It is widely accepted that calcium ions are the primary regulators of smooth muscle contraction and relaxation; however, the mechanisms by which changes in intracellular calcium ion concentration ([Ca2+ 1 ) are obtained within the cell are incompletely understood. Increases in ([Ca2+ 1 ) can be mediated either via a release of calcium from intracellular stores (1 –3 ) or via an influx of calcium ions from the extracellular fluid (4 , 5 ).