In skeletal muscle cells, action potentials cause the release of calcium from th

In skeletal muscle cells, action potentials cause the release of calcium from the SR by directly opening SR voltage-gated calcium channels. What causes the release of calcium from the SR in cardiac muscle cells?

 The mechanism is the same in cardiac muscle cells. 
 Influx of extracellular calcium ions opens ryanodine receptors allowing Ca2+ to flow out of the SR. 
 Action potentials destabilize the sarcolemma changing the permeability of the cardiac SR and allowing Ca2+ to leak out. 
 ACh from the cardiac neurons bind ACh receptors on the cardiac myocytes and cause Ca2+ to bind troponin.