SARCOPLASMIC-RETICULUM CA-2-ATPASE AND RYANODINE RECEPTOR IN COLD-ACCLIMATED DUCKLINGS AND THERMOGENESIS()

In birds, prolonged cold exposure induces the development of a nonshiv ering thermogenesis (NST) of muscular origin. NST is characterized by an increased heat production, which may be achieved by an increased AT P-dependent cycling of Ca2+ between the sarcoplasmic reticulum (SR) an d cytosolic compartments in muscle. In this study, the effects of prol onged cold exposure on SR function were assessed by determining the co ntents of the SR Ca2+-ATPase and Ca2+ release channel (ryanodine recep tor) in the gastrocnemius muscle of ducklings (Cairina moschata) kept at thermoneutrality (25-degrees-C) or cold acclimated (4-degrees-C, 5 wk). Measurement of oxalate-supported Ca-45(2+) uptake by whole muscle homogenates revealed that the SR Ca2+-ATPase activity, and fraction o f vesicles containing a ryanodine-sensitive Ca2+ release channel were increased by 30-50% in response to prolonged cold exposure. Sodium dod ecyl sulfate-polyacrylamide gel and immunoblot analysis, Ca-45(2+) upt ake, Ca2+ -ATPase activity and [H-3]ryanodine binding measurements wit h unfractionated and ''heavy'' SR membrane fractions also indicated an elevated Ca2+-ATPase and Ca2+ release channel content in cold-acclima ted ducklings. These results showed that the contents of two component s directly involved in Ca2+ cycling by the SR are increased by cold ac climation, and we suggest that this is related to NST.