Identification of vagal sensory receptors in the rat lung: are there subtypes of slowly adapting receptors?

1. We studied the characteristics of pulmonary sensory receptors whose afferent fibres are in the left vagus nerve of opened-chest rats. The activity of these receptors was recorded during mechanical ventilation approximating eupnoea, as well as during deflation, stepwise inflations and constant-pressure inflations of the lungs. Data were also collected from closed-chest rats and analysed separately. 2. Ninety-four per cent of receptors were located in the ipsilateral lung or airways with the remainder in the contralateral lung. 3. Not only were slowly adapting receptors (SARs) the most abundant pulmonary receptors but 21% of them were either exclusively or predominantly active during the deflationary phase of the ventilatory cycle. Deflationary units were found in opened- and closed-chest rats. The average conduction velocity for all fibres innervating SARs averaged 29.7 m s-1. 4. We found rapidly adapting receptors (RARs) to be extremely rare in the rat. Their activity was sparse and irregular. The conduction velocities of fibres innervating RARs averaged 12.3 m s-1. 5. Far more abundant than RARs in the remaining population of pulmonary fibres were C fibres. They were observed to have an average conduction velocity of 2.1 m s-1, base-level activity which was irregular and a high pressure threshold of activation and were stimulated by intravenous capsaicin injection. 6. Notable differences exist between pulmonary receptors in rats and those reported in other species. The variations include the abundant existence of intrapulmonary SARs with exclusively deflationary modulation and the rarity of RARs. We also encountered C fibres which have not previously been described systematically in the rat.

Effects of dipyridamole on the cardiovascular response to +Gz stress in miniature swine.

Eight conscious female miniature swine experienced acceleration levels of 3, 5, and 7 +Gz before and after infusion of dipyridamole (1-2 mg.kg-1). Each animal was instrumented to measure ECG, heart level arterial pressure (AP), eye level arterial pressure (ELBP), left arterial pressure (LAP), heart rate (HR), and regional tissue blood flows. Each was also fitted with an abdominal anti-G suit which automatically inflated. Dipyridamole infusion had no direct effect on HR or LAP but AP was significantly reduced. All cardiovascular responses to +Gz were qualitatively similar before and after dipyridamole. Tachycardia always occurred. AP and CNS blood flow were maintained better prior to dipyridamole and AP always fell in proportion to acceleration intensity. +Gz was generally associated with increased blood flow to respiratory muscles and heart, decreased blood flow throughout the viscera and to the eyes. ELBP paralleled AP, but was always lower in direct proportion to the +Gz level. We conclude that dipyridamole reduces arterial pressure thus compromising the ability of the animal to sustain cerebral perfusion pressure (ELBP) during +Gz.

Naloxone does not affect muscle blood flow during low intensity exercise in rats.

The purpose of this study was to determine whether endogenous opioids are involved in the control of skeletal muscle blood flow during locomotory exercise in rats. The radiolabeled miscrosphere technique was used to measure total and regional muscle blood flow. We first determined whether methionine enkephalin (1,000 micrograms.kg-1 I.V.) would produce vasodilation in muscle vascular beds. We found that methionine enkephalin produced a 36 mm Hg (range of 20-50 mm Hg) drop in mean arterial pressure (Pa), which was associated with decreases in calculated skeletal muscle vascular resistance in anesthetized rats, and that these effects on arterial pressure and skeletal muscle vascular resistance were blocked by the infusion of naloxone (10 micrograms.kg-1). Measurements were then made at 5 min of treadmill exercise at 15 m.min-1 (0 degree incline) and following exercise in both saline-treated (controls) and naloxone (10 micrograms.kg-1)-treated conscious rats. There were no differences between the heart rates, blood pressures, or total muscle blood flows of the two groups. There were also no significant differences between the blood flows to 32 hind limb muscle samples composed of various muscle fiber types. Since naloxone blockade did not affect total or regional muscle blood flow during low intensity exercise, it appears that the endogenous opioids are not required for the normal exercise hyperemia of skeletal muscles.

Sympathetic neural influences on muscle blood flow in rats during submaximal exercise.

These experiments were designed to estimate the involvement of the sympathetic innervation in regulation of hindlimb muscle blood flow distribution among and within muscles during submaximal locomotory exercise in rats. Blood flows to 32 hindlimb muscles and 13 other selected tissues were measured using the radiolabeled microsphere technique, before exercise and at 0.5, 2, 5, and 15 min of treadmill exercise at 15 m/min. The two groups of rats studied were 1) intact control, and 2) acutely sympathectomized (hindlimb sympathectomy accomplished by bilateral section of the lumbar sympathetic chain and its connections to the spinal cord at L2-L3). There were no differences in total hindlimb muscle blood flow among the two groups during preexercise or at 30 s or 2 min of exercise. However, flow was higher in eight individual muscles at 2 min of exercise in the sympathectomized rats. At 5 and 15 min of exercise there was higher total hindlimb muscle blood flow in the denervated group compared with control. These differences were also present in many individual muscles. Our results suggest that 1) sympathetic nerves do not exert a net influence on the initial elevations in muscle blood flow at the beginning of exercise, 2) sympathetic nerves are involved in regulating muscle blood flow during steady-state submaximal exercise in conscious rats, and 3) these changes are seen in muscles of all fiber types.

Coronary blood flow reserve during +Gz stress and treadmill exercise in miniature swine.

The purpose of this study was to compare the coronary blood flow reserve (CBFR) that exists during maximal +Gz stress to the CBFR during maximal exercise stress. Maximal exercise stress was defined as an exercise intensity greater than or equal to that necessary to produce maximal levels of O2 consumption (VO2max). Coronary blood flows (CBF) were determined with the use of the microsphere technique in chronically instrumented conscious miniature swine during +Gz stress and exercise stress at 70 and 100% of maximal tolerance (for each stress) before and after maximal coronary vasodilation with 1-2 mg/kg dipyridamole. CBFR was measured as the amount of blood flow increase produced by maximal coronary vasodilation. During exercise at VO2max, dipyridamole produced 20-30% increases in CBF, whereas it induced no coronary vasodilation or changes in CBF during +Gz stress. Dipyridamole also produced decreases in the animals' tolerance to +Gz in that all five animals could maintain a steady state for 60 s at 7 +Gz before dipyridamole, whereas only two of these animals could maintain a steady state for 60 s at 7 +Gz after dipyridamole. These results confirm that CBFR exists during maximal exercise in normal mammals. However, this dose of dipyridamole produced no coronary vasodilation during either level of +Gz stress.

Inotropic responses of the left ventricle to changes in heart rate in anesthetized rabbits.

Factors known to influence left ventricular contractility include preload, afterload, circulating catecholamine concentration, efferent sympathetic discharge, and heart rate. Heart rate influences have been primarily determined in the dog, whereas the influence of heart rate in smaller mammals has not been determined. Eight pentobarbital-anesthetized rabbits were instrumented to measure electrocardiogram, heart rate, left ventricular pressure, end-diastolic pressure, dP/dt, and mean and pulsatile aortic pressures. Systematic bradycardia was induced by stimulating the peripheral end of the sectioned right vagus nerve. Between 293 and 235 beats/min, there was no change in (dP/dt)max as heart rate was decreased. Below this range there was a direct relationship between (dP/dt)max and heart rate. Preload remained unchanged down to 132 beats/min. There was a small but significant decrease in afterload (0.09 mmHg X beat-1 X min-1; 1 mmHg = 133.32 Pa) throughout the decrease in heart rate. Infusion of propranolol (2.0 mg/kg) produced no marked change in the heart rate - (dP/dt)max relationship, although both resting heart rate and (dP/dt)max were reduced. This study demonstrates that (dP/dt)max is not influenced by changes in heart rate above 235 beats/min in the pentobarbital-anesthetized rabbit. These results differ from findings in other animals, and demonstrate that species and heart rate ranges must be considered when drawing conclusions regarding (dP/dt)max as a reliable index of contractility.

Fibrosis of extrahepatic biliary system after continuous hepatic artery infusion of floxuridine through an implantable pump (Infusaid pump).

A case of severe fibrosis of the extrahepatic biliary system after hepatic artery infusion of floxuridine is described. The clinical and pathologic features are presented, and its pathogenesis discussed. The authors suggest investigation of an alternate drug schedule and/or dosage of floxuridine to eliminate or minimize this serious complication.

Role of peripheral chemoreceptors in response to smoke-induced apnea vs tracheal occlusion.

Reflex autonomic changes which occur after cigarette smoke enters the upper airways are partially due to peripheral chemoreceptor stimulation. Chemoreceptor denervation attenuates but does not abolish smoke induced bradycardia. Denervation nearly abolishes bradycardia induced by tracheal occlusion. Hypertension accompanies smoke induced apnea but does not occur during tracheal occlusion.

Suppression of renal nerve activity by methionine enkephalin in anesthetized rabbits.

Endogenous opioid peptides such as Met-enkephalin have diverse physiological actions, which include hypotension and bradycardia in anesthetized animals. The exact sites, the mechanism and the physiological significance of the cardiodepressant effects are not known. The main purpose of this work was to correlate the effect of Met-enkephalin on the rate of sympathetic nerve discharge with its cardiodepressant actions. Anesthetized male rabbits (2-3 kg) were instrumented for the measurement of blood pressure, heart rate, electrocardiogram and multiunit renal nerve activity (RNA). Intravenous administration of norepinephrine (15 micrograms/kg) or phenylephrine (50 micrograms/kg) produced a rapid increase in blood pressure followed by a reduction of RNA and heart rate. Met-enkephalin administered via the same route in doses of 1, 10, 100 and 1000 micrograms/kg initially decreased the RNA and subsequently reduced blood pressure. The above effects of Met-enkephalin were antagonized by naloxone (1 mg/kg i.v.) given 10 min before the peptide. Naloxone methobromide (1.3 mg/kg i.v.), a quaternary derivative of naloxone, did not block the effects of Met-enkephalin under identical conditions. These results suggest that the hypotensive effect of Met-enkephalin is secondary to its effects on sympathetic outflow. Lack of naloxone methobromide action is due to its inability to cross the blood-brain barrier and is consistent with a centrally mediated action of Met-enkephalin.

Control of hemodynamic adjustments during acute volume expansion in the rabbit.

The influence of reflexes mediated by the carotid sinus, aortic, and vagus nerves on control of blood pressure were investigated in the pentobarbital-anesthetized rabbit during an acute intravascular volume expansion. Blood, kept at 37 degrees C, was gradually infused at 2.5-min intervals until the blood volume of each animal was expanded to 10, 20, 30, and 40% above normal. Responses in sinoaortic-vagally denervated rabbits were compared to intact rabbits. Both intact and denervated animals showed a significant increase in central venous pressure with each 10% addition of blood. Heart rate did fall significantly in the intact group but not in the denervated group. No significant changes were identified in mean arterial pressure (MAP) in either group. In both groups changes in cardiac output were significantly greater than control for 20, 30, and 40% expansion and calculated total peripheral resistance fell in both groups at all levels of expansion when compared to control. The absence of significant changes in MAP within the intact and denervated groups suggests that sinoaortic-vagal reflexes are not affecting control of MAP in response to slow, acute volume expansion in the intact rabbit. On the other hand, since total peripheral resistance fell in both groups, a non-sinoaortic-vagal mechanism appears to be functional. This mechanism may assist in increasing vascular capacitance in order to prevent significant increases in blood pressure.

Species differences in circulatory responses to nasopharyngeal perfusion with smoke.

Cardiovascular changes were observed in five species of laboratory mammals during upper airway perfusion with cigarette smoke. Apnoea occurred, both before and after vagotomy, in all species. Blood pressure always went up in rabbits, rats and hamsters but was not much affected in cats and guinea-pigs. Before vagotomy heart rate fell in rabbits and rats, rose in hamsters but did not change in cats and guinea-pigs. Vagotomy affected heart rate changes only in rabbits and rats. Results demonstrate markedly different autonomic responses in different species.

Differential pattern of sympathetic outflow during upper airway stimulation with smoke.

This study investigates directly the possibility that sympathetic discharge to the heart is decreased while it is increased to other organs during upper respiratory perfusion with cigarette smoke. Blood pressure (BP), heart rate, ECG, and respiratory movements were monitored in urethane-anesthetized rabbits. Insertion of two cannulas allowed respiration of room air while passing smoke across the upper respiratory irritant receptors and out through the nares. Through a retroplural incision, the left stellate ganglion was exposed and a cardiac branch isolated. Similarly, a left renal nerve was isolated. Multiunit nerve recordings were obtained from both nerves. In four control animals, cigarette smoke (50 ml) caused apnea, bradycardia (-116 beats/min) and increased BP (33 mmHg). Activity in the renal nerve increased (248% of control [C]) and activity in the cardiac nerve was reduced (62% C). In these animals after Flaxedil and artificial respiration, nerve activity responses were still pronounced (renal, 178% C; cardiac, 66% C). In four other barodenervated animals neural responses to smoke were similar to those observed with baroreceptors intact (renal, 211% C; cardiac, 51% C). In these animals after artificial ventilation and Flaxedil, responses were not significantly changed. These results indicate that smoke stimulation causes a differential pattern of sympathetic discharge. The responses observed cannot be accounted for by secondary adjustments through arterial baroreceptors, chemoreceptors, or pulmonary stretch receptors.

Lactic acidosis, neurological deterioration and compromised cellular pyruvate oxidation due to a defect in the reoxidation of cytoplasmically generated NADH.

Two patients, one dying at 25 days and one at 20 months had 'chronic' lactic acidaemia with a high lactate to pyruvate ratio. Both showed EEG abnormalities and seizure activity and both died of respiratory failure. Investigation of cultured skin fibroblasts from these patients revealed normal pyruvate dehydrogenase and pyruvate carboxylase activities but the cells showed a decreased ability to oxidase pyruvate which was returned to normal on the addition of methylene blue. Subsequent investigations revealed that the mitochondria from the patients' cells could oxidase pyruvate normally but that the cells had an abnormal NAD to NADH ratio under standard conditions of incubation. It was concluded that both children had a redox disequilibrium in the cytoplasmic compartment due to a problem in transporting reducing equivalents from the cytoplasmic to the mitochondrial compartments.

The influence of the paraventriculo-spinal pathway, and oxytocin and vasopressin on sympathetic preganglionic neurones.

In anaesthetized rats the effect of two procedures was studied on antidromically identified sympathetic preganglionic neurones (SPN) in the second thoracic (T) segment of the spinal cord: the application of iontophoresed oxytocin and vasopressin, and bipolar electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN). In the majority of cases (16/23) oxytocin inhibited SPN firing, 1/23 being excited. Vasopressin inhibited 8/14 neurones and excited 4/14. PVN stimulation inhibited SPN apparently by an action on the membrane of SPN. The possibility that oxytocin and vasopressin act as transmitters in the paraventriculo-spinal pathway, and their possible involvement in the mediation of PVN evoked inhibition of SPN activity has been discussed.

Some characteristics of sympathetic preganglionic neurones in the rat.

In anaesthetized rats sympathetic preganglionic neurones (SPN) were identified by their antidromic response to stimulation of the ipsilateral cervical sympathetic trunk (CST). Units were recorded at a depth of 0.75-1.1 mm from the dorsal surface of the spinal cord. The majority of SPN had axonal conduction velocities less than 1 m/s. Units could be routinely held for periods of up to 1 h. Spontaneously active SPN had discharge rates within the range 0.3-8.0 Hz. Application of horseradish peroxidase (HRP) to the central cut end of the CST resulted in the labelling of neurones only on the ipsilateral side, mainly in the intermediolateral cell column (IML) and the lateral funiculus and between the first and third thoracic (T) segments.

Inhibition of sympathetic activity by stimulating in the raphe nuclei and the role of 5-hydroxytryptamine in this effect.

The possibility that the putative transmitter 5-hydroxytryptamine (5-HT) is involved in the mediation of long latency to onset raphe-spinal inhibition of sympathetic preganglionic neurones was investigated in anaesthetized cats by stimulating sites located in nucleus raphe pallidus and obscurus and recording sympathetic discharge in T3 or T10 white rami evoked either reflexively or by intraspinal stimulation at cervical level. Several putative 5-HT antagonists were administered intravenously (i.v.) or topically to the spinal cord. In 7 cats lysergic acid diethylamide (LSD) in a dose range 25-50 microgram/kg i.v. or 0.6 microgram topically, reversibly reduced the raphe spinal inhibition by 40-100%. Topical application was more effective than i.v. administration. In 5 cats stimulating within the ventromedial reticular formation at sites unlikely to involved 5-HT neurones produced a short latency to onset inhibition which was unaffected by LSD. Methysergide, cinanserin and cyproheptadine depressed sympathetic discharge in the absence of brain stimulation in cats with CNS intact and in unanaesthetized decerebrate spinal cats. The results are discussed in the light of the known actions of the putative 5-HT antagonists.

Patterns of glycogen loss in muscle fibers: response to arterial occlusion during exercise.

This experiment was designed to determine if glycogen loss in active rat skeletal muscle fibers could be accentuated by periodic occlusion of blood to the muscles without significantly altering the numbers of types of fibers that lose glycogen. We attempted to augment glycogen loss by periodically occluding blood flow to the muscles while the animals ran on a treadmill. An occluder cuff was placed on the right common iliac artery of 10 rats. While the rats ran for 5 min at 26 m . min-1, blood flow to the right hindlimb was completely occluded for 16 +/- 5 (SD) s during every 30 s of the run. Glycogen loss in soleus (S), plantaris (P), and gastrocnemius (G) muscles was determined biochemically and histochemically. Muscles from both the occluded limb (S, P, and the deep red portion of G) and the nonoccluded limb (S and red G) of the runners showed significant glycogen loss compared with the controls. Furthermore, glycogen concentration was significantly lower in S, P, and red G of the occluded limb than in the same muscles of the nonoccluded limb. Significantly greater numbers of fibers within fiber type populations of P and G showed glycogen loss in the occluded limb, indicating that more motor units were recruited during the exercise bout in these muscles. The data suggest a sensitive link between motor unit recruitment and the metabolic condition of the contracting fibers, as the increased number of fibers showing glycogen loss presumably, resulted from fatigue of active units. We conclude that occlusion of blood flow to active muscles is not a feasible means of accentuating glycogen loss in active fibers while maintaining normal patterns of recruitment.

Reduction in cardiac contractility during upper respiratory stimulation with cigarette smoke.

Lightly anesthetized rabbits were instrumented to measure arterial pressure, peak left ventricular pressure (LVP) and its first derivative (LV dP/dt), electrocardiogram (ECG), heart rate (HR), and respiratory movements. Through a midtracheal incision two cannulas were inserted, one to allow spontaneous respiration of room air and the other to permit perfusion of the upper airways with unfiltered cigarette smoke. When 50 ml of smoke were passed out of the nostrils, we observed apnea (33.3 s), increases in mean arterial pressure (MAP) (31.9 mmHg) and LVP (38.3 mmHg), as well as decreases in HR (-125 beats/min) and LV dP/dtmax (-45%). After bilateral vagotomy, the responses of MAP and LVP to smoke exposure were not different from those in intact animals. The decrease in LV dP/dtmax (-27%) was, however, significantly less than in intact animals. Also, HR responses were not as pronounced after vagotomy. beta-Blockade after vagotomy abolished the LV dP/dt response to smoke, and the HR response was nearly abolished. MAP and LVP still underwent significant increases due to the smoke. After total sinoaortic denervation plus vagotomy in a second group of animals, a significant fall in LV dP/dtmax (-25%) persisted, and a small HR decrease was observed even when MAP was not allowed to change significantly. The results demonstrate that sympathetic outflow to the heart decreases while there is a net increase in sympathetic activity to the peripheral vasculature. The dramatic vagal bradycardia observed contributes part of the change in LV dP/dtmax. Reflexes originating in the circulation (vagal or sinoaortic) may also contribute to the fall in LV dP/dtmax, but a significant response persists in their absence.