In birds, prolonged cold exposure induces the development of a nonshivering thermogenesis (NST) of muscular origin. NST is characterized by an increased heat production, which may be achieved by an increased ATP-dependent cycling of Ca2+ between the sarcoplasmic reticulum (SR) and cytosolic compartments in muscle. In this study, the effects of prolonged cold exposure on SR function were assessed by determining the contents of the SR Ca(2+)-ATPase and Ca2+ release channel (ryanodine receptor) 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 45Ca2+ uptake by whole muscle homogenates revealed that the SR Ca(2+)-ATPase activity, and fraction of vesicles containing a ryanodine-sensitive Ca2+ release channel were increased by 30-50% in response to prolonged cold exposure. Sodium dodecyl sulfate-polyacrylamide gel and immunoblot analysis, 45Ca2+ uptake, Ca(2+)-ATPase activity and [3H]ryanodine binding measurements with unfractionated and "heavy" SR membrane fractions also indicated an elevated Ca(2+)-ATPase and Ca2+ release channel content in cold-acclimated ducklings. These results showed that the contents of two components directly involved in Ca2+ cycling by the SR are increased by cold acclimation, and we suggest that this is related to NST.