HYPOPINEALISM AND ARTERIAL HYPERTENSION: CHRONOBIOLOGICAL AND AGE ASPECTS *

hypopinealism IInd Hypopinealism pressure reducing. It was revealed that the blood pressure indices in the rabbits with hypopinealism and in the old animals were similar during the day, but the quantities exceeded corresponding values in the mature animals. Moreover, the indices of blood pressure circadian rhythm in the rabbits with hypopinealism and in the old animals show leveling of the biphasic circadian pattern of the parameter identified in the mature rabbits. Thereby, hypopinealism (induced by age or long-lasting 24-hour lighting) causes the development of arterial hypertension of «non-dipper» type in the rabbits. Consequently, formation of hypertension against hypopinealism induced by the long-lasting low-intensity light exposure during the darkness is one of the animal senilism symptoms.

It is known that age hypopinealism, i.e. decrease in the pineal gland functioning characterised first of all by progressive melatonin deficiency, develops in mammals at the downward ontogeny stage [1,2]. Since physiological fluctuations of the pineal gland hormone synthesis and secretion level is one of the conditions for the coherence of the daily rhythms of the organs/systems work [2,3], it is considered that pineal gland hypofunction plays an important role in the pathogenesis of age-associated diseases, in particular arterial hypertension (AH) [2]. It is known that apparently healthy elderly people have higher pineal gland melaton-forming function than the ones with AH [4]. At the same time AH is not only an old people's disease, it is also disease of mature aged persons; thus the phrase «AH has looked younger» is established.
In our previous researches done on mature rabbits with hypopinealism induced by longlasting (for 4-5 months) 24-hour lighting the development of AH was established in case of single blood pressure (BP) measuring in the day-time. In the further chronobiological studies we have found that the long-lasting disruption of light mode leads to desynchronosis in animals and against this AH develops according to the type of «non-dipper» (i.e. leveling of BP circadian rhythm) [5]. This type of AH is specific to elderly humans with melatonin deficient typical for age hypopinealism [1].
Therefore, the purpose of this study was to take a comparative assessment of changes in circadian rhythms of blood pressure in rabbits of different ages (mature and old) with induced long-lasting 24-hour lighting or age hypopinealism.

MATERIALS AND METHODS
The research is done on 30 male rabbits aged 4-5 months at the beginning of the experiment. The animals form three groups: Group I -mature, Group II -old, Group III -mature with hypopinealism. The rabbits were kept in standard vivarium conditions except the lighting mode. Animals of groups I and II were kept upon natural day-nigh cycle for 10 months and 3 years and a half respectively. In rabbits of group III hypopinealism was modeled by keeping the animals in conditions of 24-hour lighting (during the daylight it was natural sunlight, in the darkness it was electric light making the illumination of 30-40 lux) for 10 months [6].
The study of the BP daily rhythm in rabbits is based on systolic blood pressure (SBP) measurement on the central ear artery according to R.T. Grant and P. Rothschild's method in our modification [7]  1) acrophase (maximal SBP); 2) batiphase (minimal SBP); 3) mesor (average daily blood pressure); 4) amplitude (maximal SBP displacement relatively to the mesor); 5) the degree of blood pressure reducing (DBPR). BP day -BP night DBPR = × 100 % BP day An incandescent lamp with the power of 100 W was used as an artificial light source. The illumination in animals' cages was determined with luxmeter «Yu-117». BP was evaluated in the natural light during the day-time and in the red light over the night-time due to the fact that red light has minimal effect on melatonin production in the pineal gland compared with the other colors of the visible-light spectrum [8].
All procedures with the animals were done according to the European Convention for Protection of Vertebrate Animals used for experimental and other scientific purposes.
Statistical analysis of the digital material was done with parametric and non-parametric methods. The verification of the data distribution according to the Gauss law was carried out using the Shapiro-Wilk test.
The comparison of groups with the data normal distribution was done using unpaired Student's t-test. The Mann-Whitney U-test was used to analyze the data which distribution did not correspond to the Gauss law. The one-way analysis of variance was used for multiple comparisons. The results are given in the form of (X ± S X ). The level of significance (р) was excepted as р ≤ 0.05 for determining the difference between groups that was statistically significant.

RESULTS AND THEIR DISCUSSION
The features of SBP change within 24 hours in the mature and old rabbits as well as in the mature ones with AH induced by hypopinealism, are given in Table 1.
As Table 1 shows, the mature rabbits have a clearly marked SBP circadian rhythm caused by the physiological BP decrease in the darkness versus the morning figure.
The old animals have the BP level increase at the different day periods, but the most expressed in the evening and at night (by 33.8 and 35.3 % compared to the corresponding quantities in the mature rabbits) that provides levelling of the BP rhythm and gives the reasons for classifying the AH type as «non-dipper».
As shown in Table 1, the rabbits of the third group have the same SBP changes as those ones obtained in the old animals.
In other words, development of AH type «non-dipper» was revealed in the old animals with age hypopinealism and in the mature rabbits with hypopinealism induced by 24-hour lighting.
The detailed characterization of the BP circadian variations based on the most important parameters used in chronobiology is presented in Table 2.

Експериментальні дослідження
As shown in Table 2, the SBP circadian rhythm in the mature rabbits has the biphasic due to achievement of BP acrophase in the afternoon and batiphase at night. The absolute indexes of the amplitude, DBPR and mesor as well also point at the sinusoidal nature of the SBP fluctuations in the mature rabbits within 24 hours.
The results of this study fragment coincide with the scientific reports about the occurrence of the SBP circadian rhythm in the mature rabbits as well as in healthy people [9,10]. It is well-known that DBPR (about 10-20 %) at night is a physiological feature of human body [10]. Thus, it is accepted to classify this kind of BP circadian rhythm as «dipper» type using the results of daily BP monitoring as a diagnostic criterion in a clinical practice [10]. Hence, the results of the study fragment point at the fact that SBP circadian variations in the mature rabbits are similar to the BP daily rhythm of «dipper» type in humans in the appropriate age period.
Reaching old age the age-related changes occurring in rabbits' body are also reflected in the change of a BP level within 24 hours. In those rabbits the SBP mezor increase has been found versus the value in the mature rabbits as well as the acrophase displacement from the day-time to the morning ( Table 2).
Moreover as a result of the SBP circadian rhythm change in the old animals the decrease of amplitude by 3.3 times was revolved compared to corresponding quantity in the mature rabbits. The letter induces the DBPR falling at night in those animals. The specified characteristics of a SBP circadian rhythm in the rabbits at the descending ontogenesis stage refer to SBP daily rhythm leveling revealed in the mature rabbits.  The obtained data correspond to the results of the clinical research study that show the development of AH «non-dipper» type described by the insufficient DBPR at night (< 10 %) or its deficiency in healthy people at ageing [10]. The results of clinical experimental study show existence of the general biological mechanism of AH pathogenesis based on the decrease of melaton-forming function of a pineal gland [1]. The obtained data about the acrophase displacement to the morning in the old animals are representative ones taking into account the fact that age is classified as an independent risk factor of BP rapid increase in the early morning time in human that, in turns, is associated with onset of cardiovascular catastrophes at the particular day time at the descending ontogenesis stage with untreated AH [11].

Table 1 Circadian Rhythm of Systolic Blood Pressure in Rabbits During Different Physiological Development Periods and in Rabbits with Hypertension Induced by Hypopinealism (mm Hg), (X ± S X ), n = 10
The changes of a SBP circadian rhythm in the old animals are similar to those ones that belong to humans in the senility and it is classified as SBP circadian rhythm «non-dipper» type.
While comparing the studied characteristics of a SBP circadian rhythm in the mature rabbits with AH induced by long-lasting hypopinealism and in the old animals the difference between them was not revealed (see table 2). But it is to be noted that the acrophase was at the evening and batiphase was at the morning in rabbits with AH against hypopinealism when corresponding characteristics were obtained in the afternoon and at night in the mature rabbits. At the same time the displacement only maximum of SBP to the morning time was revealed in the old animals and probably that is a risk of cardiovascular disease development in those animals. The feature of the acrophase displacement to the evening in the animals with hypopinealism probably can also be interpreted as manifestation of risk of cardiovascular complication development but upon a total suppression of the pineal gland melatonin-forming function and, consequently, «loss» of the day time concept for a body. It is well-known that an important property of the circadian system is the development in body of a «biological day» and a «biological night» cyclically alternating with their transitions characterized by relatively sharp hormonal, electrophysiological, behavioral changes [3]. The revealed acrophase displacement to the evening in the rabbits with AH against hypopinealism have an extra importance taking into account the results of our previous studies that gave the evidence of chronic heart failure development, endothelial disfunction and cardiosclerosis in those animals [12].
Thus the obtained results assume that AH development in rabbits against untreated hypopinealism induced by a long-term 24-hour lighting is connected with the leveling of SBP circadian rhythm. Hence a long-term 24-hour lighting should be considered as one of their senilism risks.
In conclusion, the pathophysiological mechanisms of the insufficient DBPR at night time are not completely established currently. But it is known that pathogenesis of AH associated with melatonin deficiency and the BP circadian rhythm «non-dipper» type is related with the disruption of nervous system functioning and an excessive catecholamines production as well as the development of insulin resistance, activation of the renin-angiotensin system [13]. Taking into account the results of chronobiological studies about the changes of adrenaline and noradrenaline concentrations as well as metabolic syndrome formation in mature rabbits with hypopinealism, it is possible to confirm that the AH development mechanism against this background is connected with a long-term and excessive activation of sympathicoadrenal system in the same manner as it occurs in ageing and stress (stress-age syndrome according to V. Frolkis) and insulin resistance development [14,15].

CONCLUSIONS
1. Hypopinealism regardless of the reasons its formation (age-dependent or induced by a long-lasting 24-hour lighting) causes the development of arterial hypertension of «non-dipper» type in rabbits.
2. The occurrence of arterial hypertension associated with hypopinealism induced with a long-lasting light exposure during the darkness in mature rabbits should be consi dered as one of their premature ageing symptoms.