The role of vasopressin in the regional vascular responses evoked in the spontaneously breathing rat by systemic hypoxia.

1. In spontaneously breathing rats anaesthetized with Saffan, we have investigated the role of vasopressin in the cardiovascular responses evoked by systemic hypoxia (breathing 8 or 6% O2 for 5 min). 2. Breathing 8% O2 evoked an increase in respiratory frequency and tidal volume; arterial O2 pressure (Pa,O2) fell to 37 mmHg and arterial CO2 pressure (Pa,CO2) fell to 30 mmHg. Concomitantly, there was a fall in arterial pressure, tachycardia and increases in femoral and renal vascular conductances indicating net vasodilatation in skeletal muscle and kidney. The vasopressin V1-receptor antagonist, d(CH2)5Tyr(Me)-arginine vasopressin (20 micrograms kg-1 i.v.), had no significant effect on the baseline values of any recorded variables, nor on the respiratory or blood gas changes evoked by 8% O2. However, it accentuated the fall in arterial pressure and the increase in femoral vascular conductance (+22 vs. +77% at the 5th minute) produced by 8% O2, but had no significant effect on the increase in renal vascular conductance. 3. Breathing 6% O2 evoked qualitatively similar responses as 8% O2 but Pa,O2 fell to 33 mmHg and Pa,CO2 fell to 28 mmHg and the respiratory and cardiovascular changes tended to be larger than those evoked by 8% O2. Again the V1-receptor antagonist accentuated the hypoxia-induced fall in arterial pressure and increase in femoral vascular conductance (+5 vs. +76% at the 5th minute). 4. Infusion of vasopressin (1.5 ng min-1 kg-1 i.v.) for 5 min with the aim of producing a plasma concentration comparable to that reached during 8% O2, induced a rise in arterial pressure (9%), bradycardia (-5%) and a decrease in femoral (-11%) and renal vascular conductance (-4%). 5. These results suggest that vasopressin released during hypocapnic hypoxia helps to limit the evoked fall in arterial pressure by exerting a vasoconstrictor influence on skeletal muscle.