Vasopressin responses to peripheral and central osmotic pulse stimulation

To determine the relative importance of central and peripheral osmoreceptors in the osmotically-induced vasopressin secretion, osmosensitive areas of pentobarbital-anaesthetized rats were exposed for 5 sec to an osmotic pulse (130 mumoles NaCl in 200 microliters). The hepatic portal receptors were stimulated by superfusion of the portal vein, and the central receptors by infusion into one common carotid artery. Portal stimulation was 2.14 +/- 0.25 (mean +/- SEM, 4 groups of 5 rats) more effective than central osmotic pulse stimulation in elevating, within 1 minute, the plasma vasopressin level (measured by RIA). The central stimulus was not effective when introduced into the freely perfused vessel, although the hypothalamic extracellular NaCl concentration rose transiently (6 sec) to 2.6 +/- 0.3 w/v% (mean +/- SEM, n = 6 rats). The results show that brief osmotic pulses preferentially stimulate portal osmoreceptors to cause the release of vasopressin, and suggest that portal osmoreceptors may be involved in the dynamic regulation of plasma osmolality.