SUMMER ACCLIMATIZATION IN THE SHORT-TAILED FIELD VOLE, MICROTUS-AGRESTIS

We investigated the changes that occurred in basal and noradrenaline-i nduced metabolic rate, body temperature and body mass in short-tailed field voles, Microtus agrestis, during exposure to naturally increasin g photoperiod and ambient temperature. These parameters were first mea sured in winter-acclimatized voles (n = 8) and then in the same voles which had been allowed to seasonally acclimatize to photoperiod and am bient temperature (6 months later), Noradrenaline induced metabolic ra te, basal metabolic rate and nonshivering thermogenesis were significa ntly higher in winter-acclimatized compared to summer-acclimatized vol es. There was a significant positive relationship between basal metabo lic rate and noradrenaline-induced metabolic rate. Body mass was signi ficantly higher in summer-acclimatized compared to winter-acclimatized voles. There was a significant positive relationship between body mas s and noradrenaline-induced metabolic rate in both winter-acclimatized and summer-acclimatized voles; however, there was no relationship bet ween basal metabolic rate and body mass in either seasonal group of vo les. Body temperature after measurements of basal metabolic rate was n ot significantly different in the seasonal cohorts of voles. However, body temperature was significantly higher in winter-acclimatized compa red to summer-acclimatized voles after injection of noradrenaline. Pre viously we have found that a long photoperiod was not a sufficient sti mulus to reduce thermogenic capacity in winter-acclimatized voles duri ng cold exposure, since basal metabolic rate increased to compensate f or a reduction in regulatory nonshivering thermogenesis. Here we found that a combination of increased ambient temperature and photoperiod d id significantly reduce thermogenic capacity in winter-acclimatized vo les. This provided evidence that the two aspects of non-shivering ther mogenesis, obligatory and regulatory, are stimulated by different exog enous cues. Summer acclimatization in the short-tailed field vole is m anifest as a significant decrease in both basal and noradrenaline-indu ced metabolic rate, combined with a significant increase in body mass.