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Am. J. Respir. Cell Mol. Biol., Volume 23, Number 4, October, 2000 514-520

Inositol (1,4,5)Trisphosphate Metabolism and Enhanced Calcium Mobilization in Airway Smooth Muscle of Hyperresponsive Rats

Florence C. Tao, Barbara Tolloczko, Christine A. Mitchell, William S. Powell, and James G. Martin

Department of Medicine, Meakins-Christie Laboratories, and the Seymour Heisler Laboratory of the Montreal Chest Institute Research Centre, McGill University, Montreal, Quebec, Canada; and Department of Medicine, Monash University, Box Hill Hospital, Melbourne, Australia

Airway hyperresponsiveness (AHR) is a phenotype of asthma and can be modeled by the inbred Fisher strain of rat, which is hyperresponsive in vivo relative to the Lewis strain. Enhanced airway smooth muscle (ASM) contractility and Ca2+ mobilization are associated with the AHR observed in Fisher rats. In this study, we investigated whether the interstrain differences in Ca2+ mobilization to serotonin (5HT) result from differences in inositol (1,4,5)trisphosphate (IP3) metabolism and/or IP3 receptor (IP3R) sensitivity. Ca2+ mobilization by 5HT in cultured ASM cells from both rat strains was phospholipase C (PLC) dependent. Inositol polyphosphate accumulation, and hence PLC activity, was similar in both rat strains, but a specific IP3 transient was detectable only in Fisher myocytes in response to 5HT. These findings suggested that IP3 degradation rather than production differed between the two strains. The Vmax and Michaelis constant (Km) of IP3-specific 5-phosphatase activity were higher in the particulate fraction of Lewis than in Fisher ASM cell homogenates and appeared to be related to a greater expression of two isoforms of 5-phosphatase (type I and type II) in Lewis cells as shown by Western blot analysis. The sensitivity of the IP3R to IP3 was similar between Fisher and Lewis ASM cells, indicating that the interstrain intracellular Ca2+ differences were unrelated to IP3R function. We propose that interstrain variations in 5-phosphatase activity and expression may give rise to the interstrain differences in IP3-mediated Ca2+ release in ASM and may be a determinant of AHR.




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