Ablation of the SERCA3 gene alters epithelium-dependent relaxation in mouse tracheal smooth muscle

Sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 3 (SERCA3), an isoform of th e intracellular Ca2+ pump that has been shown to mediate endothelium-depend ent relaxation of vascular smooth muscle, is also expressed in tracheal epi thelium. To determine its possible role in regulation of airway mechanical function, we compared tracheal contractility in gene-targeted mice deficien t in SERCA3 (SERCA3(-)) with that in wild-type tracheae. Cumulative additio n of ACh elicited concentration-dependent increases in isometric force (ED5 0 = 2 mu M, maximum force = 8 mN/mm(2)) that were identical in SERCA3- and wild-type tracheae; After ACh stimulation, substance P (SP) elicited a tran sient relaxation (42.6 +/- 3.2%, n = 28) in both tracheae. However, the rat e of relaxation was significantly (P < 0.04, n = 9) more rapid in the wild- type [half-time (t(1/2)) = 34.3 s] than in the SERCA3(-) (t(1/2) = 61.6 s) trachea. The SP relaxation was reduced by rubbing the trachea, indicative o f epithelial cell involvement. This was verified using a perfused trachea p reparation. SP in the outside medium had no effect, whereas SP in the perfu sate bathing the epithelial side elicited a relaxation. Nitric oxide syntha se inhibition (0.2 mM N-omega-nitro-L-arginine) reduced the SP relaxation b y 36.5 +/- 12.5%, whereas the SP effect was abolished by eicosanoid inhibit ion (10 mu M indomethacin). ATP also elicited an epithelium-dependent relax ation similar to SP but with a more rapid relaxation in the SERCA3(-) trach ea than in the wildtype trachea. Our results indicate that SERCA3 gene abla tion does not directly affect smooth muscle, which is consistent with the d istribution of the isoform, but suggest that SERCA3 plays a role in epithel ial cell modulation of airway smooth muscle function.