Evidence for an Ultradian Secretion of Adrenocorticotropin, β-Endorphin and α-Melanocyte-Stimulating Hormone by the Ovine Anterior and Intermediate Pituitary

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Abstract
These studies were undertaken to characterize the secretion of the pro-opiomelanocortin (POMC)-peptides adrenocorticotropin (ACTH), immunoreactive (ir)-β-endorphin (β-EP), and ir-α-melanocyte-stimulating hormone (ir-α-MSH) in the conscious sheep. When blood samples were taken at 10-min intervals, a distinct POMC-peptide and cortisol ultradian rhythm was observed, and 4 of the 7 possible POMC-peptide pulse patterns were defined. Three of these patterns included pulsatile ACTH secretion, and 70% were followed by a rise in plasma cortisol. The fourth pulse pattern consisted of concordant ir-β-EPand ir-α-MSH secretion and was not temporally correlated with cortisol secretion. When an intensified rate of venous sampling was used (2-min intervals over 1 h), the POMC-peptide and cortisol pulse frequencies were increased. Although the mean POMC-peptide and cortisol pulse amplitudes remained unaltered, the mean cortisol pulse amplitude was decreased. Five of the 7 possible POMC-peptide pulse patterns were noted, and 3 of these were identical to those observed during the 10-min sampling procedure. Chronic bromocriptine treatment decreased the plasma levels and pulse amplitudes of ir-β-EP and ir-α-MSH, increased the ir-β-EP pulse interval, and abolished concordant ir-β-EP and ir-α-MSHsecretion. Neither the plasma levels nor the ultradian rhythms of ACTH and cortisol were affected by the treatment. Chronic dexamethasone administration abolished the ACTH ultradian rhythm, reduced the plasma levels of ACTH, ir-β-EP, and ir-α-MSH, while plasma cortisol levels became undetectable. Although the ir-α-MSH interpulse interval was increased, the ir-β-EP and ir-α-MSH ultradian rhythms were not abolished. We conclude that: (1) an ultradian rhythm characterizes the secretion of ACTH, ir-β-EP, and ir-α-MSH from the ovine pituitary; (2) this rhythm is due to the pulsatile secretion of peptides from the adenohypophysis and intermediate lobe; (3) the adenohypophysis and intermediate lobes appear to secrete POMC-peptides in synchrony and asynchrony, and (4) intensified rates of venous sampling unmask the appearance of high-frequency POMC-peptide and cortisol pulses in vivo in the sheep.