Activation of neurons in the rostral ventrolateral medulla increases bronchomotor tone in dogs

Previous work from this laboratory has demonstrated that the chemical activation of cell bodies in the caudal ventrolateral medulla of chloralose-anesthetized dogs decreased bronchomotor tone by withdrawing cholinergic input to airway smooth muscle. In the present study we determined the bronchomotor responses to microinjection of DL-homocysteic acid (100 mM; 25-50 nl) into the rostral ventrolateral (RVL) medulla of chloralose-anesthetized dogs. Total lung resistance was used as a functional index of bronchomotor tone. Microinjection of DL-homocysteic acid into the 20 sites located in the lateral aspect of the RVL medulla increased both total lung resistance [from 6.5 ± 0.4 to 9.1 ± 0.8 (SE) cmH₂O · 1^-1 · s; P < 0.05] and mean arterial pressure (from 125 ± 5 to 148 ± 8 mmHg; P < 0.05). Microinjection of this amino acid into nine sites located in the medial aspect of the RVL medulla increased mean arterial pressure (from 130 ± 6 to 153 ± 6 mmHg; P < 0.05) but had no effect on total lung resistance. We confirmed in three sites that the increase in total lung resistance evoked by microinjection of DL-homocysteic acid was accompanied by an increase in tracheal smooth muscle tension. The increase in total lung resistance evoked by DL-homocysteic acid was not affected by β-adrenergic blockade but was abolished by muscarinic blockade.