Ultradian adrenocortical and circulatory oscillations in conscious dogs

We examined adrenal blood flow, cortisol secretion rate, concentration of cortisol in adrenal venous blood, mean arterial blood pressure, and heart rate in unrestrained conscious dogs, sampling at 15-20 s, 5 min, or 10 min during experiments lasting from 30 min to 8 h. Time history analysis designed for short, noisy time series detected three significant ultradian oscillatory periods: approximately 3, 6, and 90 min. Circulatory variables (systemic mean arterial pressure, heart rate, and adrenal blood flow) showed all three. Cortisol secretion rate showed the 3- and 90-min oscillations but not the 6-min oscillation. Adrenal glucocorticoid secretion rate and adrenal blood flow were not strongly coupled. However, at one extreme of blood flow (close to zero) and at the opposite extreme (very high blood flow stimulated by adrenocorticotropic hormone) adrenal blood flow and cortisol secretion were tightly coupled. In the normal physiological range, the multiperiodic, rhythmic organization of circulatory variables and adrenal glucocorticoid function arises from independent or only weakly coupled oscillators, not necessarily harmonically related, manifesting near-periodicity with wobble and intermittency.