Effect of an Osmotic Stimulus on the Secretion of Arginine Vasopressin and Adrenocorticotropin in the Horse*

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Abstract
Arginine vasopressin (AVP) is released in response to changes in blood osmolality and is also a putative secretagogue for ACH. However, it is unclear whether osmotically generated increases in AVP in the physiological range influence ACTH secretion. We have studied this question using our unique noninvasive technique for collecting pituitary venous blood in six normal conscious horses that received an iv infusion of hypertonic saline (HS; 5%, 0.07 ml/kg.cntdot.min) for 45-60 min. Pituitary and jugular venous samples were collected every 5 min for 40 min before, during, and for 20 min after HS. During HS, mean blood osmolality rose (P < 0.01), with a mean peak increase of 14.8 mosmol/kg (range, +6-+37 mosmol/kg). Jugular AVP rose (P < 0.01) from 0.56 .+-. 0.18 pmol/liter (mean .+-. SEM) before HS to 2.16 .+-. 0.86 pmol/liter during HS. Mean jugular AVP and osmolality were correlated (r = 0.82; P < 0.05) during HS. Mean jugular ACTH concentrations increased (P < 0.01) from 49 .+-. 9 ng/liter before HS to 148 .+-. 54 ng/liter during HS, while mean cortisol levels during and after HS exceeded basal levels (P < 0.05). Pituitary AVP and ACTH concentrations exceeded jugular concentrations by up to 100-ofold, and mean (P < 0.01 for both) and peak (P < 0.001 for both) levels increased during HS. AVP and ACTH secretion during HS were pulsatile. The mean and peak changes in pituitary AVP were significantly correlated with those in ACTH. For the six horses together, pituitary ACTH and AVP concentration changes occurred synchronously during the experiment (P < 0.001), and the paired AVP and ACTH concentrations were highly correlated (r = 0.73; n = 129 pairs; P < 0.001). We conclude that 1) physiological changes in AVP secretion are closely associated with comparable changes in ACTH secretion, and 2) osmotic signals that presumably activate the magnocellular neurons of the suproptic and paraventricular nuclei may be physiologically relevant regulators of corticotrope function.