Cholecystokinin-induced residual stimulation of enzyme secretion from mouse pancreatic acini

When dispersed acini from mouse pancreas are first incubated with cholecystokinin octapeptide, washed and then reincubated with no additions there is significant stimulation of amylase secretion during the second incubation (residual stimulation of enzyme secretion). Cholecystokinin-induced residual stimulation of enzyme secretion is modified, but not abolished, by reducing the temperature of the first incubation from 37 degrees C to 4 degrees C. Measurement of binding of 125I-labeled cholecystokinin octapeptide indicated that maximal cholecystokinin induced residual stimulation of enzyme secretion occurs when 12-20% of cholecystokinin receptors are occupied by cholecystokinin octapeptide. Moreover, maximal cholecystokinin-induced residual stimulation of amylase secretion is 25% greater than maximal cholecystokinin-induced direct stimulation of amylase secretion. Cholecystokinin tetrapeptide, which causes the same maximal direct stimulation of amylase secretion as does cholecystokinin octapeptide, causes a maximal residual stimulation of enzyme secretion that is only 30% of that caused by a maximally effective concentration of cholecystokinin octapeptide. Adding dibutyryl cyclic GMP to the second incubation can reverse the residual stimulation caused by adding cholecystokinin to the first incubation. The pattern and extent of the dibutyryl cyclic GMP-induced reversal of residual stimulation varies, depending on the temperature and concentration of cholecystokinin octapeptide in the first incubation. The present results are compatible with the hypothesis that mouse pancreatic acini possess two classes of cholecystokinin receptors. One class has a relatively high affinity for cholecystokinin and produces stimulation of enzyme secretion; the other class has a relatively low affinity for cholecystokinin and produces inhibition of enzyme secretion.     

Effect of alveolar hypoxia on pulmonary fluid filtration in in situ dog lungs

We have studied the effect of alveolar hypoxia on fluid filtration characteristics of the pulmonary microcirculation in an in situ left upper lobe preparation with near static flow conditions (20 ml/min). In six dogs (group 1), rate of edema formation (delta W/delta t, where W is weight and t is time) was assessed over a wide range of vascular pressures under two inspired O2 fraction (FIO2) conditions (0.95 and 0.0 with 5% CO2-balance N2 in both cases). delta W/delta t was plotted against vascular pressure, and the best-fit linear regression was obtained. There was no significant difference (paired t test) in either threshold pressure for edema formation [18.3 +/- 1.8 and 17.1 +/- 1.2 (SE) mmHg, respectively] or the slopes (0.067 +/- 0.008 and 0.073 +/- 0.017 g.min-1. mmHg-1.100g-1, respectively). In another seven dogs (group 2), delta W/delta t was obtained at a constant vascular pressure of 40 mmHg under four FIO2 conditions (0.95, 0.21, 0.05, and 0.0, with 5% CO2-balance N2). Delta W/delta t for the four conditions averaged 0.60 +/- 0.11, 0.61 +/- 0.11, 0.61 +/- 0.10, and 0.61 +/- 0.10 (SE) g.min-1.mmHg-1.100g-1, respectively. No significant differences (ANOVA for repeated measures) were noted. We conclude that alveolar hypoxia does not alter the threshold for edema formation or delta W/delta t at a given microvascular pressure.     

Steady-state response of quadriplegic subjects to inspiratory resistive load

Normal subjects preserve tidal volume (VT) in the face of added inspiratory resistance by increasing maximal amplitude and duration of the rising phase of respiratory driving pressure (DP) and by changing the shape of this phase to one that is more concave to the time axis. To explore the possible role of chest wall afferents in mediating these responses, we determined averaged DP in eight quadriplegic subjects during steady-state unloaded breathing and while breathing through an inspiratory resistance (8.5 cmH2O X 1(-1) X s). As with normal subjects, quadriplegics preserved VT (loaded VT = 106% control) by utilizing all three mechanisms. However, prolongation of the inspiratory duration derived from the DP waveform (+22% vs. +42%) and shape response were significantly less in the quadriplegic subjects. Shape response was completely absent in subjects with C4 lesions. The results provide strong evidence that respiratory muscle spindles are responsible for shape response and that changes in afferent feedback from the chest wall play an important role in mediating inspiratory prolongation.     

The role of iron in the paracetamol- and CCl4-induced lipid peroxidation and hepatotoxicity

Treatment of non-induced or phenobarbital-induced, glutathione-depleted mice with 400 mg/kg paracetamol led to a marked ethane exhalation as an index of in vivo lipid peroxidation (LPO) and to a significant elevation of liver-specific serum enzyme activities. Similar effects were seen with rats treated with 0.5 ml/kg CCl4. Pretreatment with the iron-chelating agent desferrioxamine (DFO) clearly suppressed lipid peroxidation in all cases, but inhibited only the CCl4-induced hepatotoxicity. Treatment of mice with desferrioxamine alone showed no hepatotoxicity at all, nor did it influence liver GSH-levels. In addition, DFO had no effect on hepatic microsomal enzyme activities responsible for the bioactivation of both paracetamol and CCl4. These findings are consistent with the theories which indicate that lipid peroxidation requires the presence of Fe2+-ions, regardless of the initiating agent, and that LPO is involved in CCl4-toxicity, but most probably not in paracetamol-induced liver damage. Furthermore, Fe2+-ions might play a role as mediators of CCl4-hepatotoxicity.     

Central adaptation to inspiratory-inhibiting expiratory-prolonging vagal input

We reported earlier on the changes in excitability of central respiratory switching mechanisms in the course of a brief inspiratory-inhibiting vagal stimulus (J. Appl. Physiol. 50: 1183-1192, 1981). To further define the dynamics of central processing of such input we studied the changes in the excitability of timing mechanisms in the immediate (less than 1.0 s) and late (1-20 s) periods after stimulus removal. We also examined the changes in respiratory timing in the course of protracted (greater than 20 s) stimulation. Studies were done using pentobarbital-anesthetized cats. For studies involving long-term stimulation or late off responses, cats were paralyzed, vagotomized, carotid denervated, and artificially ventilated. We found that the inspiratory inhibitory influence of a brief stimulus continues, in a declining fashion, for 0.3-10 s after removal of the stimulus. This was followed by a paradoxical response, inspirations were prolonged and expirations were shortened, which was maximal 1-2 s after stimulus removal and which declined gradually over a period of 6-16 s. There was progressive decline in inspiratory-shortening expiratory-prolonging influence in the course of sustained stimuli. These results indicate substantial adaptation in the course of even brief stimuli and provide an explanation for inspiratory-expiratory duration and expiratory-inspiratory duration linkages.     

Temporal changes in effectiveness of an inspiratory inhibitory electrical pontine stimulus

We determined the temporal changes in effectiveness of inspiratory-shortening expiratory-prolonging stimulus trains delivered in the region of the nucleus parabrachialis medialis and compared the responses to those observed during trains delivered to the vagus in the same animals (pentobarbital, sodium-anesthetized paralyzed cats). The inspiratory inhibitory effect of the pontine stimulus was assessed from the effect the stimulus has on threshold for terminating inspiration. Stimulus effect increased gradually, reached a peak at 0.2-0.4 s, and declined thereafter. The time of occurrence of peak effect was different from that observed in the course of vagal stimulus trains. With long stimulus trains (19-40 s), the initial effect on inspiratory duration (TI) (i.e., shortening) rapidly subsided and, in six of eight animals, was replaced by TI prolongation. The initial effect on expiratory duration (TE) (i.e., prolongation) also gradually declined with time but TE remained above control throughout. The time constant of adaptation was very similar with vagal and pontine stimulus trains (12.2 and 11.0 s, respectively), but the gain of the adapting response was much more pronounced with pontine stimuli, resulting in a paradoxical effect while stimulation continued. We conclude that the response to pontine stimuli, as with vagal stimuli, displays both integrative and adaptive characteristics. The similarity of the time constants for vagal and pontine adaptation responses suggests that these two inputs share common processing pathways.     

Effect of slowly increasing elastic load on breathing in conscious humans

Because the detection of added loads is a function of the step change in load relative to background level of load (Weber's law), we reasoned that detection may be delayed if the load is increased in very small steps over a protracted period. This would permit the study of subliminal load responses over a greater range than would otherwise be possible. In 13 healthy males, an external elastic load (delta E) was added in very small steps (load increased every few seconds) such that delta E increased from zero to 6.0 cmH2O/l in 18 min. Each subject underwent a control study in which an identical protocol was followed but no load was added. Five subjects sensed the load after a variable period of load application. Their results were discarded. In the remaining eight subjects, there was no perception of increased load throughout the 18 min. By comparison with the control studies in the same subjects, there was a progressive reduction in tidal volume and breath duration with loading. To assess the response to an increase in load associated with perception, in five of the eight "nonsensers" the load was suddenly doubled (delta E = 6 cmH2O/l) at the end of the 18 min of progressive nonsensed loading. This evoked perception in all subjects and was associated with highly variable responses in tidal volume and breath duration. We conclude that tachypnea associated with elastic loading in conscious humans is a reflex response that is facilitated by consciousness but does not require perception.     

Respiratory mechanics and breathing pattern during and following maximal exercise

We looked for evidence of changes in lung elastic recoil and of inspiratory muscle fatigue at maximal exercise in seven normal subjects. Esophageal pressure, flow, and volume were measured during spontaneous breathing at increasing levels of cycle exercise to maximum. Total lung capacity (TLC) was determined at rest and immediately before exercise termination using a N2-washout technique. Maximal inspiratory pressure and inspiratory capacity were measured at 1-min intervals. The time course of instantaneous dynamic pressure of respiratory muscles (Pmus) was calculated for the spontaneous breaths immediately preceding exercise termination. TLC volume and lung elastic recoil at TLC were the same at the end of exercise as at rest. Maximum static inspiratory pressures at exercise termination were not reduced. However, mean Pmus of spontaneous breaths at end exercise exceeded 15% of maximum inspiratory pressure in five of the subjects. We conclude that lung elastic recoil is unchanged even at maximal exercise and that, while inspiratory muscles operate within a potentially fatiguing range, the high levels of ventilation observed during maximal exercise are not maintained for a sufficient time to result in mechanical fatigue.     

Mechanism of detection of resistive loads in conscious humans.

Conscious humans easily detect loads applied to the respiratory system. Resistive loads as small as 0.5 cmH2O.l-1.s can be detected. Previous work suggested that afferent information from the chest wall served as the primary source of information for load detection, but the evidence for this was not convincing, and we recently reported that the chest wall was a relatively poor detector for applied elastic loads. Using the same setup of a loading device and body cast, we sought resistive load detection thresholds under three conditions: 1) loading of the total respiratory system, 2) loading such that the chest wall was protected from the load but airway and intrathoracic pressures experienced negative pressure in proportion to inspiratory flow, and 3) loading of the chest wall alone with no alteration of airway or intrathoracic pressure. The threshold for detection for the three types of load application in seven normal subjects was 1.17 +/- 0.33, 1.68 +/- 0.45, and 6.3 +/- 1.38 (SE) cmH2O.l-1.s for total respiratory system, chest wall protected, and chest wall alone, respectively. We conclude that the active chest wall is a less potent source of information for detection of applied resistive loads than structures affected by negative airway and intrathoracic pressure, a finding similar to that previously reported for elastic load detection.