ACTIVATION OF THE SKELETAL-MUSCLE CALCIUM-RELEASE CHANNEL BY A CYTOPLASMIC LOOP OF THE DIHYDROPYRIDINE RECEPTOR

Expression studies with skeletal and cardiac muscle cDNAs have suggest ed that the putative cytoplasmic loop region of the dihydropyridine re ceptor (DHPR) alpha 1 subunit between transmembrane repeats II and III (DCL) is a major determinant of the type of excitation contraction co upling (skeletal or cardiac) in rescued dysgenic muscle cells (Tanabe, T., Beam, K. G., Adams, B. A., Niidome, T., and Numa, S. (1990) Natur e 346, 567-569). In this study, the possibility of a direct functional interaction with the sarcoplasmic reticulum ryanodine receptor/Ca2+ r elease channel has been tested by expressing the DCLs of the mammalian skeletal and cardiac muscle DHPR alpha 1 subunit in Escherichia coli. The purified peptides activated the skeletal muscle ryanodine recepto r/Ca2+ release channel in single channel and [H-3]ryanodine binding me asurements, by increasing channel open probability and the affinity of [H-3]ryanodine binding, respectively. The two peptides did not activa te the cardiac muscle Ca2+ release channel. Other proteins (polylysine , serum albumin) also increased [H-3]ryanodine binding and Ca2+ releas e channel activity, but their activation mechanisms were distinguishab le from DCLs. These results show that the II-III cytoplasmic loop of t he skeletal and cardiac DHPR alpha 1 subunit functionally interacts wi th the skeletal, but not cardiac, muscle Ca2+ release channel. Further more, our studies suggest that in addition to the DHPR, the sarcoplasm ic reticulum Ca2+ release channel may determine the type of E-C coupli ng that exists in muscle.