Inositolpolyphosphate binding sites and their likely role in calcium regulation in smooth muscle.

Inositol 1,4,5-trisphosphate (InsP 3 ) and inositol 1,3,4,5-tetrakisphosphate (InsP 4 ) binding sites have been identified in smooth muscle and other tissues. Subcellular localization of these receptors in smooth muscle indicates that they are present in both the sarcoplasmic reticulum membrane and the plasma membrane, although the InsP 3 receptor appears predominantly localized in the sarcoplasmic reticulum membrane. The heterogeneity of InsP 3 binding sites is confirmed by radioligand binding and molecular cloning studies. It is now clear that InsP 3 , in addition to releasing intracellular Ca 2+ , can also stimulate Ca 2+ entry across the plasma membrane. Although the mechanism of Ca 2+ entry remains a matter for much debate, what is not in doubt is that increases in InsP 3 , perhaps acting together with InsP 4 , can maintain a constant influx of Ca 2+ across the cell membrane. Compared to the InsP 3 receptor, our understanding of the InsP 4 binding site is limited. In most cases, including release of intracellular Ca 2+ or Ca 2+ entry, the major role of InsP 4 appears to be the potentiation of the InsP 3 -induced response. Future studies of the InsP 4 binding site by purification and molecular cloning, as well as subcellular localization, are needed to clarify the role for InsP 4 in the regulation of intracellular Ca 2+ .