1. Arginine vasopressin was infused at 0.5, 2, 6, 18 or 54 ng min−1 kg−1 for 1 h into normal, sham-operated and DOCA—salt hypertensive rats. Complete vasopressin/blood pressure dose—response curves were constructed from circulating plasma vasopressin concentrations measured at the end of each infusion.
2. DOCA—salt hypertensive rats had a higher basal plasma vasopressin concentration (11.1 ± sd 3.7 fmol/ml) than either the normal (3.9 ± 2.3, P < 0.01) or the sham-operated rats (4.5 ± 2.4, P < 0.01).
3. The DOCA—salt hypertensive rats did not have any detectable enhancement of pressor sensitivity, compared with either of the two normotensive groups.
4. There was no significant increase in blood pressure in either the normal rats or sham-operated rats until vasopressin was infused at 2 ng min−1 kg−1, when the plasma concentration was between 30 and 40 fmol/ml.
5. Subpressor infusion of vasopressin in the normal and sham-operated rats, which gave plasma concentrations of 22–23 fmol/ml, completely suppressed plasma angiotensin II to levels similar to the basal values found in the DOCA—salt hypertensive rats (10.5 ± 2.3, 14.5 ± 4.5 and 8.0 ± 1.6 fmol/ml respectively).
6. These findings suggest that the mechanism of vasopressin involvement in DOCA—salt hypertension is as yet unclear, that short-term changes in vasopressin concentration appear unimportant in the regulation of normal blood pressure, that small physiological changes of vasopressin in normal rats may be important in the regulation of renin secretion, and that the increase in vasopressin concentration seen in DOCA—salt hypertension may contribute to the suppression of renin and angiotensin II in this state.