VENTILATION IS STIMULATED BY SMALL REDUCTIONS IN ARTERIAL-PRESSURE IN THE AWAKE DOG

Changes in arterial pressure commonly accompany respiratory adaptations. The purpose of this study was to determine, in awake dogs (n = 6), the degree to which small acute decreases in arterial pressure affect ventilation and acid-base balance. Mean arterial pressure (MAP) was reduced by 6 +/- 2, 10 +/- 3, and 16 +/- 2% by intravenous infusion of sodium nitroprusside for sequential 20-min periods. In another experiment, the ventilatory response to hypercapnia was determined during MAP reduction of 16 +/- 3%. Step reductions in MAP were accompanied by increases in minute ventilation (maximum increase 152 +/- 75%) and step reductions in arterial PCO2 (Pa(CO2); maximum reduction -4.8 +/- 0.8 Torr). Although eupneic Pa(CO2) threshold was lowered during MAP reduction, ventilatory sensitivity to CO2 remained unchanged. Despite the lowered Pa(CO2), arterial [H+] remained constant (acid-base balance was maintained) as a result of a concurrent decrease in strong ion difference. Plasma renin activity increased during MAP reduction (93 +/- 39%) and may have contributed to the increase in minute ventilation, inasmuch as angiotensin II can stimulate respiration by a central mechanism. Evidence is provided that nitroprusside is unlikely to be a primary factor in these hypotensive responses. We conclude that relatively modest decreases in MAP have a consistent stimulatory effect on respiratory control. Therefore it is important to take into account effects of small changes in MAP when interpreting mechanisms for respiratory responses in awake animals.