Carbachol, norepinephrine, and hypocapnia stimulate phosphatidylinositol turnover in rat tracheal slices

Tidal breathing pattern differentially antagonizes bronchoconstriction in C57BL/6J vs. A/J mice

There is abundant evidence that tidal breathing, and especially tidal breathing at elevated minute ventilation, antagonizes the development and persistence of airflow obstruction during bronchoconstrictor stimulation in normal animals and people. Here, we studied the antiobstructive effect of different tidal breathing patterns in C57Bl/6J and A/J mice during bronchoconstriction induced by continuous or bolus infusion of methacholine. Anesthetized, paralyzed mice were mechanically ventilated at 1,500 ml.kg(-1).min(-1), using each of three breathing patterns: 5 ml/kg, 300 breath/min; 10 ml/kg, 150 breath/min; or 20 ml/kg, 75 breath/min. Changing from 10 ml/kg, 150 breath/min to 20 ml/kg, 75 breath/min, breathing functionally antagonized bronchoconstriction, reducing the level of airflow obstruction induced by methacholine infusion or boluses equivalently in both strains. In marked contrast, changing from 10 ml/kg, 150 breath/min to 5 ml/kg, 300 breath/min, breathing substantially exacerbated methacholine-induced airflow obstruction in A/J mice, whereas it had no significant effect in C57Bl/6J mice. Our results therefore demonstrate that 1) even at moderate, fixed minute ventilation, the precise breathing pattern can influence the degree of airflow obstruction substantially, and 2) the influence of breathing pattern on bronchoconstriction differs considerably between genetically diverse inbred mouse strains. These findings imply that differences in antiobstructive effects of breathing can contribute to differences in apparent airway constrictor responsiveness. Much attention has been placed on dysregulation of contractile function of airway smooth muscle in human disease. We suggest that important pathophysiology might also be found in impairment of the functional antagonist effect of tidal breathing on airflow obstruction.

Cellular mechanisms of acrolein-induced alteration in calcium signaling in airway smooth muscle

Acrolein, an unsaturated aliphatic aldehyde, is a potent respiratory irritant. We have previously observed that acrolein administered ex vivo to isolated airways alters subsequent airway responsiveness to muscarinic agonists in terms of both mechanical activity of rings and calcium signaling in isolated cells. In the present study, we have examined the mechanisms by which acrolein alters Ca(2+) signaling. In freshly isolated rat tracheal smooth muscle cells, preexposure to acrolein increased the [Ca(2+)](i) oscillation frequency in response to endothelin 1 (ET-1, 0.1 microM), a contractile agonist that acts via the activation of a receptor different from the muscarinic cholinoceptor. We then studied acrolein-induced alteration in cell signaling with special attention to the steps downstream of membrane receptor activation i.e., the inositol 1,4,5-trisphosphate (InsP(3)) signaling pathway. Pretreatment of cells with LiCl (20 mM), a modulator of InsP(3) concentration, mimicked the effect of acrolein exposure on agonist-induced [Ca(2+)](i) response, i.e., increased the amplitude of the first Ca(2+) rise and the oscillation frequency in response to 0.1 and 10 microM acetylcholine (ACh), respectively. Moreover, in tracheal smooth muscle, preexposure to acrolein significantly increased carbachol-induced [(3)H]inositol-phosphates accumulation, up to 34 +/- 11% above unexposed tissue values. Finally, in beta-escin permeabilized cells, injection of InsP(3) (0.1-10 microM) induced a concentration-dependent [Ca(2+)](i) rise followed, for high InsP(3) concentration, by [Ca(2+)](i) oscillations, a calcium response whose pattern was similar to that induced by ACh. Exposure to acrolein did not alter the InsP(3)-induced [Ca(2+)](i) response. These results indicate that the effect of acrolein exposure on Ca(2+) responses in airway smooth muscle is not restricted to activation of the muscarinic cholinoceptor and is due to an enhancement in agonist-induced InsP(3) production. Since acrolein does not modify InsP(3) receptor channel sensitivity, we conclude that acrolein-induced alteration in calcium signaling can be ascribed to its sole effect on InsP(3) production.

The hazardous effects of alveolar hypocapnia on lung mechanics during weaning from cardiopulmonary bypass

The bronchoconstrictive effects of alveolar hypocapnia during weaning from cardiopulmonary bypass (CPB) were investigated in patients undergoing elective coronary artery revascularization. Thirty patients were randomly assigned into two equal groups. In both groups, mechanical ventilation was initiated for 3 min prior to weaning from CPB with the venous pressure low. This kept the pulmonary vascular bed empty, resulting in alveolar hypocapnia (ETCO2 < 2 kPa). Peak airway pressure (P(peak)) and plateau pressures (P(plateau)) were recorded. In group 1, 5% CO2 was added to the inspiratory gas mixture and the ETCO2 allowed to rise (ETCO2 > 3.3 kPa). The ventilation pressure measurements were recorded again after 3 min stabilization. In group 2, the venous pressure was increased to allow the pulmonary venous bed to fill and the ventilation pressures recorded after a 3 min period of stabilization. In group 1, the ventilatory pressures dropped significantly (p < 0.001) when the alveolar hypocapnia was reversed with added CO2 (P(peak) 19.71 +/- 5.7 to 12.31 +/- 2.8 cmH2O and P(plateau) 13.15 +/- 3.28 to 9.15 +/- 2.23 cmH2O). In group 2, a similar effect was achieved by allowing filling of the pulmonary vascular bed (P(peak) 17.46 +/- 4.72 to 11.92 +/- 3.03 cmH2O and P(plateau) 13.93 +/- 4.10 to 9.37 +/- 3.00 cmH2O). These results suggest that filling the pulmonary vascular bed prior to initiating ventilation, when weaning from CPB, prevents the otherwise deleterious effects of alveolar hypocapnia, resulting in raised bronchomotor tonus and raised airway pressures.

Stimulation of cyclic AMP accumulation and phosphoinositide hydrolysis by M3 muscarinic receptors in the rat peripheral lung

The effects of oxotremorine-M (oxo-M), a muscarinic agonist, on cyclic AMP (cAMP) accumulation in slices of the rat peripheral lung were investigated. Oxo-M stimulated cAMP accumulation in a concentration-dependent manner with an EC50 value of 4.2 microM and a maximal effect of 2.4 +/- 0.39-fold over basal. In the presence of forskolin (25 microM), the maximal effect of oxo-M was increased to 14.1 +/- 4.0-fold over basal. Forskolin alone caused a 5.9 +/- 2.2-fold increase in cAMP relative to basal; therefore, the combination of both drugs was more than additive. The effects of oxo-M on cAMP accumulation were unaffected by tetrodotoxin, indicating that the action of oxo-M was not mediated by neuronal release of neurotransmitters. Oxo-M had a small inhibitory effect on cAMP in a homogenate preparation, indicating that the stimulatory response to oxo-M in slices of the lung is not due to direct stimulation of adenylyl cyclase. Characterization of the oxo-M potentiation of forskolin-stimulated cAMP accumulation using different muscarinic antagonists yielded calculated pKB values that agreed with binding affinities for the M3 subtype. Oxo-M elicited phosphoinositide hydrolysis in the lung, and the nature of the antagonism of this response was also consistent with that expected for an M3-mediated response. cAMP accumulation in the presence of oxo-M (100 microM), forskolin (12 microM), or both drugs combined was inhibited by indomethacin (1 microM). These results demonstrate that the M3 receptor stimulates cAMP accumulation and phosphoinositide hydrolysis in the rat peripheral lung, and the mechanism for cAMP stimulation may involve arachidonic acid metabolites.