MATURATION OF INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR-BINDING IN RABBIT TRACHEAL SMOOTH-MUSCLE

The mechanisms underlying maturational changes in agonist-mediated airway contractility remain to be identified. Since the signal transduction process coupled to airway contraction involves the Ca2+-mobilizing action of the second messenger, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], the present study examined 1) whether Ins(1,4,5)P3 binding to its intracellular receptor varies with age in rabbit tracheal smooth muscle (TSM), and 2) whether Ca2+ exerts a modulatory effect on Ins(1,4,5)P3 receptor binding that is age dependent. [H-3]Ins(1,4,5)P3 binding was assayed in crude TSM membrane preparations isolated from 2-week-old and adult rabbits. Monophasic Scatchard plots were obtained, reflecting a single binding site, with Hill coefficients of 0.988-0.996. The mean +/- SE values for receptor density (B(max)) and binding affinity [i.e., the dissociation constant (K(d))] were similar in the adult and immature tissues, wherein B(max) = 211 +/- 6 and 238 +/- 60 fmol/mg protein, respectively; and K(d) = 14.1 +/- 0.2 and 11.6 +/- 1.2 nM, respectively. Addition of Ca2+ (10(-8)-10(-3) M) significantly modulated Ins(1,4,5)P3 binding, with opposing maturational effects. In adult tissues, Ca2+ produced dose-dependent inhibition of Ins(1,4,5)P3 binding to 59.5% control, whereas Ins(1,4,5)P3 binding in response to Ca2+ was significantly enhanced in the 2-week-old tissues to 183% control. Collectively, these observations demonstrate that the inherent basal binding characteristics of the Ins(1,4,5)P3 receptor are similar in maturing TSM, but that Ca2+ exerts opposite modulatory actions on Ins(1,4,5)P3 receptor binding in immature and adult tissues. These findings support the notion that maturational differences in agonist-mediated TSM contractility are attributed, at least in part, to age-related changes in Ca2+-induced modulation of Ins(1,4,5)P3 binding to its receptor.