RESPONSE OF PULMONARY CIRCULATION OF RESTING, UNANESTHETIZED DOGS TO ACUTE HYPOXIA

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Modulation of converting enzyme activity by hypoxia and its physiological effects

To explore the haemodynamic consequences of the reduction in converting enzyme activity by acute alveolar hypoxia we made sequential haemodynamic observations in seven saline-infused and 12 bradykinin-infused anaesthetized, catheterized dogs. They were ventilated initially with room air and then for 50 minutes with hypoxic gas mixtures. Within two minutes after starting hypoxic ventilation, converting enzyme activity decreased, arterial angiotensin II concentrations dropped, and, in the bradykinin-infused dogs, arterial bradykinin concentrations rose. Both groups of dogs experienced a rise in systemic and pulmonary arterial blood pressure in response to hypoxia, but by different mechanisms. In the saline-infused (control) dogs there was increased systemic (+40%) and pulmonary (+90%) vascular resistance while cardiac output was unchanged or slightly reduced. Bradykinin-infused dogs demonstrated reduced systemic vascular resistance (-40%), no increase in pulmonary vascular resistance and a 100% increase in cardiac output. Return to room air breathing restored converting enzyme activity, releasing high concentrations of angiotensin II. Oxygen tension thus regulates converting enzyme activity and hence the circulating levels of angiotensin II and bradykinin.

Canine nonresponders to alveolar hypoxic vasoconstriction and quantitative restoration of the response by aspirin I-3

Hypoxic pulmonary vasoconstriction is characterized by considerable variability in rate of response (pulmonary vascular resistance [PVR] as a function of time under hypoxia). To further define this response, forty-five closed chest dogs were anaesthetized (pentobarbitone sodium), intubated, and mechanically ventilated. Constant left lower lobar pulmonary artery flow was maintained through a balloon tipped 14F catheter via an extracorporeal pump at a rate to achieve lobar pulmonary artery pressure (Plobar) equal to main pulmonary artery pressure (PPA) and thereafter held constant. Left ventricular end diastolic pressure (PLVED) was measured by left ventricular catheter and lobar pulmonary artery flow rate (Q) by flow meter. Lobar PVR (mmHg/min per 1) was calculated every 15 min. Ventilation with 10% oxygen (O2) separated two groups based on the increase in PVR over time: twenty-two rapid hypoxic responders [HR] (slope= delta PVR/delta min greater than 0.3) and twenty-three slow or nonresponders [NR] (slope less than 0.1). The twenty-three NR were divided into two groups. Ten NR dogs (NR control) had no change in mean PVR (23.9, s.d. = 8.2, to 24.1, s.d. = 9.6) over a mean of 78 min and were used as controls. Thirteen NR dogs (NR ASA) had no change in mean PVR (32.9, s.d. = 9.5, to 32.3, s.d. = 9.8) over 75 min and were given aspirin (ASA), 10-15 mg/kg intra-arterially. The NR ASA group mean PVR then increased from 32.3 (s.d. = 9.8) to 59.1 (s.d. = 23.9, 82.9% increase, P less than 0.01) over a mean of 54 min. The mean PVR for the twenty-two HR rose from 39.8 (s.d. = 34.0) to 64.5 (s.d. = 36.6, 62.1% increase, P less than 0.01) over a mean of 72 min. The slopes of rate of response for HR (0.66) and for NR ASA (0.88) were not significantly different. The absolute values of PVR reached after plateau for HR and for NR ASA (after ASA) were also not different. Aspirin restored the NR capability to develop pulmonary vasoconstriction in response to alveolar hypoxia. The rate of response and the absolute level of response reached were also restored by aspirin.

Mechanisms of O2 transport in Andean dogs

Using previously inserted catheters, 11 dogs native to high altitude (7.5-23 kg bwt) were studied standing and unsedated in Cerro de Pasco, Peru at 4350 meters. Hemoglobin (Hb), hematocrit (Hct), O2 and CO2 contents, PO2, PCO2 and pH were measured in simultaneously obtained arterial and mixed venous blood samples. Blood pressures were measured in the pulmonary artery and the left ventricle and cardiac output (Q) was determined by dye dilution. Moderately higher values for Hb and Hct were found in these dogs. Hb-O2 affinity was no different than that found in sea level dogs: the P50 in the Andean dogs was 31.6 mm Hg at 38 degrees C and pH of 7.4. Because of the low barometric pressure at 4350 m (458 mmHg) the partial pressures of oxygen in inspired and in alveolar air were lower than at sea level: 84.3 and 56.4 mm Hg, respectively. PAO2 and PVO2, were 55.5 and 32.9 mm Hg while the SAO2 and SVO2 were 79.5 and 50.7%, respectively. Marked hyperventilation was observed (PACO2, 25.6 mm Hg) however, pH was normal. Cardiac output was normal (average 162 plus or minus 39 ml/min/kg). Moderate pulmonary arterial hypertension was observed in the presence of normal left ventricular end diastolic pressure suggesting increased pulmonary vascular resistance.

Studies on hypoxia. II. Autoradiographic quantitation of proline-3H incorporation by connective tissue cells of the neonatal hamster

Quantitative evaluation of autoradiographic grains from anoxia-treated and control neonatal hamster indicated that anoxia impairs collagen synthesis and significantly suppresses the incorporation of proline-sH in the odontoblast, fibroblast, and osteoblast by 24 hours after the insult. However, the rapid and apparently total recovery of the dental and paradental connective tissue cells suggests that anomalies of dentition, heretofore attributed to brief spells of anoxia at birth, may not be due to such a hypoxic insult during the perinatal period.

Response of the pulmonary vasculature to hypoxia and H+ ion concentration changes

Images