[Tracheobronchial muscle tonus and its reflex control]

Assessment of an airway-to-pulmonary circulation reflex in cats

In anesthetized, mechanically ventilated cats we studied the effects on pulmonary circulation of sustained distension or increased air flow stream in an isolated, innervated upper airway segment constituted by the cervical trachea, the larynx and the pharynx. Direct and averaged pulmonary artery pressures and blood flows were measured. In intact cats, sustained distension as well as increased air flow stream in the upper airway segment induced significant, marked increase in pulmonary artery pressure with concomitant decrease in blood flow. No change in heart rate occurred. Section of the superior laryngeal nerves (SLNs), which denervated the tracheolaryngeal segment but not the epipharynx, markedly reduced and delayed the pulmonary vascular response to mechanical stimulation of the upper airways. The response was not found after cervical bivagotomy. The present observations demonstrate the existence of an airway-to-pulmonary circulation reflex, largely mediated through SLNs afferent pathways, the vagus nerve constituting the efferent arm of the reflex.

[Cellular mechanism of muscle contraction of bronchial smooth muscle]

Airway smooth muscle is one of the main effector of bronchial reactivity. The understanding of the cellular mechanisms involved in the contraction of this muscle has advanced in the recent past since isolated cells in culture can now be studied. Extracellular messengers (neurotransmitters and mediators) as well as their specific membrane receptors have been analyzed in some details. The membrane transduction of extracellular messengers brings about the formation (or the increase in the concentration) of the intracellular second messenger which, in airway smooth muscle, is the cytosolic calcium (Ca2+i) via activation of calcium channels which depend on surface membrane potential changes (electromechanical coupling) on the one hand and mainly via mechanisms independent of surface membrane potential changes-so-called the pharmacomechanical coupling--which involves membrane phosphoinositides metabolism. Changes in Ca2+i activate contractile proteins leading the muscle to shorten and to develop force via several controlled steps such as phosphorylation of myosin or changes in the sensitivity to Ca2+ of the contractile elements. Experimental techniques that enable to simultaneously study different aspects of the cellular response are being developed in airway smooth muscle and are likely to provide complementary information about the cellular physiology and pathophysiology of this muscle.