Temporal structure of endocrine secretion: circannual changes in insulin responsiveness to tolbutamide in man

In our previous observations we could detect the existence of circadian rhythms of either tolbutamide-induced insulin release with the acrophase early in the morning (at 04.20) or an insulin-induced hypoglycemia with the acrophase at 10.30 a.m. Aim of our work was to extend the observation to circannual rhythms of the same parameters. Four healthy young volunteers were submitted to the study in the early months of the year and observed in a 24 hrs-period, every other month, in the course of one year. 250 mg tolbutamide were injected i.v. in each day of the experiment every four hours with starting points at a random sequence. The data were analyzed by microscopic analysis: Halberg's Cosinor method. The highest insulin release occurred in winter (Acrophase: Jan. 19 with 95% confidence limits: Oct. 31 - April 1).