Postabsorptive Undulations and Oscillations in Blood Glucose

Abstract

The postabsorptive blood glucose level is practically never static for longer than a few moments at a time. It is a constantly changing phenomenon with sharp diphasic oscillations, each to-and-fro oscillation taking approximately 30 seconds, more or less, for its completion. The only realistic picture of a continuously moving function, which is obviously so dynamically active as is glucose exchange across barriers, would seem to be one represented by the concept of the motion-picture, an assembled run in rapid succession of a large number of single “snap-shots” of blood-glucose status. Eighty-one per cent of 100 unselected humans in whom successive venous true-glucose determinations were made at two-minute intervals over a four- or a six-minute period, showed significant variation from reading to reading. The intraindividual mean significant variation among these successive readings was found to be 22.2 mg per 100 ml blood, which is over four times the actual error (5.3 mg per 100 ml) incidental to the methods employed. When these venous glucose readings are plotted at two-minute intervals, they assume the form of smooth aperiodic undulations with wave-lengths of from two to seven minutes. Repeated break-down of these undulations at random into their component fluctuations (readings at 15-second intervals) shows each undulation to be made up of a succession of these two-phased oscillations. It is proposed that the sharp venous descent in each completed oscillation represents the momentary transfer of glucose across cellular or molecular barriers. Each precipitous descent is immediately followed by an equally sharp rebound in venous glucose, a finding which can reasonably be interpreted only as a resurgence of glucose back into the venous channels from behind the barriers. The speed of the rebound (when compared with the over-all circulation time) would preclude there being any central origin for this precipitous rise in venous glucose. Even with careful correction for circulatory lag, the venous undulations and their component oscillations are identical at no two points throughout the entire venous system (except by accident). They are thought to vary in accordance with the tissue uptake and with the resurgence of glucose at each tissue point (with the peripheral function of insulin). Similar aperiodic arterial undulations with component oscillations were found to arise in the liver. By contrast with the venous fluctuations, however, arterial undulations are identifiable as being uniform throughout the entire arterial system by careful circulatory timing. During postabsorption (at rest), the mean glucose level of all arterial undulations and oscillations tends to equal rather precisely that of the fluctuations on the venous side. There is, however, a gradual descent of this mean level over the hours of postabsorption, the difference probably representing the net captivity of glucose by the tissues in phosphorylative esterification, the over-all deficit between the transfer across barriers and the venous resurgence of glucose. The reciprocal relationship found to exist between arterial and venous glucose levels, with the maintenance of almost identical mean values for each, further suggests that the oscillatory activity characteristic of the postabsorptive period is one means by which the body maintains blood glucose homeostatically constant between its production by the liver and its transfer across cellular or molecular spacial barriers. The phenomenon of insulin action in the periphery is far more rapid than has generally been appreciated. In vivo, the transfer of glucose across peripheral barriers takes place rather massively within a matter of seconds of time after exposure of the peripheral tissues to insulin. The total effect of insulin in this respect would seem to be registered as the summation of a continuous succession of brief spurts of the hormone rather than the single precipitous dumping out into the blood stream of a reservoir content of the hormone. The over-all glucose-lowering effect of the hormone is, accordingly, under normal conditions not linear between two widely separated points of time, but is reflected as the ultimate mean of a constantly fluctuating blood glucose level. The same aperiodic principle applies to A-V difference in respect to the tissue “utilization” of glucose.