A influência da hipóxia severa na sensibilidade barorreflexa de tilápia-do-Nilo (Oreochromis niloticus)

Abstract

The importance of the circulatory system in homeostasis of different animals is unquestionable. Given that one of the basic factors that determine blood flow is blood pressure, the short- and long-term regulation of this variable is essential for the various functions of the circulation to be performed adequately. The baroreceptor reflex is one of the most known mechanisms of rapid control of blood pressure, causing any change in this variable to result in an inversely proportional change in heart rate and in systemic vascular resistance, aiming at pressure normalization. However, another factor that can cause reflex changes in heart rate is the oxygen environmental concentration, with hypoxia being relatively common in several aquatic environments. The basic response to hypoxia in teleosts is the reduction of heart rate by chemoreflex; being attributed to this bradycardia possible physiological roles, such as improving oxygen uptake and protection of the myocardium. Thus, with the knowledge that both baro and chemoreflex are physiologically important for the animal and, in certain situations, may trigger antagonistic responses in heart rate, the present study aimed to investigate whether the condition of severe hypoxia has an influence on baroreflex sensitivity of Nile tilapia (Oreochromis niloticus). Therefore, administrations of phenylephrine and sodium nitroprusside were carried out in animals in the condition of normoxia and severe hypoxia; being heart rate and mean arterial pressure in the ventral aorta continuously recorded. Then, the baroreflex function of all animals was investigated by analysis of the sigmoid curve of the barostatic reflex and the baroreflex gain was calculated by equations proposed in literature. The present study was the first to analyze the barostatic reflex by means of the sigmoidal curve in fish and revealed that severe hypoxia increases the baroreflex sensitivity of Nile Tilapia (Oreochromis niloticus), which occurs due to an increase in the animal's regulatory capacity against hypotension.