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

Kunihiko Iizuka, Akihiro Yoshii, Kunio Dobashi, Takeo Horie, Masatomo Mori and Tsugio Nakazawa *

1. 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.

2. In the presence of 50 µM GTP, CCh and InsP₃ contracted -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.

3. However, cADPR (10-300 µM), -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. -Toxin-permeabilized bronchial smooth muscle and ileum smooth muscle also responded to caffeine, InsP₃ and CCh but not to cADPR.

4. 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.

5. 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.

6. Heparin (300 µg ml^-1) blocked InsP₃ (300 µM) and CCh (3 µM) responses in -escin-permeabilized tracheal smooth muscle, while Ruthenium Red (100 µM) partially inhibited the CCh response.

7. 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.

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