The effector mechanism of hypoxic pulmonary vasoconstriction (HPV) nucleotides.10 Furthermore, lung ATP content is unchanged by hypoxia until the alveolar 02 tension is reduced below 1 mm Hg.11 Depletion of high-energy phosphates in rat lungs, accomplished by chronic ingestion of a diet rich in the false precursor /3-guanidinopropionic acid, does not alter HPV.12 Finally, the doses of several ETC inhibitors that alter vascular tone fail to deplete ATP.l0,13 An 02 sensor that monitors a redox parameter (eg, a redox ratio of couples such as GSH/ GSSG, NADH/NAD, or activated 02 species [AOS] production) rather than ATP levels might be advantageous. Changes caused by mild or moderate hypoxia could be rapidly sensed while preserving ATP at levels necessary to sustain HPV. More severe hypoxia (anoxia) might lower ATP levels and activate ATP-sensitive K' (KATp) channels, which would inhibit HPV.Another means by which hypoxia and ETC inhibitors might alter vascular tone is proposed. The ETC is responsible for the oxidation of NADH and is the major site of 02 consumption in the lung, accountXig iui greater than 56% of total lung O2 consumption.14 A small but significant portion of 02 consumed results in the production of radicals and peroxides.15-16 We hypothesized that inhibition of the proximal ETC would have two effects that tend to make the net cytosolic redox status more reduced (eg, increased ratios of NADH to NAD and GSH to GSSG). First, inhibition of by guest on

SUMMARY1. We studied the characteristics of pulmonary sensory receptors whose afferent fibres are in the left vagus nerve of opened-chest rats. The activity of these receptors was recorded during mechanical ventilation approximating eupnoea, as well as during deflation, stepwise inflations and constant-pressure inflations of the lungs. Data were also collected from closed-chest rats and analysed separately.2. Ninety-four per cent of receptors were located in the ipsilateral lung or airways with the remainder in the contralateral lung.3. Not only were slowly adapting receptors (SARs) the most abundant pulmonary receptors but 21 % of them were either exclusively or predominantly active during the deflationary phase of the ventilatory cycle. Deflationary units were found in opened-and closed-chest rats. The average conduction velocity for all fibres innervating SARs averaged 29-7 m s-'. 4. We found rapidly adapting receptors (RARs) to be extremely rare in the rat. Their activity was sparse and irregular. The conduction velocities of fibres innervating RARs averaged 12 3 m s-'.5. Far more abundant than RARs in the remaining population of pulmonary fibres were C fibres. They were observed to have an average conduction velocity of 2-1 m s-1, base-level activity which was irregular and a high pressure threshold of activation and were stimulated by intravenous capsaicin injection.6. Notable differences exist between pulmonary receptors in rats and those reported in other species. The variations include the abundant existence of intrapulmonary SARs with exclusively deflationary modulation and the rarity of RARs. We also encountered C fibres which have not previously been described systematically in the rat.

No abstract

These experiments were designed to estimate the involvement of the sympathetic innervation in regulation of hindlimb muscle blood flow distribution among and within muscles during submaximal locomotory exercise in rats. Blood flows to 32 hindlimb muscles and 13 other selected tissues were measured using the radiolabeled microsphere technique, before exercise and at 0.5, 2, 5, and 15 min of treadmill exercise at 15 m/min. The two groups of rats studied were 1) intact control, and 2) acutely sympathectomized (hindlimb sympathectomy accomplished by bilateral section of the lumbar sympathetic chain and its connections to the spinal cord at L2-L3). There were no differences in total hindlimb muscle blood flow among the two groups during preexercise or at 30 s or 2 min of exercise. However, flow was higher in eight individual muscles at 2 min of exercise in the sympathectomized rats. At 5 and 15 min of exercise there was higher total hindlimb muscle blood flow in the denervated group compared with control. These differences were also present in many individual muscles. Our results suggest that 1) sympathetic nerves do not exert a net influence on the initial elevations in muscle blood flow at the beginning of exercise, 2) sympathetic nerves are involved in regulating muscle blood flow during steady-state submaximal exercise in conscious rats, and 3) these changes are seen in muscles of all fiber types.