## 4 Discussion
Despite the alterations discovered in four out of twelve respiratory parameters, it has to be highlighted that all disorders observed after 14 days of inhalation subsided with continued inhalation--there were no side effects in the lung function of animals exposed to the tested metabolites for 28 days. Unfortunately, we were not able to compare our results or, more importantly, their interpretation with other scientific data because of the lack of any such study. As mentioned before, there are only three studies that investigated the influence of inhalation of vitamin D3 metabolites on the respiratory system (improvement in development of neonatal rat lungs [19], fighting with _Mycobacterium tuberculosis_[20], and attenuation of LPS-induced acute lung inflammation [21]). Despite obvious differences in our study and that by [PERSON] [19], such as species of animals (mice vs. rats), age of animals (adults vs. neonates), purpose of the study (restoration of physiological calcitriol levels in the lung tissue of mice with vitamin D3 deficiency vs. stimulation of lung maturation in neonatal rats with unknown vitamin D3 status), and measured parameters (concentration of calcitriol in the lungs and serum and respiratory parameters vs. concentration of calcidiol in serum and markers of lung maturation), it is worth mentioning this research because of a similar therapeutic strategy, i.e., direct administration into the respiratory tract of the biologically active form of vitamin D3 and its precursor in similar concentration ranges (calcidiol doses: 100, 500, and 1000 ng/kg; calcitriol doses: 1, 10, and 50 ng/kg), as well as the approximate duration of the experiment (14 days). The cited study proved that chronic exposure of neonatal rats to calcidiol and calcitriol in the whole range of tested concentrations did not affect the serum levels of both calcium and 25(OH)-vitamin D3 and did not impact on the expression of VDR and PPARy (peroxisome proliferatorator-activated receptors) in kidneys, which, according to the authors, proved the low possibility of systemic side effects of the investigated therapeutic strategy. Furthermore, [PERSON] et al. discovered that neonatal rats' chronic inhalations with vitamin D3 metabolites enhanced lung maturation, which was associated with increased surfactant production, improvement in alveogenesis, as well as stimulation of differentiation of alveolar type II cells, lipofibroblasts and, endothelial cells [19]. Consequently, the mentioned research also supports our observations that the presented therapeutic strategy is effective and safe. It has to be also emphasized that, in the majority of experiments conducted by [PERSON]'s team, vitamin D3 metabolites in the lowest investigated concentrations (calcidiol at 100 ng/kg and calcitriol at 1 ng/kg) revealed the greatest impact on lung maturation [19]. Our research goals were achieved when we used calcidiol in the same concentration, while calcitriol at a five-times higher dose than [PERSON]'s team [19], but the range of potential therapeutical concentration was still quite similar. Furthermore, our study, as well as the cited research [19], clearly demonstrated that the bioactive form of vitamin D3 could be used in significantly lower doses (1/20 or even 1/100, respectively) than its precursor to induce a similar beneficial effect in pulmonary compartments. Of course, this conclusion is not surprising considering the metabolic changes necessary for calcidiol transformation into an active hormone; however, in our opinion, it is a valuable tip regarding the dosage of vitamin D3 metabolites for inhalations in future studies. As indicated above, the second study examining the influence of vitamin D3 inhalation on respiratory tracks was conducted by [PERSON]'s team, who focused on the possibility of using calcitriol in the treatment of tuberculosis [20]. The cited studies contain even more differences from our research than the previously discussed study by [PERSON]'s team [19].