Inferior petrosal sinus AVP in patients with Cushing's syndrome

OBJECTIVE In both normal volunteers and patients with Cushing's disease, one dominant inferior petrosal sinus (IPS) contains higher concentrations of AVP and ACTH than the contralateral (non‐dominant) IPS, but ovine corticotrophin‐releasing hormone (oCRH)‐stimulated AVP in the petrosal sinuses is greater in Cushing's disease than in normal volunteers. To distinguish whether greater oCRH‐releasable AVP might be specifically related to the presence of a pituitary corticotrophinoma, or be due to hypercortisolism per se , we compared IPS AVP in patients with Cushing's disease with those of patients with other causes of Cushing's syndrome. PATIENTS Twenty‐three patients with Cushing's disease, 16 patients with the syndrome of ectopic ACTH and seven patients with Cushing's syndrome of adrenal origin. MEASUREMENTS AVP and ACTH, measured both before and 3, 5 and 10 minutes after oCRH in the petrosal sinuses, and in a peripheral vein. RESULTS In all three groups, AVP concentrations were lateralized such that most of the AVP was found in one, dominant IPS. oCRH significantly increased IPS ACTH only in patients with Cushing's disease ( P < 0.001), whereas it significantly increased dominant IPS AVP levels in all three patient groups ( P < 0.01). However, neither dominant nor non‐dominant IPS AVP (basal or oCRH‐stimulated) were significantly different among patients with Cushing's disease, ectopic ACTH or Cushing's syndrome of adrenal origin. Basal and oCRH‐stimulated IPS AVP were negatively correlated with urine free cortisol. CONCLUSIONS Inferior petrosal sinus AVP levels are similar in all forms of Cushing's syndrome, and thus the higher inferior petrosal sinus AVP levels in patients with Cushing's disease compared with normal volunteers are unlikely to be related specifically to the presence of the pituitary corticotrophinoma. While AVP may play a role in pituitary corticotroph tumourigenesis or may be secreted by some pituitary corticotroph tumours, the observation that CRH‐stimulated inferior petrosal sinus AVP levels are higher in Cushing's disease than in normal volunteers appears most likely to be related to the low endogenous CRH levels induced by hypercortisolism, rather than a consequence of Cushing's disease itself. We hypothesize that low endogenous CRH leads to increased sensitivity of central nervous system CRH receptors to exogenous CRH, and thus to greater ovine CRH‐stimulated AVP.