InsP3, but not novel Ca2+ releasers, contributes to agonist‐initiated contraction in rabbit airway smooth muscle

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Abstract
To examine the contributions of the putative Ca2+ releasers, inositol 1,4,5‐trisphosphate (InsP3), cyclic ADP ribose (cADPR), and nicotinate adenine dinucleotide phosphate (NAADP), to carbachol (CCh)‐induced contraction in airway smooth muscle, we measured force development of permeabilized rabbit tracheal smooth muscle, human bronchial smooth muscle and guinea‐pig ileum longitudinal smooth muscle. In the presence of 50 μm GTP, CCh and InsP3 contracted α‐toxin‐permeabilized tracheal smooth muscle dose dependently; the EC50 values for CCh and InsP3 were 1.84 μm and 363 μm, and the maximum responses (normalized to the 30 mM caffeine response) to 100 μm CCh and to 800 μm InsP3 were 206 ± 13.4 % (mean ± s.e.m.) and 84.4 ± 5.3 %, respectively. However, cADPR (10‐300 μm), β‐NAD+ (2.5 mM), FK506 (30 μm) and NAADP (100 μm) neither contracted the strip by themselves nor affected the subsequent CCh (1 μm) response. α‐Toxin‐permeabilized bronchial smooth muscle and ileum smooth muscle also responded to caffeine, InsP3 and CCh but not to cADPR. Both 100 μm 8‐amino‐cADPR, a selective cADPR antagonist, and 100 μm thionicotinamide‐NADP, a selective NAADP antagonist, failed to inhibit the CCh response, although procaine abolished the caffeine, InsP3 and CCh responses in the permeabilized tracheal smooth muscle. Although inhibition of the caffeine response by 30 μm ryanodine was nearly complete, approximately 30 % of the InsP3 (300 μm) plus GTP (50 μm) response was retained, and the resultant response disappeared after the caffeine response was evoked in the presence of ryanodine. Heparin (300 μg ml−1) blocked InsP3 (300 μm) and CCh (3 μm) responses in β‐escin‐permeabilized tracheal smooth muscle, while Ruthenium Red (100 μm) partially inhibited the CCh response. Collectively, InsP3 but not cADPR or NAADP plays a key role in CCh‐initiated contraction, and InsP3 utilizes a single compartment of the caffeine/ryanodine‐sensitive stored Ca2+ in airway smooth muscle.