Structure of Rhythms of Blood Pressure, Heart Rate, Excretion of Electrolytes, and Secretion of Melatonin in Normotensive and Spontaneously Hypertensive Rats Maintained under Conditions of Prolonged Daylight Duration.

We studied the structure of rhythms of BP, HR (by telemetric monitoring), electrolyte excretion (by capillary electrophoresis), and products of epiphyseal melatonin (by the urinary concentration of 6-sulfatoxymelatonin measured by ELISA) in normotensive Wistar-Kyoto rats and spontaneously hypertensive SHR rats maintained at 16/8 h and 20/4 h light-dark regimes. In Wister-Kyoto rats exposed to prolonged daylight, we observed changes in the amplitude, rhythm power (% of rhythm), and range of oscillations of systolic BP; HR mezor decreased. In SHR rats, mezor of HR also decreased, but other parameters of rhythms remained unchanged. Changes in electrolyte excretion were opposite in normo- and hypertensive rats. Under conditions of 20/4 h light-dark regime, daytime melatonin production tended to increase in normotensive rats and significantly increased in SHR rats. At the same time, nighttime melatonin production did not change in both normotensive and hypertensive animals. As the secretion of melatonin has similar features in animals of both lines, we can say that the epiphyseal component of the "biological clock" is not the only component of the functional system that determines the response of the studied rhythms to an increase in the duration of light exposure.