Putative ryanodine receptors in the sarcolemma of ventricular myocytes.

The activity of caffeine-activated large conductance channels was recorded in whole-cell, patch-clamped, isolated ventricular myocytes from rabbit heart. The channels were permeable to monovalent and divalent cations and had a unitary monovalent cation conductance of 300-400 pS. Extracellular ruthenium red reduced the unitary conductance of the caffeine-activated channel in a concentration- and voltage-dependent manner. Ryanodine locked the caffeine-activated channels into a subconductance state. Elevating intracellular Ca2+ by photolysis of "caged calcium" increased the number of channel openings. The properties of this caffeine-activated channel were remarkably similar to those of cardiac ryanodine receptors (RyR) and support the novel finding that these channels may also be found on the sarcolemmal membrane.