Cold acclimation of guinea pig depressed contraction of cardiac papillary muscle

Guinea pigs were exposed to 5 degrees C for 3 wk, and the contractions of m yocardial papillary muscle were compared with preparations dissected from c ontrol animals kept at similar to 25 degrees C. Developed tension of the pa pillary muscle per cross-sectional area was significantly (t-test, P < 0.05 ) decreased after cold exposure (19,200 +/- 8,160 vs. 3,020 +/- 2,890 dyne/ cm(2); 1 Hz). Time to peak tension was significantly faster in cold-exposed guinea pigs (126.4 +/- 11.1 ms; 1 Hz) than in controls (162.7 +/- 8.7 ms). The magnitude of the developed tension after application of ryanodine (2 m M) to muscles from cold-exposed animals was decreased to 37.5 +/- 8.3% of c ontrol at 1 Hz, whereas in muscles from control animals, tension was decrea sed to 82.4 +/- 7.7%. The ryanodine-sensitive component of contraction was not significantly changed in control guinea pigs at frequencies >0.5 Hz, wh ereas in muscles from cold-acclimated guinea pigs, there was a "positive st aircase." These results suggested that reversal of the Na+/Ca2+ exchanger i s predominantly involved in the positive staircase in control guinea pigs, whereas rate-dependent increases in the Ca2+ store in the sarcoplasmic reti culum may be involved in the staircase after cold acclimation.