic post-ganglionic neurones of the rabbit.5. Repetitive stimulation of the preganglionic nerve at low frequencies augmented the after-discharge in some cases and depressed it in others; stimulation at high frequencies invariably depressed the after-discharge. Repetitive stimulation of the post-ganglionic nerve at low or high frequencies depressed the after-discharge.6. If the normal preganglionic activity was interrupted abruptly by an anode block, the post-ganglionic discharge also stopped abruptly.7. A ganglion-stimulating action of hexamethonium is described.8. Adrenaline inhibited the post-ganglionic neurones with no sign of excitation. This effect was antagonized by Rogitine without affecting the resting discharge, the action of the depressor nerve or the after-discharge in any way.

The characteristics of spontaneous and evoked action potentials recorded from the rabbit's uterine nerves

1. A method is described for recording in vivo the action potentials of afferent and efferent fibres in whole nerves supplying the rabbit's uterus.2. Examination of these nerves under the light microscope and the electron microscope showed them to be composed almost entirely of non-myelinated fibres.3. Two types of spontaneous action potential were observed; one travelled at about 4 m/sec and probably came from the myelinated fibres, the other travelled at 0.4-1.4 m/sec and certainly came from non-myelinated fibres.4. The efferent fibre spikes were shown to be faster and higher than the spikes from uterine afferent fibres, but slower and smaller than spikes from broad ligament afferent fibres.5. Apart from differences in conduction velocity and height, all spikes were basically similar, lasting about 1.5 msec. The height was related to the square of the velocity. Some more complex spikes were also observed.6. The compound action potential evoked by stimulation of the uterine nerve had three peaks, conducted at 1.3, 0.8 and 0.6 m/sec, respectively, and thought to correspond to the fast afferent fibres, the efferent fibres and the slow afferent fibres, respectively. There were also some late peaks due to reflexion of the antidromic action potentials from the ganglion cells.7. Stimulation of the hypogastric nerve also evoked a compound action potential in the uterine nerves. Stimulation of the pelvic nerves had no effect.8. By means of ganglion blocking agents, the uterine ganglia were shown to lie in the pelvic plexus, peripheral to the hypogastric nerve, but central to the uterine nerves.9. It is argued that the spontaneous action potentials came from individual fibres rather than Remak bundles, and that the recording technique used detected the activity of all but the smallest fibres.