The function of the sarcoplasmic reticulum is not inhibited by low temperatures in trout atrial myocytes

The effect of temperature on sarcoplasmic reticulum (SR) Ca2+ uptake and release was measured in trout atrial myocytes using the perforated patch-clamp technique. Depolarization of the myocyte for 10 s to different membrane potentials (Vm) induced SR Ca2+ uptake. The relationship between Vm and SR Ca2+ uptake was not significantly changed by lowering the experimental temperature from 21 to 7°C, and the relationship between total cytosolic Ca2+ and SR Ca2+ uptake was similar at the two temperatures with a pooled Vmax = 66 amol/pF and K0.5 = 4 amol/pF. Quantification of the Ca2+ release from the SR elicited by 10-ms depolarizations to different Vm showed an increasing SR Ca2+ release at more positive Vm between -50 and +10 mV, whereas SR Ca2+ release stagnated between +10 and +50 mV. Lowering of the temperature did not affect this relationship significantly, giving an SR Ca2+ release of 1.71 and 1.54 amol/pF at 21 and 7°C, respectively. Furthermore, clearance of the SR Ca2+ content slowed down inactivation of the L-type Ca2+ current at both temperatures (the fast time constant increased significantly from 10.4 ± 1.9 to 15.0 ± 2.0 ms at 21°C and from 38 ± 15 to 73 ± 24 ms at 7°C). Thus the SR has the capacity to remove the entire Ca2+ transient at physiologically relevant stimulation frequencies at both 21 and 7°C, although it is estimated that -40% of the total Ca2+ transient is liberated from and reuptaken by the SR with continuous stimulation at 0.5 Hz independently of the experimental temperature.