D. Bee

Endothelial control of the pulmonary circulation in normal and chronically hypoxic rats.

1. The effect of blockade of nitric oxide synthesis in pulmonary endothelium by two L‐arginine analogues was tested in isolated blood‐perfused lungs of normal rats and rats exposed chronically to 10% O2. 2. In both groups of rats the analogues (N‐monomethyl‐L‐arginine (L‐NMMA) and N‐nitro‐L‐arginine methyl ester (L‐NAME)) enhanced hypoxic vasoconstriction. In normal rats, with rare exceptions, these analogues had little or no effect on pulmonary artery pressure (Ppa) at constant blood flow during normoxia. However, chronically hypoxic rats have pulmonary hypertension and in these rats the analogues always raised Ppa; the rise in Ppa after L‐NMMA but not L‐NAME could be partially reversed by L‐arginine. L‐NAME was more potent than L‐NMMA. 3. To see whether the difference between rat groups was due to the high Ppa in chronically hypoxic rats, in control rats we raised Ppa passively by lung inflation to values higher than found in chronically hypoxic rats. L‐NAME did not alter the effects of lung inflation on Ppa. 4. Ppa was also raised passively by plotting pressure‐flow lines up to high flow rates; the lines were changed minimally by both analogues in control rats but in chronically hypoxic rats the lines were raised to higher pressures and steepened substantially. 5. In control rats, during vasoconstriction caused by hypoxia, endothelin 1 and almitrine, L‐NAME caused further rises in pressure. We conclude that a stimulus for nitric oxide release in control rats is the narrowing of vessels caused by vasoconstriction rather than passive increases in intravascular pressure. 6. In chronically hypoxic rats arterioles are narrowed by growth of new muscle and there is some muscle tone even in normoxia. Thus narrowing of the vascular lumen is the stimulus common to both groups of rats which leads to nitric oxide synthesis and attenuation of Ppa by a negative feedback process. Narrowing is associated with a large increase in shear stress due to two factors; the pressure drop along a vessel segment is increased and the surface area of the lining of the affected segment is decreased. 7. Atrial natriuretic peptide caused dose‐dependent pulmonary vasodilation in both rat groups but had a greater effect in chronically hypoxic rats. The action persisted and was enhanced after blockade of NO synthesis.

Contribution of polycythaemia to pulmonary hypertension in simulated high altitude in rats.

A rat model was used to assess the viscosity factor in pulmonary hypertension of high altitude. Rats exposed to 10% O2 for three weeks developed increased pulmonary vascular resistance (p.v.r.) and polycythaemia; the haematocrit (Hct) was 50‐60%, values similar to those in normal men at high altitudes. The contribution of high Hct to the increased p.v.r. was assessed in isolated perfused lungs of chronically hypoxic rats perfused with their own high Hct blood, or normal Hct blood from control rats. Pressure/flow relationships were measured over a wide range and the slope (P/Q) of this relationship and its extrapolated intercept on the pressure axis were increased by high Hct blood. A return to low Hct blood did not restore initial conditions although a second perfusion with high Hct blood again increased p.v.r. and intercept. Lack of reversibility was attributed to changes with time in blood or lung. In a second experiment designed to eliminate time changes, chronically hypoxic or litter‐mate control rats were each perfused with only one blood, their own or each others and P/Q relations were rapidly measured. The P/Q slope and pressure intercept increased progressively in the following groups: control rats perfused with their own blood (Hct 34%), control rats perfused with chronically hypoxic blood (Hct 56%), chronically hypoxic rats perfused with control blood (Hct 35%) and chronically hypoxic rats perfused with chronically hypoxic blood (Hct 53%). To exclude factors in chronically hypoxic blood other than high Hct which might increase p.v.r., control rats were perfused with blood of different Hct obtained by centrifugation. High Hct again increased p.v.r. There was a significant relationship in all rats between pulmonary artery pressure (Ppa), which takes into account both P/Q slope, intercept and Hct. There was substantial batch variation which may reflect sensitivity to hypoxia. In chronically hypoxic rats with high Hct blood, Ppa varied from 29‐47 mmHg; with low Hct blood the range was 26‐38 mmHg. Comparable values for control rats were 21‐29 and 17‐20 mmHg. We conclude that the polycythaemic blood of chronic hypoxia contributes substantially to pulmonary hypertension. Where it is excessive, it may prejudice tissue blood flow.

Division of Type I and Endothelial Cells in the Hypoxic Rat Carotid Body

The mammalian carotid body is enlarged under conditions of chronic hypoxaemia. There has been some discussion as to whether this is due to hypertrophy or to hyperplasia. We have subjected rats to 1, 2 or 7 days of 10% oxygen and, 4 h before removing the carotid bodies, injected each animal with vincristine sulphate, an inhibitor of mitosis. The results of this study indicate that numerous mitoses can be found in the carotid bodies of rats exposed to 10% oxygen, but not in control animals maintained in air. These experiments thus provide direct evidence that at least a proportion of the increase in size of the carotid body induced by chronic hypoxaemia is due to a cellular hyperplasia.

ACTION OF ALMITRINE BISMESYLATE ON VENTILATION-PERFUSION MATCHING IN CATS AND DOGS WITH PART OF THE LUNG HYPOVENTILATED

1. Ventilation to one lobe of lung was reduced in anaesthetized open‐chest cats and dogs to simulate the ventilation/perfusion (V̇/Q̇) mismatching of chronic lung disease. Blood flow to this lobe fell less than ventilation; thus lobar V̇/Q̇ diminished.

AN ANALYSIS OF THE ACTION OF AN ANALOGUE OF ALMITRINE BISMESYLATE IN THE RAT MODEL OF HYPOXIC LUNG DISEASE

Chronically hypoxic (CH) and normoxic control rats were used to assess the action of S9581, a water‐soluble analogue of almitrine bismesylate. S9581 increased ventilation (Ve) by 34% in control and 20% in CH rats. During acute hypoxia Ve was raised and S9581 caused a further increase of 20% in both groups. Low doses of S9581 and almitrine enhanced the hypoxic ventilatory response in CH rats while high doses depressed it in both groups. Effects of S9581 on the pulmonary circulation were assessed in the isolated perfused lung of rats. As with almitrine a complex relationship of dose‐dependent vasoconstriction and dilatation was revealed. In low doses, S9581 enhanced the hypoxic pulmonary vasoconstrictor response to 2% O2 whilst this was attenuated by high doses in both control and CH rats. S9581 seemed to act like almitrine bismesylate on both the ventilation (peripheral chemoreceptor) and the pulmonary circulation. For studying almitrine‐like activity the water solubility of S9581 provides considerable advantages for the researcher.

Ca2+ Channel Currents in Type I Carotid Body Cells from Normoxic and Chronically Hypoxic Rats

Rats born and reared under chronically hypoxic (10% O2) conditions do not respond to acute hypoxia with an increased ventilation. Their carotid bodies undergo hyperplasia and hypertrophy and we have recently shown that K+ channels recorded in type I carotid body cells isolated from normal and chronically hypoxic (CH) rats show marked differences (Wyatt et al, 1995): normoxic type I cells express Ca2+-activated K+ (KCa) channels which are inhibited by acute hypoxia, leading to cell depolarization, opening of voltagegated Ca2+ channels (VGCCs) and the consequent influx of Ca2+ to trigger neurotransmitter release (Peers & Buckler, 1995). In type I cells from CH rats, there is far less expression of KCa channels, and, whilst the remaining K+ channels are inhibited by hypoxia, this does not lead to cell depolarization, which may explain the lack of ventilatory response to acutely inspired hypoxia in intact CH rats (Wyatt et al, 1995). An important factor in the response of normal type I cells to hypoxia is the activation of VGCCs, of which numerous sub-types exist in various tissues. It is known that L-type VGCCs are involved in hypoxic chemotransduction (Buckler & Vaughan-Jones, 1994), but very little is known about the possible presence of other VGCCs, whether they may be involved in chemotransduction, and whether they are affected by chronic hypoxia. We have therefore compared the properties of VGCCs in type I carotid body cells isolated from normoxically-reared and CH rats.

Attenuation of the Hypoxic Ventilatory Response in Awake Rabbit Pups; Possible Role of Dopamine

A failure of some part of the respiratory control apparatus has been quoted as the underlying cause of sudden and unexplained death in infants (Busuttil, 1992). This could be due to either a prevailing immaturity of central mechanisms or an inability of the carotid body to detect hypoxia. The ventilatory response to hypoxia was found to be poor in a group of near miss sudden infant death syndrome (SIDS) infants (Hunt et al., 1981). Bolton (1990) also found that 2% of infants in a normal population also demonstrated an attenuated response to hypoxia, which he suggested may help to identify the individuals at risk. Investigations of the animal carotid body have shown that this organ resets its function after birth (Eden & Hanson, 1987) but within a few days the “adult” responses are achieved (Hertzberg et al., 1990). However, SIDS deaths have a peak incidence at around 3’4 months of age, which suggests that the period of risk is not associated with this early resetting of the hypoxic response. This led us to investigate the longitudinal development of the hypoxic ventilatory response (HVR) in rabbits from birth until weaning. In a study of 10 rabbit pups we have established that a severe attenuation of the HVR occurred around week 6 of life (Bee et al., 1993) which showed partial recovery at weaning. The current investigation is aimed at elucidating the role of the inhibitory amine, dopamine, in this attenuated HVR.