Circular smooth muscle cells from the feline newborn antrum, unlike the adult, are unable to respond to myogenic agonists in the absence of extracellular calcium or to exogenous inositol 1,4,5-trisphosphate(IP3). This study examined the reasons behind the relative inaccessibility of intracellular calcium stores in the newborn period. IP3 binding was determined in antral smooth muscle homogenates from adult cats and newborns by evaluating the competitive binding of D-myo-[3H]IP3 and unlabeled IP3. Receptor density (Bmax) (fmol/mg of protein) and binding affinity(Kd) were determined. The Kd was similar in adults (31 ± 4 nM) and newborns (28 ± 7 nM); however, the Bmax was markedly decreased in the newborn (647 ± 181.0 fmol/mg) compared with the adult (1755 ± 275 fmol/mg). In adult and newborn antral cells, thapsigargin, which causes a net release of Ca2+ from intracellular stores by inhibiting Ca2+-ATPase-dependent re-uptake activity, caused an early contraction at 30 s that was maintained for at least 20 min. We conclude that, in the newborn, dynamic intracellular calcium stores are present in the smooth muscle of the feline antrum and that differences in accessibility of intracellular calcium stores may be related to changes in the release of calcium from IP3-sensitive stores.