Ventilatory changes associated with changes in pulmonary blood flow in dogs

To examine the influence of pulmonary blood flow (Qp) on spontaneous ventilation (VE), we isolated the systemic and pulmonary circulations and controlled the arterial blood gases and blood flow (Q) in each circuit as we measured VE. Each dog was anesthetized with ketamine and maintained with halothane. Systemic Q was drained from the right atrium and pumped through an oxygenator and heat exchanger and returned to the aorta. An identical bypass was established for the pulmonary circulation, draining blood from the left atrium and pumping it to the pulmonary artery. The heart was fibrillated, all cannulas were brought through the chest wall, and the median sternotomy was closed. The dog was then allowed to breathe spontaneously. The arterial O2 partial pressure (PO2) of both circuits was maintained greater than 300 Torr. Systemic Q was maintained at 0.080 l X min-1 X kg-1. Initially the arterial CO2 partial pressure (PCO2) of both circuits was set at 40 Torr as Qp was varied randomly between approximately 0.025 and 0.175 l X min-1 X kg-1. The average VE-Qp relationship was linear with a slope of 1.45 (P less than 0.0005). Increasing the arterial PCO2 of both circuits to 60 Torr elevated VE an average of 0.37 l X min-1 X kg-1 at each level of Qp (P less than 0.0005). Vagotomy abolished the effect of Qp on VE. Increasing Qp affected the systemic arterial PCO2-VE response curve by shifting it upward without altering its slope. These results demonstrate that increases in Qp are associated with increases in VE. This phenomenon may contribute to exercise hyperpnea.     

Impaired erythrocyte deformability precedes vascular changes in experimental diabetes mellitus.

The effect of diabetes on the red blood cell (RBC) deformability and its association with histological vascular changes was investigated in 35 streptozotocin-induced diabetic Wistar rats in a 30-day experiment and compared to 10 controls. RBC deformability was significantly impaired in the diabetic rats on day 5 (p < 0.001) and continued to deteriorate until day 20. On the 20 (th) day, the diabetic rats were randomly divided into two groups (group A: insulin-treated; group B: non-insulin-treated). A slight, non-significant (p = 0.20) improvement in RBC deformability was noticed in the insulin-treated group. In vitro incubation of RBCs with insulin did not improve the acquired RBC rigidity in either diabetic group. In contrast, it caused a significant reduction in RBC-deformability in the controls. On day 30, histological examination of arterial specimens from various sites revealed moderate to significant thickening in medium- and small-size artery and arteriole walls in both diabetic groups, with no evidence of diabetes-related changes in large, elastic-type arteries. No vascular changes were noticed in nine diabetic rats that succumbed between days 10 and 15. The results of this study indicate that reduced RBC deformability is an early manifestation of abnormal blood rheology in experimental diabetes, and precedes the evolution of vascular changes.     

Inhibition of NOS enhances pulmonary vascular changes in stroke-prone spontaneously hypertensive rats

To determine the effects of chronic nitric oxide (NO) blockade on the pulmonary vasculature, 58-day-old spontaneously hypertensive rats of the stroke-prone substrain (SHRSP) and Wistar-Kyoto rats (WKY) received N(omega)-nitro-L-arginine (L-NNA; 15 mg. kg(-1). day(-1) orally for 8 days). Relaxation to acetylcholine (ACh) in hilar pulmonary arteries (PAs), the ratio of right ventricular (RV) to body weight (RV/BW) to assess RV hypertrophy (RVH), and the percent medial wall thickness (WT) of resistance PAs were examined. L-NNA did not alter the PA relaxation, RV/BW, or WT in WKY. Although the PA relaxation and RV/BW in control SHRSP were comparable to those in WKY, the WT was increased (31 +/- 2 vs. 19 +/- 1%). L-NNA-treated SHRSP showed two patterns: in one group, the relaxation, RV/BW, and WT were comparable to those in the control SHRSP; in the other, impaired relaxation (36 +/- 7 vs. 88 +/- 4% for WKY) was associated with an increase in WT (37 +/- 1%) and RV/BW (0. 76 +/- 0.05). Thus the abnormal pulmonary vasculature in SHRSP at <10 wk of age is not accompanied by impaired relaxation in PAs or RVH; however, impaired relaxation is associated with increased WT and RVH.     

Selective right, but not left, coronary endothelial dysfunction precedes development of pulmonary hypertension and right heart hypertrophy in rats

We investigated a causal role for coronary endothelial dysfunction in development of monocrotaline (MCT)-induced pulmonary hypertension and right heart hypertrophy in rats. Significant increases in pulmonary pressure and right ventricular weight did not occur until 3 wk after 60 mg/kg MCT injection (34 +/- 4 vs. 19 +/- 2 mmHg and 37 +/- 2 vs. 25 +/- 1% septum + left ventricular weight in controls, respectively). Isolated right coronary arteries (RCA) showed significant decreases in acetylcholine-induced NO dilation in both 1-wk (33 +/- 3% with 0.3 microM; n = 5) and 3-wk (18 +/- 3%; n = 11) MCT rats compared with control rats (71 +/- 8%, n = 10). Septal coronary arteries (SCA) showed a smaller decrease in acetylcholine dilation (55 +/- 8% and 33 +/- 7%, respectively, vs. 73 +/- 8% in controls). No significant change was found in the left coronary arteries (LCA; 88 +/- 6% and 81 +/- 6%, respectively, vs. 87 +/- 3% in controls). Nitro-L-arginine methyl ester-induced vasoconstriction, an estimate of spontaneous endothelial NO-mediated dilation, was not significantly altered in MCT-treated SCA or LCA but was increased in RCA after 1 wk of MCT (-41 +/- 6%) and decreased after 3 wk (-18 +/- 3% vs. -27 +/- 3% in controls). A marked enhancement to 30 nM U-46619-induced constriction was also noted in RCA of 3-wk (-28 +/- 6% vs. -9 +/- 2% in controls) but not 1-wk (-12 +/- 7%) MCT rats. Sodium nitroprusside-induced vasodilation was not different between control and MCT rats. Together, our findings show that a selective impairment of right, but not left, coronary endothelial function is associated with and precedes development of MCT-induced pulmonary hypertension and right heart hypertrophy in rats.     

Ventilatory changes of pulmonary capillary blood volume assessed by arterial density

By use of an improved density measuring system, we found that the gravimetric density of arterial blood of dogs fluctuates at the same frequency as the spontaneous or mechanical ventilation. Similar density fluctuations were observed in the blood leaving isolated, perfused lobes of dogs that were ventilated cyclicly. Employing an analysis that balanced the erythrocyte and plasma flows through distensible capillaries containing blood with a tube hematocrit lower than the hematocrit in large blood vessels, we derived a relationship to estimate from the density fluctuation the change in pulmonary capillary blood volume (Vc). For mechanical ventilation, the maximum change in density over one ventilation cycle increased from 0.084 +/- 0.01 to 0.47 +/- 0.05 (SE) g/l as the frequency decreased from 29 to 6 cycles/min. These density changes were estimated to be the result of an 1-16% change in Vc. A larger tidal volume for the mechanical ventilation led to a larger density fluctuation. The maximum density change of spontaneous respiration of 6 cycles/min was one-sixth of the mechanical case, indicating a much smaller change in Vc during spontaneous respiration. When the airway flow resistance was increased for spontaneous respiration, larger density fluctuations were observed.     

Maturational changes in the regulation of pulmonary vascular tone by nitric oxide in neonatal rats

Nitric oxide (NO) is an important regulator of vasomotor tone in the pulmonary circulation. We tested the hypothesis that the role NO plays in regulating vascular tone changes during early postnatal development. Isolated, perfused lungs from 7- and 14-day-old Sprague-Dawley rats were studied. Baseline total pulmonary vascular resistance (PVR) was not different between age groups. The addition of KCl to the perfusate caused a concentration-dependent increase in PVR that did not differ between age groups. However, the nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine augmented the K(+)-induced increase in PVR in both groups, and the effect was greater in lungs from 14-day-old rats vs. 7-day-old rats. Lung levels of total endothelial, inducible, and neuronal NOS proteins were not different between groups; however, the production rate of exhaled NO was greater in lungs from 14-day-old rats compared with those of 7-day-old rats. Vasodilation to 0.1 microM of the NO donor spermine NONOate was greater in 14-day lungs than in 7-day lungs, and lung levels of both soluble guanylyl cyclase and cGMP were greater at 14 days than at 7 days. Vasodilation to 100 microM of the cGMP analog 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphate was greater in 7-day lungs than in 14-day lungs. Our results demonstrate that the pulmonary vascular bed depends more on NO production to modulate vascular tone at 14 days than at 7 days of age. The observed differences in NO sensitivity may be due to maturational increases in soluble guanylyl cyclase protein levels.     

Split-dose sparing of gamma-ray-induced pulmonary endothelial dysfunction in rats

The purpose of this study was to determine whether radiation-induced pulmonary endothelial dysfunction exhibits split-dose sparing. Rats were sacrificed 2 months after a range of 60Co gamma-ray doses (0-40 Gy) delivered to the right hemithorax in either a single fraction or in two equal fractions separated by 24 h. Pulmonary angiotensin converting enzyme (ACE) activity, plasminogen activator (PLA) activity, and prostacyclin (PGI2) and thromboxane (TXA2) production served as indices of lung endothelial function. There were dose-dependent decreases in ACE and PLA activity and increases in PGI2 and TXA2 production after both single and split-dose exposures. The D2-D1 values determined from the two-fraction minus single-fraction isoeffective doses were 3.9 Gy for ACE activity, 7.2 Gy for PLA activity, 4.8 Gy for PGI2 production, and 4.7 Gy for TXA2 production. Thus these data demonstrate that over the present range of radiation doses approximately 4-7 Gy is repairable as subeffective endothelial damage during the 24-h interval between fractions. These values agree with previously published estimates of split-dose sparing in mouse lung based on lethality and breathing rate assays.     

Ventilatory long-term facilitation in unanesthetized rats

Wetested the hypothesis that unanesthetized rats exhibit ventilatorylong-term facilitation (LTF) after intermittent, but not continuous,hypoxia. Minute ventilation (E) and carbon dioxide production (CO₂) were measured inunanesthetized, unrestrained male Sprague-Dawley rats via barometricplethysmography before, during, and after exposure to continuous orintermittent hypoxia. Hypoxia was either isocapnic [inspiredO₂ fraction (FI_O2) = 0.08-0.09 and inspired CO₂ fraction(FI_CO2) = 0.04] or poikilocapnic(FI_O2 = 0.11 andFI_CO2 = 0.00). Sixty minutes afterintermittent hypoxia, E orE/CO₂ wassignificantly greater than baseline in both isocapnic and poikilocapnicconditions. In contrast, 60 min after continuous hypoxia,E andE/CO₂ were notsignificantly different from baseline values. These data demonstrateventilatory LTF after intermittent hypoxia in unanesthetized rats.Ventilatory LTF appeared similar in its magnitude (after accounting forCO₂ feedback), time course, and dependence on intermittenthypoxia to phrenic LTF previously observed in anesthetized,vagotomized, paralyzed rats.

     

Location and mechanisms of pulmonary vascular volume changes

We examined the influence of changing outflow pressure, P out, on the vascular and extravascular volumes (QV and QEV, respectively, as measured by indicator dilution) and on the outflow occlusion pressures in isolated dog lung lobes perfused with constant flow. Changing P out had a substantial effect on QV, but not on QEV, whether P out was less than or greater than alveolar pressure, PA. Since QEV did not change with QV, recruitment of previously unperfused vessels did not appear to contribute substantially to the increases in QV when P out was increased. The rapid jump in P out immediately following outflow occlusion was virtually independent of the difference between PA and P out suggesting that the alveolar vessels were an important volume storage site when P out was low relative to PA. We conclude that, over a certain range of pressures, alveolar vessel volume can be controlled by venous pressure even when the change in venous pressure has little effect on arterial pressure (zone 2). Further, we conclude that in zone 3 and within the transition from zone 2 to zone 3 increases in the intralobar blood volume occurring within the alveolar vessels may not require recruitment in the sense of opening of previously unperfused vessels.     

Ventilatory effect of acute pulmonary hypothermia

The isolated effect of cooling the pulmonary circulation on ventilation was quantified in nine anesthetized dogs. The right pulmonary artery (RPA) was cannulated within the pericardium, and systemic blood was pumped from the left atrium to the RPA between, but not during, periods of cooling. Cooled blood boluses were injected into the RPA under conditions in which either bolus temperature (5-35 degrees C) or volume (0-1.5 ml/kg body wt) varied. Inspiratory time (TI), expiratory time (TE), breath duration (TT), and peak integrated activity (PEAK) were determined from diaphragm EMG. Results for five postinjection breaths were converted to a percent of the values from five preinjection breaths. There was a linear relationship between bolus temperature and TI [r = 0.61, slope (x) = 0.59%/degrees C, P less than 0.001), TE (r = 0.73, x = 1.43%/degrees C, P less than 0.001] as well as TT (r = 0.74, x = 1.10%/degrees C, P less than 0.001), whereas PEAK was unaffected (n = 9). When injection temperature was 5 degrees C, an inverse linear relationship existed between bolus volume and TI (r = 0.75, x = -15.2%.ml-1.kg-1, P less than 0.001) and TE (r = 0.78, x = -23.4%.ml-1.kg-1, P less than 0.001) (n = 4). In two dogs tested the effect of bolus injection was minimal at residual volume and progressively increased with lung volume. The effect of cold bolus injection was eliminated after right vagotomy in three dogs. Results indicate that cooling of some vagal receptor in the lung increases breathing frequency primarily by shortening TE.     

Ventilatory CO2 response in endurance-trained rats

A CO2 rebreathing technique was used to assess possible changes in the ventilatory response to CO2 in rats following a 14-week swim training program. Over the final 9 weeks, the rats swam 1 hr per day with a weight of 2.5% of the body weight attached to the tail. Ventilation was measured by a barometric method in awake, restrained rats in a total body plethysmography at CO2 concentrations of 0, 2, 4, 6, and 8%, with an initial O2 concentration of approximately 100%. Ventilation increased in the trained rats with increasing CO2 from 775ml . min-1 . kg-1 at 0% CO2 to 1,387 ml . min-1 . kg-1 at 8% CO2. This increase was a consequence of a 34% increase in tidal volume and a 32% increase in breathing frequency. In comparison with a group of sedentary control rats, there was a significantly higher ventilation and tidal volume at 0% CO2; however, this difference disappeared with increasing levels of CO2. A significantly lower resting heart rate was observed in the exercised (296 +/- 44 beats . min-1, mean +/- SD) compared to the sedentary control rats 380 +/- 42). It was concluded that, while the normal training response of resting bradycardia was observed following this duration and intensity of training, endurance swimming had no significant effect on the ventilatory response to CO2 in the rat.     

Quantitative radionuclide assessment of total pulmonary vascular volume changes

Current techniques do not permit continuous and noninvasive assessments of changes in total pulmonary intravascular volume. Hence, the present study was undertaken to determine whether quantitative radionuclide imaging can be used to determine the direction and estimate the magnitude of total pulmonary vascular volume changes. The pulmonary circulation was separately perfused at a constant rate via the pulmonary artery and drained at a constant pressure via the left atrium in nine dogs. Changes in pulmonary intravascular volume were recorded as reciprocal changes in extracorporeal reservoir volume during phenylephrine or isoproterenol administration, a 20% increase in pulmonary artery flow or a 5 mmHg (1 mmHg = 133.32 Pa) decrease in left atrial pressure. Erythrocytes were labeled with technetium-99m and pulmonary volume changes were determined from tissue attenuation, blood radioactivity, and changes in total pulmonary radioactivity obtained with a gamma-camera. During each of the interventions, count changes correlated with volume changes (r greater than or equal to 0.75). The technique reliably detected volume changes as small as 10 mL. For all 531 individual pairs of radionuclide- and reservoir-determined volume changes, the correlation between reservoir-determined and radionuclide-estimated pulmonary intravascular volume changes was 0.87. The standard error of the radionuclide estimate was 21 mL. Hence, the present study demonstrates that quantitative radionuclide imaging can be used to continuously and noninvasively determine total pulmonary vascular volume changes.