Rapidly alternating photoperiods disrupt central and peripheral rhythmicity and decrease plasma glucose, but do not affect glucose tolerance or insulin secretion in sheep. Issue 9 (25th July 2014)

• Record Type: Journal Article
• Authors: Varcoe, Tamara J.; Gatford, Kathryn L.; Voultsios, Athena; Salkeld, Mark D.; Boden, Michael J.; Rattanatray, Leewen; Kennaway, David J.
• Is Part Of: Experimental physiology. Volume 99:Issue 9(2014:Sep.)
• Page Start: 1214
• Page End: 1228
• Publication Date: 2014-07-25
• Subjects: Physiology, Experimental -- Periodicals; 571.0724
• DOI: 10.1113/expphysiol.2014.080630
• Languages: English
• ISSNs: 0958-0670

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <sec id="eph1476-sec-0010" sec-type="section"> <title>New Findings</title> <p> <list id="eph1476-list-0001" list-type="bullet"> <list-item> <p> <bold>What is the central question of this study?</bold> </p> <p>Disrupting circadian rhythms in rodents perturbs glucose metabolism and increases adiposity. In this study, we asked whether circadian rhythm disruption, induced by exposure of sheep to rapidly alternating photoperiods (RAPs), also disrupts metabolic homeostasis in a large diurnal animal model.</p> </list-item> <list-item> <p> <bold>What is the main finding and its importance?</bold> </p> <p>Exposure to RAPs disrupted central (melatonin and core body temperature) and peripheral rhythmicity (skeletal muscle clock gene expression). This led to reduced nocturnal plasma glucose concentrations, but did not affect glucose tolerance and glucose‐stimulated insulin secretion. These results suggest that RAP‐induced circadian rhythm disruption has minimal effect on glucose homeostasis in the sheep.</p> </list-item> </list> </p> </sec> <sec id="eph1476-sec-0020" sec-type="section"> <p>Disrupting circadian rhythms in rodents perturbs glucose metabolism and increases adiposity. To determine whether these effects occur in a large diurnal animal, we assessed the impact of circadian rhythm disruption upon metabolic function in sheep. Adult ewes (<italic>n</italic> = 7) underwent 3 weeks of a control 12 h light–12 h dark photoperiod, followed by 4 weeks of rapidly alternating photoperiods (RAPs) whereby the time of light exposure was reversed twice each week. Measures of central (melatonin secretion and core body temperature) and peripheral rhythmicity (clock and metabolic gene expression in skeletal muscle) were obtained over 24 h in both conditions. Metabolic homeostasis was assessed by glucose tolerance tests and 24 h glucose and insulin profiles. Melatonin and core body temperature rhythms resynchronized within 2 days of the last photoperiod shift. High‐amplitude <italic>Bmal1</italic>, <italic>Clock</italic>, <italic>Nr1d1</italic>, <italic>Cry2</italic> and <italic>Per3</italic> mRNA rhythms were apparent in skeletal muscle, which were phase advanced by up to 3.5 h at 2 days after the last phase shift, whereas <italic>Per1</italic> expression was downregulated at this time. <italic>Pparα, Pgc1α</italic> and <italic>Nampt</italic> mRNA were constitutively expressed in both conditions. Nocturnal glucose concentrations were reduced following chronic phase shifts (zeitgeber time 0, −5.5%; zeitgeber time 12, −2.9%; and zeitgeber time 16, −5.7%), whereas plasma insulin, glucose tolerance and glucose‐stimulated insulin secretion were not altered. These results demonstrate that clock gene expression within ovine skeletal muscle oscillates over 24 h and responds to changing photoperiods. However, metabolic genes which link circadian and metabolic clocks in rodents were arrhythmic in sheep. Differences may be due to the ruminant <italic>versus</italic> monogastric digestive organization in each species. Together, these results demonstrate that despite disruptions to central and peripheral rhythmicity following exposure to rapidly alternating photoperiods, there was minimal impact on glucose homeostasis in the sheep.</p> </sec> </abstract>