Modulation of airway smooth muscle β-adrenoceptor function by a muscarinic agonist

We have investigated the effect of a muscarinic agonist and protein kinase C (PKC) activation on β-adrenoceptors and their coupling to adenylyl cyclase in bovine tracheal smooth muscle. There was a significant reduction in maximum binding capacity of [125I]iodocyanopindolol ([125I]ICYP) after exposure to carbachol and 4β-phorbol 12β-myristate 13α-acetate (PMA). Similarly both carbachol and PMA inhibited the 5'-guanylylimidodiphosphate-induced shift in [125I]ICYP binding by isoproterenol and significantly decreased isoproterenol-induced cyclic AMP accumulation. A phorbol ester, 4α-phorbol 12,13-didecanoate which does not activate PKC had no effect on β-receptor binding or coupling. These results suggest that PKC activation directly via a phorbol ester and indirectly via muscarinic receptor stimulation may lead to reduction and uncoupling of β-receptors in airway smooth muscle. We suggest that this mechanism may be relevant to the reduction in β-receptor coupling in asthmatic airways as an effect of PKC activation by inflammatory mediators and neurotransmitters.link_to_subscribed_fulltex

Mucus secretion from individual submucosal glands of the ferret trachea

Mucus secretion from individual tracheal glands in adult ferrets was studied with time-lapse optical imaging of mucus droplets under an oil layer. Density of functional glands (determined by responses to 1 μM carbachol) was 1.5 ± 0.3 per mm2 (n = 6). Secretion rates (in pl·min−1·gland−1) were as follows: 4.1 ± 0.7 basal (unstimulated; n = 27, 669 glands), 338 ± 70 to 10 μM forskolin (n = 8, 90 glands), 234 ± 13 to 1 μM VIP (n = 6, 57 glands), 183 ± 92 to 10 μM isoproterenol (n = 3, 33 glands), 978 ± 145 to 1 μM carbachol (n = 11, 131 glands), and 1,348 ± 325 to 10 μM phenylephrine (n = 7, 74 glands). The potency (EC50, in μM) and efficacy (Vmax, in pl·min−1·gland−1) were 7.6 (EC50) and 338 ± 16 (Vmax) to forskolin, 1.0 (EC50) and 479 ± 19 (Vmax) to VIP, 0.6 (EC50) and 1,817 ± 268 (Vmax) to carbachol, and 3.7 (EC50) and 1,801 ± 95 (Vmax) to phenylephrine. Although carbachol and phenylephrine were equally effective secretagogues, only carbachol caused contractions of the trachealis muscle. Synergy was demonstrated between 300 nM isoproterenol and 100 nM carbachol, which, when combined, produced a secretion rate almost fourfold greater than predicted from their additive effect. The dependence of fluid secretion on Cl− and HCO3− varied depending on the mode of stimulation. Secretion stimulated by VIP or forskolin was reduced by ∼60% by blocking either anion, while carbachol-stimulated secretion was blocked 68% by bumetanide and only 32% by HEPES replacement of HCO3−. These results provide parametric data for comparison with fluid secretion from glands in ferrets lacking CFTR