Differential bronchodilatory effects of terbutaline, diltiazem, and aminophylline in canine intraparenchymal airways

A Comprehensive Overview of Respiratory Compliance in Dogs Under General Anesthesia: Clinical Factors and Future Perspectives

Respiratory compliance reflects the ability of the lungs and chest wall to expand in response to increases in pressure. In this review, relevant studies were selected through a comprehensive literature search with the aim of summarizing and generalizing them to describe the relevant factors that may be present in veterinary clinical practice and affect respiratory compliance in dogs. Individual factors, including breeds, disease background, drugs administered, and especially surgical procedures, can result in alterations to respiratory compliance due to their impact on the respiratory system in dogs. Despite its potential clinical utility, such as in anesthesia monitoring, respiratory compliance remains underutilized in veterinary medicine, and further research is necessary to support its future clinical applications.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

The gastrointestinal effects that may follow the administration of theophylline reflect the pharmacodynamic profiles of both the parent drug and its metabolites

This study investigates the effect of theophylline along the rabbit gastrointestinal tract in comparison with the pharmacodynamic effect produced by the combined application of its three major metabolites. At concentrations up to 10(-3) m, theophylline relaxed, in a declining order from the lower oesophageal sphincter (LOS) to pylorus, all regions of the upper gastrointestinal tract, but only the ascending colon from the intestinal regions studied. At concentrations higher than 10(-3) m, instead of relaxing, theophylline strongly contracted the antrum and pylorus. In all three small intestinal regions, at concentrations up to 10(-3) m, theophylline produced a weak contraction, which at higher concentrations became very strong, and at 10(-2) m was comparable to that produced by a supramaximal dose of acetylcholine. The additive relaxing effect resulting from the combined application of the theophylline's metabolites was, from oesophagus to pylorus, weaker than that produced by theophylline, while on the ascending colon it was comparable to that of the parent drug. In contrast, the additive contractile effect of the metabolites on the three small intestinal regions was four to five times higher the one produced by theophylline. In conclusion, this study shows that the additive effect of the combined application of theophylline's major metabolites on the rabbit gastrointestinal tract plays a major role in the final response of the intestine, and a minor one in the final responses of the gastric regions, while both the parent drug and the metabolites contribute to the final responses of the oesophagus and LOS.