Filling of the bladder with urine stretches the myocytes in the wall. Stretch activates nonselective cation channels (SACs) thereby constituting a pacemaking mechanism. Once action potentials are triggered, Ca2+ influx through nifedipine-sensitive Ca2+ channels provides activator Ca2+ for the stretch-induced increase in wall tension (myogenic response). An additional component of myogenic response is independent of nifedipine and membrane potential; Ca2+ influx through SACs is large enough to induce Ca2+ release from intracellular stores.