There are previous data on synergistic action of both CRH and AVP on ACTH release. Inder et al. [
23] demonstrated a respectively highly significant rise in cortisol, ACTH, AVP and CRH following hypoglycaemia. They interestingly demonstrated as well increases in AVP following administration of ovine CRH in some subjects. In vitro studies confirm a close interaction between CRH and AVP. This is exemplified by treatment of pituitary cells for 1 h with CRH, which increases the percentage of corticotrophs that bind AVP [
24]. The reverse phenomenon also occurs; treatment of pituitary cells for 1 h with AVP increased CRH binding per corticotroph and the percentage of cells that bound CRH [
25]. More recent studies [
26] demonstrated that vasopressin V1b receptor and CRH receptor 1 are capable of forming constitutive homo- and heterodimers, and that this interaction does not affect the binding properties of the receptors. Clinical studies suggest that AVP rather enhances the effects of CRH, while isolated stimulatory effects of AVP on ACTH release are much more modest, i.e., relatively high intravenous AVP doses are necessary to stimulate ACTH and cortisol secretion in healthy volunteers [
27]. On the other hand, ACTH responses to ITT are higher than maximal ACTH responses to CRH [
28]. Comparable to intramuscular glucagon injection, ACTH responses were higher, than after isolated administration of either human CRH or AVP [
29]. Differential effects of CRH and AVP on corticotrophs, were confirmed by recent electrophysiological studies, that demonstrated that corticotroph cells of the anterior pituitary are electrically excitable. In corticotrophs this bursting is primarily controlled by activation of the CRH-signalling pathways, whereas AVP promotes an increase in action potential frequency [
30,
31].
Yet, while “classical” dynamic studies of pituitary function (i.e., ITT or GST) result in simultaneous release of both CRH and AVP, in our study we investigated isolated effects of CRH on AVP/copeptin secretion. Our study demonstrates for the first time that CRH stimulates copeptin release; thus, this phenomenon is likely to be, at least partially, responsible for an increase in copeptin concentrations observed during ITT or GST that was noted before [
17,
20]. Simultaneously we also observed a significant, though rather moderate (
r = 0.406,
p < 0.001), correlation between copeptin and plasma ACTH concentrations. The observed increase in serum copeptin in the control group was simultaneous with ACTH, similar to observations of Demiralay et al. [
32], who also observed simultaneous release of copeptin and ACTH during stress, i.e., during CCK-4-induced panic symptoms. Yet, CRH-dependent stimulation of ACTH is largely independent of AVP/copeptin, as in a group of subjects with a history of pituitary disease, but normal ACTH–cortisol responses to CRH, we demonstrated no significant increase in copeptin despite highly significant ACTH–cortisol response of the same magnitude as in healthy controls (i.e., approximately a fourfold increase in a mean ACTH concentrations). On the other hand in a group of subjects, that failed to obtain satisfactory ACTH/cortisol release during CRH, we had a significant number of subjects (five out of nine) with panhypopituitarism and diabetes insipidus (DI). In our opinion, the presence of diabetes insipidus, that is associated with low AVP/copeptin secretion, was the main factor responsible for lower copeptin concentrations in that group. We therefore confirm that subjects with a history of pituitary disease have lower copeptin secretion after CRH stimulation, even in the setting of the absence of clinically significant abnormalities in ACTH–cortisol axis. Lower copeptin secretion was also seen in subjects with mildly impaired pituitary function during ITT [
18], as well as during GST [
20]. Hence, we can conclude that copeptin appears to be a sensitive marker of alterations of anterior pituitary function, even below a threshold that warrants glucocorticoid substitution. The reason for this phenomenon remains to be fully elucidated. There are, however, data that AVP secretion in the response to hypoglycaemia is blunted by somatostatin-induced inhibition of growth hormone secretion [
33,
34]. There is a possibility that some subjects in group P1 had a subtle growth hormone deficiency. Hence, we can speculate whether GH deficiency might contribute to blunted AVP/copeptin response after CRH stimulation. This hypothesis, however, requires further study (e.g., assessment of copeptin secretion after CRH in healthy subjects before and after somastostatin). Our subjects, however, were not formally tested for GH deficiency, as GH treatment in adults is not covered by the Polish state insurance.
In summary, we have demonstrated that CRH is able to stimulate copeptin release in healthy controls suggesting a direct interaction of CRH and AVP/vasopressin. Interestingly, this relation is altered already in the group of pituitary patients who pass the standard CRH test in terms of satisfactory ACTH and cortisol secretion. In our opinion, this indicates that the CRH–ACTH–cortisol response is largely independent from the AVP system, yet simultaneously, CRH–AVP interaction reflected by copeptin may be much more sensitive to reveal subtle alterations in the regulation of pituitary function.