In cardiac muscle, Ca2+ entry through the L-type Ca2+ channel during excitation triggers Ca2+ release from sarcoplasmic reticulum (SR) via the ryanodine receptor, Causing muscle contraction. This Ca2- induced Ca2+ release (CICR) is locally controlled by openings of the single L-type Ca2+ channel that triggers the opening of ryanodine receptors located within the dyadic junction The unitary Current through the single Ca2+ channel determines the open probability of the ryanodine receptors, and hence the efficacy of CICR Here, we report findings of multiple conductance levels of L-type Ca2+ channels in freshly isolated rat ventricular myocytes. When 10 mM Ca2+ was used in the pipette solution as the charge carrier in the cell-attached configuration, the most frequently Occurring conductance levels are 6.9 pS and 2 9 pS Three distinct conductance levels were also observed, although infrequently, in the same cell, corresponding to unitary currents of 0.33 pA, 0.24 pA, and 0 17 pA upon depolarization to -10 mV In conclusion, our data demonstrate the existence of multiple single L-type Ca2+ channel conductance levels in the adult mammalian ventricular myocytes with Ca2+ as the charge carrier The multiple conductance levels present heterogeneity in the Ca2+ trigger signal strength at local dyadic junctions. The physiological significance of this heterogeneity on affecting the efficacy of CICR in the cardiac muscle is discussed. (C) 2009 Elsevier Inc. All rights reserved