The main problem in applied lung resection patients, who indeed have a limited capacity of respiration, is the respiration functioning efficiency problems that can be encountered after the operation. This situation is not so common because preoperatively the patients are subjected to respiratory function tests by spirometry, their arterial blood gas is examined and if needed, their quantitative ventilation-perfusion scintigraphies are taken. Surgical process is performed according to these results if patients are suitable. In addition, the remaining lung tissue is also not healthy in these patients and most of which cilliary functions are out of order, compliance is decreased, and have damage due to chronic infection . In conclusion, residual lung may be in difficulty in meeting the sufficient respiration during an effort postoperatively. Such situation decreases the functional capacity and lengthens the time to return to daily activities .
Such problems developing postoperatively are usually seen in pneumonectomy patients. But, also lobectomy sometimes leads to reduction of lung function whether patients complain of dyspnea on effort or not. In addition, compensatory lung growth is not a response only peculiar to pneumonectomy patients. Regenaration of the lungs also occur in patients who had undergone whether anatomical surgical procedure such as lobectomy and segmentectomy or nonanatomical surgical procedure like wedge resection. Rannels et al. reported that this regeneration response occurs via adrenal steroids and growth hormone . Postlobectomy lung growth is a more rapid and restorative process. On the contrary, the space occurring in hemithorax due to compensatory lung growth fills in time and no air space remains in most of the patients . In addition, Kaza et al. have reported that, compensatory lung growth after lobectomy occurs with cellular hyperplasia in early period and with cellular hypertrophy in late period [12, 13]. In this study, compensatory lung growth after pneumonectomy is just a model to evaluate the possible role of retinoic acid in regeneration of the lung.
Postpneumonectomic compensation develops in two phases. In the initial phase of active cellular growth, septal thickening, lengthening in bronchioalveolar tract and limited functional adaptation occur together. In the late phase, septal remodeling restores the normal alveolar structure, and this phase occurs together with great functional increase. When the compensatory lung growth is complete; alveolar and capillary cells and the surface area of blood gas barrier protect their position mostly [14, 15].
Gaining a dimensional increase in residual lung tissue contributes to the improvement of respiratory functions. A number of pharmacologic agents have been used for this purpose. One of these is RA [16, 17]. Cell culture proliferation of Type 2 pneumonocytes can be induced by means of RA treatment. Besides, RA increases the production of surfactant and alveolization. It also increases the collagen on the walls of respiratory tract. In addition, it plays an important role in the alveolar epithelial cell proliferation. It increases the mechanical tightness in lung tissue and induces the progressive compensatory growth of new alveolar tissue. RA increases the endothelial cells and capillary growth selectively. Yet, it does not affect the growth of the other alveolar cell types .
RA has been used in lots of studies because of these known effects. One of these studies is randomized, double-blind, feasibility study carried out by Roth et al. In this study, 148 subjects with chronic obstructive pulmonary disease and whose primary component was emphysema were divided into three groups and received low, high doses of RA and placebo, respectively. This treatment lasted for six months. In this study, retinoid-related side effects were found to be common but mild. In conclusion, with the administration of RA, there were no definitive clinical benefits in the patients. However, time and dose-dependent changes in diffusing capacity of the lung for carbon monoxide and health related quality of life were observed in RA group. The authors stated that, these results supported the possibility of biological activity by means of the administration of RA and for this reason there is a need for more investigation .
In this study, the compensatory growth occurring in the contralateral lung by administering RA to the rats after left pneumonectomy was controlled. It was administered to the Group C as 0.5 microgram/g for 10 days. Lung volume indices and lung weight indices were used for the evaluation of the results. These are the values that can be calculated quantitatively and that can give information about lung growth.
There was some weight increase in the group to which RA was administered. This may be explained by the knowledge that, Vitamin A generally plays a part in the cellular growth of all tissues. However, when the weight gain is compared with the other groups, it is seen that it is not significant statistically (p > 0.05).
When the rats were sacrificed, right lung tissues were examined macroscopically both when those were in thorax and those were removed. It was observed that right lung tissue showed herniation to the left hemithorax much more in Group C, and partially in Group B and C anteriolaterally, and more from posterior prevertebral space. Those posterior herniations seen more commonly after left pneumonectomy are more in Group C shows a proportionally increased herniation with right lung growth.
Right lung tissues in each of the three groups were removed by turning from the right main bronchus and separating the surrounding fibrotic adhesions. A very careful dissection should be performed in all these steps as paranchyme destruction may affect the lung weight, cause air leakage and thus lead to wrong volume measurements.
Total volume of the lungs of rats is generally between 5.0–5.5 ml although it shows changes from the point of age and sex. Right lung volumes in Group B showed increase compared to Group A, and in Group C when compared to Groups A and B. Average right lung volume to which RA administered reached above 4 ml. This situation verifies the alveolar expansion and the size increase due to this. The reason for choosing the 10th day as the measurement time is that the upregulation of molecular mediators in growth period occurs in this duration. It was reported previously that the right lung growth reached plateau between 10th and 21th days . Therefore, a 50% or more volume increase was determined with a ten-day-treatment protocol. This is a little more above than expected. However, how long the RA treatment would continue and when the maximum response would be were not known thoroughly, a ten-day-treatment program was applied. Some more studies should be carried out on RA in respect to duration and response axis depending on this time.
Lung volume index is a useful parameter that can be calculated quantitatively. Since there is the body weight in denominator part, it is affected negatively by the weight increase occurred in rats. Yet, the weight increase and the volume growth will lead to some increase in lung tissue of the rats. Thus, the changes in these two ratios will balance each other, and a reliable value will be obtained as a result. In our study, the lung volume indices in Group B increased compared to Group A, and in Group C when compared to Groups A and B. The mean increase in index of Group C is about 45–50%. This shows that, when the most weight increase in Group C is taken into account, the lung volume increase in the group administered RA is much more than the others. The amount in the volume increase also affects the index. As the lung volume index determines the volume amount of lungs according to the body weight, it can give information only about the lung volume amount, and it is not a functional indicator.
Mean rat dry lung weight is between 5–9 g though it shows differences from the point of weight, age and sex. When the right lung weights are evaluated, an increase about 20% in Group B, and about 40% in Group C were observed. From this point of view it is understood that the compensatory lung growth caused some increase in the lung weight. The administration of RA doubled this increase. Of course, as the weight increase in Group C is more than the other groups, the lung weight to occur would be affected from this situation. In other words, the final lung weight increase should be accepted a little lower than the value for Group C. However, this situation should not overshadow the existing significant difference.
In the evaluation of the lung tissue growth, the lung weight index value can be calculated quantitatively and similarly to the volume index. It can be obtained dividing the provided lung weight by total body weight. As in the volume index, since the body weight is in the denominator, it is negatively affected by the weight increase occurred in rats. However, since the increase occurred in lung weight shows a parallelism to total weight gain, lung weight index may remain fixed without being affected from these. The lung weight indices calculated at the end of the experiment increased in Group B compared to Group A, and in Group C compared to other groups. The increase in this index in Group C is about 40–45% as an average. That is, lung weight increase is more in the rats of administered RA. Since the lung weight index determines the weight amount of lungs according to the body weight, it can give information only about lung size, and it is not a functional indicator. On the other hand, the information reached shows that RA leads to a real growth in the lung tissue, and that the alveolar surface on the expands as its tissue weight has increased, and thus these result in an increase in the total effective lung volume. This shows that the lung growth by administration of RA after pneumonectomy is increased significantly as a result of the evidences of the increase in these two indices.
In the investigation of the experiments carried out on the postpneumonectomic lung growth, the studies of Kaza et al. attract attention . Kaza et al. indicated the positive effects of RA and claimed that it provides a significant functional contribution. The same authors used epidermal growth factor and keratinocyte growth factor for this purpose, and again reached successful results [12, 20].
Epidermal growth factor normally plays an important role in prenatal and postnatal lung development. In postresectional lung growth, on the other hand, this can be explained by the increase in the amount of growth factor receptors of residual lung tissue. On the other side, keratinocyte growth factor plays a role in pneumocyte proliferation and lung development. Postresectional lung tissue also induces new alveolar formation.
Sakamaki et al. used hepatocyte growth factor for the same purpose . Hepatocyte growth factor has mitogenic and morphogenic actions on lung epithelial cells. As a result of the study, it was found that hepatocyte growth factor increased the deoxyribonucleic acid synthesis in lung epithelial cells in rats undergone pneumonectomy and that it was a pulmotrophic factor.
Desai et al. investigated RA from the point of its effect on lung morphogenesis in mouse foregut cultures and reported that RA affected the mesodermal proliferation and induced fibroblast growth factor expression . Nevertheless, Sakurai et al. observed in their experimental study that compensatory lung growth was angiogenesis- dependent and was accelerated by vascular endothelial growth factor but not affected by fibroblast growth factor .
Finally, all these growth factors can be effective on compensatory lung growth. They achieve these effects usually by providing the upregulation of their own receptors in lung tissue. Among these, the lung growth most resembling to the result obtained by RA was achieved by using keratinocyte growth factor and epidermal growth factor. RA was preferred in this study due to its easy availability, cheapness, easy usage and low side effect incidence.
In our study, histopathological results were evaluated in addition to lung weight index and lung volume index. There was no pathological finding in Group A (Fig 4). An increase in the dimensions of alveoli in the right lung tissue of Group B was observed (Fig 5). We also observed a significant increase in the dimension of the alveoli in Group C. Besides, a thickening in the walls of alveoli was determined (Fig 6).
In the histopathological examination, the mean alveoli number in per 5 high-power fields was found to be the most in Group A, and this value got less in Group B and Group C. If the mean alveolar dimension values are to be compared among groups, there was nearly a 50% increase in Group B compared with Group A, and this increase outran the value of Group A twice as much as Group C.
When the mean alveoli thickening values were compared among groups, there was a 30% increase in Group B compared with Group A, and a 70% increase in Group C compared with Group A.
In conclusion, we think that the fall in the mean alveoli number in Group B and C results from the alveolar hypertrophy developing in these groups. The increases in the dimension and wall thickness of the alveoli in Group B and C lead to decrease in the number of alveoli for per unit field.
The mean alveolus dimension and the mean alveolar wall thickness were found increased in Group B compared with sham group and this supports the postpneumonectomic compensatory lung growth expected to occur to some degree normally. These increases were observed to be more in Group C. This aspect partially supports the positive effects of RA on compensatory lung growth. However, the mean alveolar wall thickness of Group B and C was compared statistically, and there was not a statistically significant difference between Group B and Group C (p > 0.05). This situation may support that, the increase of the mean alveolar wall thickness on pneumonectomy normally occurs, and RA has not much affected this value.
The initial stimulus of the compensatory lung growth to occur during postpneumonectomic period is the mechanical tightening in the residual lung tissue. This tightening will increase the wall tightening of the cells in the existing tissue and this will possibly lead to expansion in volume. The compliance in the expanded lung tissues will increase and try to fill the area by moving towards the pneumonectomic space. These areas physiological changes are observed as routine. Yet, the compensation occurred differs from an individual to another, and its amount and effectiveness are different. In order to solve this problem, RA can be used in the light of the information we obtained. RA administration in the early postoperative period increases the alveolar capillary amount and the size of endothelial cells and causes the formation of new capillaries. As a result, the celluler growth increases and lung functions improve . Septum thickens is an important ratio. The increase occurred in the amount of capillaries can be explained by the increase in the vascular endothelial growth factor. Administration of RA increases both the upregulation of its own receptors and the amount of growth receptors in the residual lung tissue. Epidermal growth factor may play a role in this process. In this way, there occurs a restoration in the levels of all protein, Deoxyribonucleic acid, collagen and elastin in the residual lung due to this complex receptor-mediator relation that took place [15, 25].
It is controversial how long the RA treatment will last. In some studies, it is indicated that there is a ten-day-treatment period, and if more, that there occurs a stagnation period in lung growth . An evident growth takes place at the end of the first week. The remodeling of the lung takes place in 21 days in normal people. As a result, it is thought that during postpneumonectomic early period, a few weeks treatment is enough. The usage of RA for more than one month is controversial. The dosage to be used is also an important subject. A suitable dosage not having side effects and showing physiological effects should be determined. Finally, RA is deposited in fat, and high dosage for a long time may lead to toxicity.