Enhanced Myosin Phosphatase and Ca²⁺-Uptake Mediate Adrenergic Relaxation of Airway Smooth Muscle

Abstract

We examined the mechanisms underlying relaxations evoked by isoproterenol (Iso) in isolated porcine, bovine, or human tracheal and bronchial tissues (TSM and BSM, respectively). Iso had little effect against contractions evoked by high KCl, indicating that it does not directly suppress voltage-dependent Ca(2+)-influx nor directly inhibit myosin light chain kinase. Furthermore, Iso was equally potent against carbachol (CCh) contractions in the presence versus absence of nifedipine (10(-6) M), establishing that the primary action of Iso is not through membrane hyperpolarization. However, Iso relaxations in porcine/bovine BSM were significantly suppressed by inhibitors of the internal Ca(2+) pump (cyclopiazonic acid; 10(-5) M) or of myosin light chain phosphatase (calyculin; 10(-6) M). Myosin light chain phosphatase activity was assayed directly (using (32)P-labeled myosin) and found to be enhanced in a time- and concentration-dependent fashion by Iso. Iso relaxations in human airway tissues, on the other hand, were not significantly affected by either calyculin or cyclopiazonic acid. Thus, we conclude that Iso acts largely in a voltage-independent fashion: in nonhuman airways, this involves enhanced Ca(2+) pump activity (to decrease [Ca(2+)](i)) and myosin light chain phosphatase activation (to decrease Ca(2+)-sensitivity of the contractile apparatus), whereas in human airways the underlying mechanisms are still unclear.

Regulation of airway smooth muscle RhoA/ROCK activities by cholinergic and bronchodilator stimuli

“…The current authors have previously shown that Rho/ROCK activities are in part Ca 2+ dependent [28,40], and b-agonists and NO are known to decrease intracellular calcium levels in airway smooth muscle; however, the present study did not control for changes in intracellular calcium. Future studies of the mechanisms by which b-adrenoceptors couple to the Rho/ROCK signalling pathway will require a careful pharmacological dissection using agents that abrogate one or another of the excitation-contraction coupling pathways.…”

“…Generally speaking, bronchodilators trigger signalling events opposite to those summarised above, including stimulation of the enzymatic activities of adenylate cyclase/protein kinase (PK)A (b-agonists) or of guanylate cyclase/PKG (NO), Ca 2+ uptake and Ca 2+ extrusion, membrane hyperpolarisation, inhibition of MLCK and stimulation of MLCP [7,10,[26][27][28][29]. Interestingly, some of these effects depend upon whether cholinergic stimulation precedes b-adrenoceptor stimulation, or vice versa [7,26,27].…”

Epigenetic Alterations by DNA Methylation in House Dust Mite–Induced Airway Hyperresponsiveness

“…PDE4 is shown to be the major PDE subtype in human ASM cells (36). It hydrolyzes cAMP, which is known to activate protein kinase A (PKA)-and exchange protein directly activated by cAMP (Epac)-dependent pathways to regulate ASM functions, such as inhibition of ASM cell proliferation and airway constriction (37)(38)(39). The dramatic increase in pde4 d expression in lungs of HDM-challenged mice, therefore, could cause a significant reduction in cAMP and enhance airway reactivity and remodeling.…”

Beyond the dogma: novel 2-adrenoceptor signalling in the airways

“…4) [70]. With respect to ASM relaxation, there are data to suggest that b-adrenoceptor agonists may activate MLCP and that this may play an important role in reducing tone [71]. Moreover, MLCP can also be activated by the cyclic guanosine monophosphate (cGMP)/cGMP-dependent PKG pathway.…”