The maturation of the neuromodulatory action of substance P (SP) was investigated in tracheal smooth muscle (TSM) segments isolated from rabbits aged 2-24 wk. The tissues were placed in baths containing Krebs-Ringer solution and contracted with electrical field stimulation (ES) with ES frequencies ranging from 1 to 75 Hz. In tissues > 1 mo of age, the ES frequency-response relationships were progressively shifted in the presence of a maximally effective neuromodulatory SP dose (HT-7 M) such that by 24 wk of age the mean (±SEM) maximal tension (Tmax) significantly increased from 380.4 (±41.9) to 502.3 (±64.2) g/g TSM, and the corresponding mean (±SEM) log ES frequency producing 50% of Tmax (log ES50) significantly decreased from 1.209 (±0.069) to 1.055 (±0.046) Hz. By contrast, relative to methacholine, the direct contractile effects of SP did not significantly vary with age. In further analyzing the basis for the above age-related difference in the neuromodulatory action of SP, we found that the magnitude of SP-induced neuromodulation was highly correlated to the tissue's intrinsic sensitivity to ES. Indeed, after accounting for the tissue's sensitivity to ES, the effect of age alone on the magnitude of SP-induced neuromodulation was not statistically significant. These findings provide new evidence that: (a) SP-induced neuromodulation of acetylcholine release at the airway neuromuscular junction is significantly enhanced during postnatal development; and (b) that the latter age-dependent action of SP is based on a close coupling of the magnitude of SP-induced neuromodulation to the tissue's intrinsic sensitivity to neurally mediated contraction.
