Schematic of the events involved in excitation-contraction coupling. A L-type Ca channel 3Na* A:Excitation-Ca Release. NCX (a)Calcium enters the cell via L-type Ca channels situated in the surface membrane and (as shown) Ca2 SR in transverse tubules. RyR SERCA (b)Some of this Ca binds to the ryanodine receptor (RyR)making it open and thereby releasing much phospholamban more Ca in a process known as calcium-induced calcium release. (c)Ca release results in the systolic Ca transient and thence contraction. B 0.8r (d)Relaxation occurs by Ca being reduced to resting Ca transient levels by a combination of uptake into the amplitude sarcoplasmic reticulum(SR)via SERCA and (umol/L) removal from the cell by the electrogenic Na-Ca exchange(NCX). 0.0 (B)The steep dependence of the amplitude of the 0 25 50 systolic Ca transient on SR Ca content. SR Ca content(umol/L)