Cold acclimation of guinea pig depressed contraction of cardiac papillary muscle

Abstract

Guinea pigs were exposed to 5°C for 3 wk, and the contractions of myocardial papillary muscle were compared with preparations dissected from control animals kept at ∼25°C. Developed tension of the papillary 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/cm2; 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 mM) to muscles from cold-exposed animals was decreased to 37.5 ± 8.3% of control at 1 Hz, whereas in muscles from control animals, tension was decreased to 82.4 ± 7.7%. The ryanodine-sensitive component of contraction was not significantly changed in control guinea pigs at frequencies >0.5 Hz, whereas in muscles from cold-acclimated guinea pigs, there was a “positive staircase.” These results suggested that reversal of the Na+/Ca2+exchanger is predominantly involved in the positive staircase in control guinea pigs, whereas rate-dependent increases in the Ca2+ store in the sarcoplasmic reticulum may be involved in the staircase after cold acclimation.