Developmental aspects of bladder contractile function: evidence for an intracellular calcium pool

Recent studies indicate that specific functional changes in smooth muscle accompany development and that these functional differences may relate to developmental alterations in intracellular calcium content. The possibility that neonatal bladder smooth muscle might be more permeable to calcium ions was tested using the radioligand H3 PN-200 and subsequent Scatchard analyses. No differences in either receptor density or the dissociation constants were noted in comparisons between neonatal and mature rabbit bladder smooth muscle. The sensitivity to calcium in isolated bladder smooth muscle strips exposed to high potassium was tested following calcium depletion. Calcium repletion was performed and dose response curves were generated, which showed that the neonatal strips had a significantly lower effective dose producing 50% of maximal response than their mature counterparts (1.5 versus 7.5 mM., p < 0.05). These findings are consistent with the hypothesis that intracellular binding and secondary calcium release are low in neonatal bladder smooth muscle, and that they increase with maturation. These studies are also consistent with the studies showing maturational increases in the intracellular binding and storage of calcium (sarcoplasmic reticulum), which have been reported in the developing rabbit and sheep myocardium.