Diurnal episodic pattern of insulin secretion in the dog

Sampling portal blood every 15 min by means of indwelling cannulae, we have found evidence that basal insulin secretion in nonanesthetized dogs takes place in six or seven major secretory episodes over a 24-h period. Reproducible patterns were obtained in four experiments conducted on three normal animals. When frequency of peaks was plotted against the log of insulin concentration, a normal distribution was obtained (lognormal distribution). The means and variances were nearly the same in the three animals. Low levels of insulin (10--25 microU/ml) alternated with peaks which were occasionally higher than 200 microU/ml. The average duration of a peak was 30 min and seemed to be independent of ambient glucose levels. It is suggested that these peaks are the result of surges in plasma growth hormone concentrations and/or spontaneous repetitive discharges along neuronal pathways functionally related to insulin secretion.