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

Sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 3 (SERCA3), an isoform of the intracellular Ca2+ pump that has been shown to mediate endothelium-dependent relaxation of vascular smooth muscle, is also expressed in tracheal epithelium. To determine its possible role in regulation of airway mechanical function, we compared tracheal contractility in gene-targeted mice deficient in SERCA3 (SERCA3(-)) with that in wild-type tracheae. Cumulative addition of ACh elicited concentration-dependent increases in isometric force (ED50 = 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 transient relaxation (42.6 +/- 3.2%, n = 28) in both tracheae. However, the rate 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 of epithelial cell involvement. This was verified using a perfused trachea preparation. SP in the outside medium had no effect, whereas SP in the perfusate bathing the epithelial side elicited a relaxation. Nitric oxide synthase inhibition (0.2 mM N-omega-nitro-L-arginine) reduced the SP relaxation by 36.5 +/- 12.5%, whereas the SP effect was abolished by eicosanoid inhibition (10 mu M indomethacin). ATP also elicited an epithelium-dependent relaxation similar to SP but with a more rapid relaxation in the SERCA3(-) trachea than in the wildtype trachea. Our results indicate that SERCA3 gene ablation does not directly affect smooth muscle, which is consistent with the distribution of the isoform, but suggest that SERCA3 plays a role in epithelial cell modulation of airway smooth muscle function.