Citations

To assess the reliability of electron beam computed tomography (EBCT), measurements of single-kidney renal blood flow (RBF), glomerular filtration rate (GFR), and intratubular contrast medium concentration (ITC) of radiographic contrast media were quantified in anesthetized pigs before and after acetylcholine-induced vasodilation and diuresis. EBCT measurements were compared with those obtained with intravascular Doppler and inulin clearance. The capability of EBCT to detect chronic changes in single-kidney function was evaluated in pigs with unilateral renal artery stenosis, and their long-term reproducibility in normal pigs was studied repeatedly at 1-mo intervals. EBCT-RBF (ml/min) correlated with Doppler-RBF as RBFEBCT= 45 + 1.07 ∗ RBFDoppler, r = 0.81. EBCT-GFR (ml/min) correlated with inulin clearance as GFREBCT= 11.7 + 1.02 ∗ GFRinulin, r = 0.80. During vasodilation, RBF and GFR increased, whereas ITC decreased along the nephron. In renal artery stenosis, single-kidney GFR decreased linearly with the degree of stenosis, and ITC increased along the nephron, indicating increased fluid reabsorption. EBCT-RBF, GFR, and ITC were similar among repeated measurements. This approach might be invaluable for simultaneous quantification of regional hemodynamics and function in the intact kidneys, in a manner potentially applicable to humans.

The mechanism of pressure-induced natriuresis remains controversial. To assess whether intracortical or medullary renal blood flows (RBF) change with changes in renal perfusion pressure (RPP), global and regional RBFs were measured using the dynamic spatial reconstructor, a fast computed tomography scanner, in eight anesthetized dogs (group B) within the range of RBF autoregulation (RPP of 153.5 and 114.4 mmHg). Similar measurements were obtained in seven control dogs (group A) in which RPP was not manipulated. In group B, only inner medullary perfusion decreased (from 0.84 to 0.51 ml/min per cm3 of tissue, P = 0.03) with reduction of RPP, whereas global renal, intracortical, and outer medullary perfusions remained unaltered. In group A there was no change in global or regional renal perfusion. The change in inner medullary perfusion in group B (-34.7%) was significantly different (P = 0.021) from that in group A (+27.4%). Global, cortical, and total medullary RBFs (ml/min) and volumes did not change in either group. These results suggest that with changes in RPP, the only detectable change in intrarenal perfusion occurs in the inner medulla.

The effects of the β2-adrenoceptor agonist salbutamol (Slb) on isometric and isotonic contractile properties of the rat diaphragm muscle (Diamus) were examined. A loading dose of 25 μg/kg Slb was administered intracardially before Diamusexcision to ensure adequate diffusion. Studies were then performed with 0.05 μM Slb in the in vitro tissue chamber. cAMP levels were determined by radioimmunoassay. Compared with controls (Ctl), cAMP levels were elevated after Slb treatment. In Slb-treated rats, isometric twitch and maximum tetanic force were increased by ∼40 and ∼20%, respectively. Maximum shortening velocity increased by ∼15% after Slb treatment, and maximum power output increased by ∼25%. During repeated isotonic activation, the rate of fatigue was faster in the Slb-treated Diamus, but both Slb-treated and Ctl Diamusfatigued to the same maximum power output. Still, endurance time during repetitive isotonic contractions was ∼10% shorter in the Slb-treated Diamus. These results are consistent with the hypothesis that β-adrenoceptor stimulation by Slb enhances Diamuscontractility and that these effects of Slb are likely mediated, at least in part, by elevated cAMP.

Three anesthetics (pentobarbital, halothane, and isoflurane) were studied in six mongrel dogs to systematically compare their effects on chest wall function during spontaneous breathing. Each dog received each anesthetic on separate occasions. Electrical activities of several respiratory muscles were measured with chronically implanted electrodes, and chest wall motion was assessed by high-speed three-dimensional computed tomography scanning. Phasic expiratory muscle activity was markedly depressed by volatile anesthetics halothane and isoflurane compared with pentobarbital. In contrast, inspiratory activity in parasternal intercostal muscles was relatively well preserved during anesthesia with these volatile agents. The contribution of expiratory muscles to tidal volume was diminished during halothane and isoflurane compared with pentobarbital anesthesia. As anesthesia was deepened, expiratory muscle activity was unchanged during pentobarbital anesthesia, enhanced in some dogs during isoflurane anesthesia, and remained absent during halothane anesthesia. Activity in parasternal intercostal muscle was depressed as inspired concentration of halothane or isoflurane was increased, whereas diaphragmatic activity was unchanged. Depression of expiratory muscle activity by halothane persisted when breathing was stimulated by positive end-expiratory pressure, with significant mechanical consequences for chest wall configuration. Many of these findings are in contrast with previous observations in humans and suggest that the dog is not a suitable model for the study of the effects of anesthetic drugs on the pattern of human respiratory muscle activity.

Prakash, Y. S., H. F. M. van der Heijden, M. S. Kannan, and G. C. Sieck. Effects of salbutamol on intracellular calcium oscillations in porcine airway smooth muscle. J. Appl. Physiol. 82(6): 1836–1843, 1997.—Relaxation of airway smooth muscle (ASM) by β-adrenoceptor agonists involves reduction of intracellular Ca2+concentration ([Ca2+]i). In porcine ASM cells, acetylcholine induces [Ca2+]ioscillations that display frequency modulation by agonist concentration and basal [Ca2+]i. We used real-time confocal microscopy to examine the effect of salbutamol (1 nM to 1 μM), a β2-adrenoceptor agonist, on [Ca2+]ioscillations in freshly dissociated porcine ASM cells. Salbutamol decreased the frequency of [Ca2+]ioscillations in a concentration-dependent fashion, completely inhibiting the oscillations at 1 μM. These effects were mimicked by a cell-permeant analog of adenosine 3′,5′-cyclic monophosphate. The inhibitory effect of salbutamol was partially reversed by BAY K 8644. Salbutamol reduced [Ca2+]ieven when sarcoplasmic reticulum (SR) Ca2+reuptake and Ca2+influx were blocked. Lanthanum blockade of Ca2+efflux attenuated the inhibitory effect of salbutamol on [Ca2+]i. The [Ca2+]iresponse to caffeine was unaffected by salbutamol. On the basis of these results, we conclude that β2-adrenoceptor agonists have little effect on SR Ca2+release in ASM cells but reduce [Ca2+]iby inhibiting Ca2+influx through voltage-gated channels and by enhancing Ca2+efflux.

To determine the relative contribution of rib cage and abdominal muscles to expiratory muscle activity during quiet breathing, we used lumbar epidural anesthesia in six pentobarbital sodium-anesthetized dogs lying supine to paralyze the abdominal muscles while leaving rib cage muscle motor function substantially intact. A high-speed X-ray scanner (Dynamic Spatial Reconstructor) provided three-dimensional images of the thorax. The contribution of expiratory muscle activity to tidal breathing was assessed by a comparison of chest wall configuration during relaxed apnea with that at end expiration. We found that expiratory muscle activity was responsible for approximately half of the changes in thoracic volume during inspiration. Paralysis of the abdominal muscles had little effect on the pattern of breathing, including the contribution of expiratory muscle activity to tidal breathing, in most dogs. We conclude that, although there is consistent phasic expiratory electrical activity in both the rib cage and the abdominal muscles of pentobarbital-anesthetized dogs lying supine, the muscles of the rib cage are mechanically the most important expiratory muscles during quiet breathing.

A microcomputed tomography (micro-CT) scanner, which generates three-dimensional (3-D) images consisting of up to a billion cubic voxels, each 5–25 μm on a side, and which has isotropic spatial resolution, is described. Its main components are a spectroscopic X-ray source that produces selectable primary emission peaks at ∼9, 18, or 25 keV and a fluorescing thin crystal plate that is imaged (at selectable magnification) with a lens onto a 2.5 × 2.5-cm, 1,024 × 1,024-pixel, charge-coupled device (CCD) detector array. The specimen is positioned close to the crystal and is rotated in 721 equiangular steps around 360° between each X-ray exposure and its CCD recording. Tomographic reconstruction algorithms, applied to these recorded images, are used to generate 3-D images of the specimen. The system is used to scan isolated, intact, fixed rodent organs (e.g., heart or kidney) with the image contrast of vessel lumens enhanced with contrast medium. 3-D image display and analysis are used to address physiological questions about the internal structure-to-function relationships of the organs.

We employed high-speed multisliced X-ray-computed tomography to determine the relative volume contributions of rib cage (delta Vrc) and diaphragmatic motion (delta Vdi) to tidal volume (VT) during spontaneous breathing in 6 anesthetized dogs lying supine. Mean values were 40 +/- 6% (SE) for delta Vrc and 62 +/- 8% of VT for delta Vdi. The difference between VT and changes in thoracic cavity volume was taken to represent a change in thoracic blood volume (2 +/- 3% of VT). To estimate how much of delta Vrc was caused by diaphragmatic contraction and how much of delta Vdi was caused by rib cage motion, delta Vrc and delta Vdi were determined during bilateral stimulation of the C5-C6 phrenic nerve roots in the apneic dog and again during spontaneous breathing after phrenicotomy. Thoracic cavity volume (Vth) measured during hypocapnic apnea was consistently larger than Vth at end expiration, suggesting that relaxation of expiratory muscles contributed significantly to both delta Vrc and delta Vdi during spontaneous inspiration. Phrenic nerve stimulation did not contribute to delta Vrc, suggesting that diaphragmatic contraction had no net expanding action on the rib cage above the zone of apposition. Spontaneous breathing after phrenicotomy resulted in small and inconsistent diaphragmatic displacement (8 +/- 4% of VT). We conclude that the diaphragm does not drive the rib cage to inflate the lungs and that rib cage motion does not significantly affect diaphragmatic position during spontaneous breathing in anesthetized dogs lying supine.

We measured the volume change of the thoracic cavity (delta Vth) and the volumes displaced by the diaphragm (delta Vdi) and rib cage (delta Vrc) in six pentobarbital-anesthetized dogs lying supine. A high-speed X-ray scanner (dynamic spatial reconstructor) provided three-dimensional images of the thorax during spontaneous breathing and during mechanical ventilation with paralysis. Tidal volume (VT) was measured by integrating gas flow. Changes in thoracic liquid volume (delta Vliq, presumably caused by changes in thoracic blood volume) were calculated as delta Vth - VT. Absolute volume displaced by the rib cage was not significantly different during the two modes of ventilation. During spontaneous breathing, thoracic blood volume increased during inspiration; delta Vliq was 12.3 +/- 4.1% of delta Vth. During mechanical ventilation, delta Vliq was nearly zero. Configuration of the relaxed chest wall was similar during muscular relaxation induced by either pharmacological paralysis or hyperventilation. Expiratory muscle activity produced 50 +/- 11% of the delta Vth during spontaneous breathing. We conclude that at constant VT the volume displaced by the rib cage is remarkably similar during the transition from spontaneous breathing to mechanical ventilation, while both diaphragmatic volume displacement and changes in intrathoracic blood volume decrease by a similar amount.