Effects of acute warming on cardiac and myotomal sarco(endo)plasmic reticulum ATPase (SERCA) of thermally acclimated brown trout (Salmo trutta)

Abstract

At high temperatures, ventricular beating rate collapses and depresses cardiac output in fish. The role of sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) in thermal tolerance of ventricular function was examined in brown trout (Salmo trutta) by measuring heart SERCA and comparing it to that of the dorsolateral myotomal muscle. Activity of SERCA was measured from crude homogenates of cold-acclimated (+ 3 °C, c.a.) and warm-acclimated (+ 13 °C, w.a.) brown trout as cyclopiazonic acid (20 µM) sensitive Ca²⁺-ATPase between + 3 and + 33 °C. Activity of the heart SERCA was significantly higher in c.a. than w.a. trout and increased strongly between + 3 and + 23 °C with linear Arrhenius plots but started to plateau between + 23 and + 33 °C in both acclimation groups. The rate of thermal inactivation of the heart SERCA at + 35 °C was similar in c.a. and w.a. fish. Activity of the muscle SERCA was less temperature dependent and more heat resistant than that of the heart SERCA and showed linear Arrhenius plots between + 3 and + 33 °C in both c.a. and w.a. fish. SERCA activity of the c.a. muscle was slightly higher than that of w.a. muscle. The rate of thermal inactivation at + 40 °C was similar for both c.a. and w.a. muscle SERCA at + 40 °C. Although the heart SERCA is more sensitive to high temperatures than the muscle SERCA, it is unlikely to be a limiting factor for heart rate, because its heat tolerance, unlike that of the ventricular beating rate, was not changed by temperature acclimation.

Keywords: Calcium management; Fish heart; Heart rate; Temperature tolerance.

Fig. 1

pH dependence of SERCA activity in heart ventricle and dorsolateral myotomal muscle of the brown trout acclimated at + 3 °C (c.a.) or + 13 °C (w.a.). The experiments were conducted at + 23 °C. The results are means ± SEM of 5 and 8 homogenates (= fish) for heart and muscle, respectively. SERCA activity was measured as Ca²⁺-ATPase activity inhibited by either cyclopiazonic acid (CPA, 20 µM) or thapsigargin (TG, 20 µM). Activities were normalized to maximum activity, regardless of the concentration at which it occurred. pH dependence of SERCA activity was statistically significant (two-way ANOVA, F = 68.61, p = 0.000) and there was a difference between heart and muscle (two-way ANOVA, F = 4,55, p = 0.037)

Fig. 2

Concentration-dependent inhibition of heart ventricle (a, b) and dorsolateral myotomal muscle (c, d) SERCA by thapsigargin (TG) and cyclopiazonic acid (CPA). The experiments were carried out in preparations from both cold-acclimated (+ 3 °C, c.a.) (a, c) and warm-acclimated (+ 13 °C, w.a.) (b, d) brown trout. The experiments were made at + 23 °C and at pH 7.2. The results are means ± SEM of 5–8 homogenates (= fish) for the muscle and 3–4 homogenates (= fish) for the heart. The activities were normalized to the control value in the absence of blockers. TG and CPA significantly inhibited heart SERCA in w.a. (two-way ANOVA, F = 31.32, p < 0.001) and c.a fish (two-way ANOVA, F = 229.08, p = 0.001) and muscle SERCA in w.a. (two-way ANOVA, F = 148.77, p < 0.001) and c.a. (two-way ANOVA, F = 320.57, p < 0.001) fish. Effects of CPA and TG were statistically different in w.a. muscle SERCA (two-way ANOVA, F = 13.58, p < 0.001)

Fig. 3

Temperature dependence of SERCA activity in heart ventricle (a, b) and dorsolateral myotomal muscle (c, d) from cold- (+ 3 °C, c.a.) and warm- (+ 13 °C, w.a.) acclimated brown trout. Panels a and c show SERCA activity (µmols of inorganic phosphate liberated by mg tissue wet weight in 1 min) as a function of temperature. Panels b and d show Q₁₀ values of SERCA. The results are means ± SEM of 11 and 8 preparations for muscle and heart, respectively. Temperature significantly affected SERCA activity in both heart (two-way ANOVA, F = 82.67, p < 0.001) and muscle (two-way ANOVA, F = 249.48, p < 0.001) and the activities were different between acclimation groups both for heart (two-way ANOVA, F = 26.33, p < 0.001) and muscle (two-way ANOVA, F = 6.21, p = 0.015) SERCA

Fig. 4

The rate of thermal inactivation of heart ventricle (a) and dorsolateral myotomal muscle (b) SERCA activity of cold-acclimated (+ 3 °C, c.a.) and warm-acclimated (+ 13 °C, w.a.) brown trout. The results are means ± SEM of 11 and 8 preparations for muscle and heart, respectively. The activities were normalized to the value of the untreated preparations. The rates of inactivation (τ) were not different between w.a. and c.a. fish either in heart (Mann–Whitney, p = 0.11) or muscle (Mann–Whitney, p = 0.16)