Circadian Variation of the Serum Glucose and Immunoreactive Insulin Levels

Abstract

The serum glucose and immunoreactive insulin levels were determined in mice during a 24-hr period. Both parameters showed a clear circadian rhythm, having their highest and lowest values located in the (light) and (darkness) respectively. However, they were not hourly coincident, since the glucose peak preceded the insulin peak by 4 hr. This fact is explained on the basis of hormone interaction during the day. The present results show the importance of chronobiology in endocrine studies. {Endocrinology 88:1529,1971) A LTHOUGH glucose has been pointed out as J\. the most physiological stimulus for insulin release, several other substances have been described as having such effect upon this process. Agents capable of inducing the secretion of insulin are certain aminoacids (1), short chain fatty acids (2), tolbutamide (3), glucagon (4), cyclic AMP (5). On the other hand, several hormones are able to modify the pancreatic beta cell behavior; e.g., catecholamines block the glucose-induced insulin secretion (6), whereas this secretion is enhanced in the presence of hormones like growth hormone (7) and corticoids (8). Hence, both the serum insulin levels and the pancreatic response in the whole animal must be considered a multiple-dependent process rather than a simple stimulus-secretory response interaction. On the basis of this assumption, the beta cell effect of any substance administered to an experimental animal would be conditioned by the action of the animal's intrinsic regulators at that moment. Since our main field of interest is related to the regulatory mechanisms of insulin secretion, we assumed that it should be important to study the spontaneous fluctuations of both the serum glucose and immunoreactive insulin (IRI) levels during the 24-hr period. Materials and Methods Female mice of the C3H-S strain, 10 weeks old, from the Instituto de Embriologia, Biologia e Histologia, Facultad de Ciencias Medicas, UniReceived December 4, 1970. This paper was partially supported with funds from the Consejo Nacional de Investigaciones Cientificas y Tecnicas de la Republica Argentina (Grant 3477a/69), Comisidn de Investigaciones Cientificas de la Universidad Nacional de La Plata and Comisidn de Investigaciones Cientificas de la Provincia de Buenos Aires. versidad Nacional de La Plata, were used. They were caged in a room at a temperature of 25 ± 1 C with water and food ad lib. and illumination (fluorescent light 40W) from 0600 (6 AM) to 1800 (6 PM) alternating with 12 hr darkness. Since the animals engage in exercise and food intake only during the darkness this is called the activity period, the other one being the resting period (9). This nomenclature provides an appropriate way to compare animal and human experiments. Lots of 40 animals each were bled through a retro-orbital plexus with a heparinized capillary tube at 0000, 0400, 0800, 1200, 1600 and 2000 hr on different days. The blood collected at every time point was divided into several pools in order to obtain a suitable volume for the glucose and IRI determinations. The serum obtained after centrifugation was kept frozen at — 20 C until the determinations were performed. Glucose concentration and IRI levels were determined by the Otoluidine (10) and the charcoal-dextran immunoassay method (11), respectively.