Evidence that the Physiological Pulse Frequency of Glucagon Secretion Optimizes Glucose Production by Perifused Rat Hepatocytes*

We have reported that glucagon administered to perifused rat hepatocytes as a series of pulses at 15-min intervals is a more effective stimulus for hepatocyte glucose production (HGP) than is continuous glucagon infusion. To test whether the efficiency of HGP depends upon the frequency of pulsatile glucagon delivery, we administered glucagon to perifused rat hepatocytes as a series of pulses of fixed amplitude 922 +/- 30 (+/- SE) pg/ml at eight separate pulse intervals ranging from 3-45 min. Compared to continuous infusion of the same total amount of hormone, pulsatile glucagon administration clearly enhanced HGP in a frequency-dependent fashion. At pulse intervals between 10 and 20 min, pulsatile HGP exceeded continuous HGP by a factor of 1.5-2. This range of optimal intervals compared favorably with the glucagon secretory period of 10 min observed in nonhuman primates and that of 13-20 min observed in humans. We noted a desensitization of the hepatocyte response to glucagon that was directly proportional to the log of the time-averaged hormone concentration. Since the magnitude of the desensitization elicited by pulsatile glucagon delivery exceeded the desensitization elicited by continuous hormone delivery regardless of pulse frequency, differential desensitization could not explain the frequency dependency of the pulse enhancement effect. A mathematical simulation of our data demonstrated that the asymmetry of the HGP waveform elicited by a brief glucagon pulse could account for the observed frequency dependency of HGP. Pulse to pulse summation and the desensitization phenomenon modulated both the magnitude of the pulse enhancement effect and the frequency range over which the effect was manifest. We conclude that the enhancement of HGP by glucagon pulses is a frequency-dependent phenomenon and that the physiological glucagon secretory period optimizes HGP.