Steroid-induced cardiac contractility requires exogenous glucose,glycolysis and the sarcoplasmic reticulum in rainbow trout

Abstract

SUMMARY Recent data from our laboratory suggest that sex steroids promote contractile function in cardiac muscle of rainbow trout (Oncorhynchus mykiss Walbaum), and there are sex differences in hormone signaling and cardiac function. The current study investigated whether steroid-induced inotropism in electrically paced (0.5 Hz, 14°C) ventricle strips at 90%Lmax (1) has a metabolic requirement for exogenous glucose and (2) is associated with enhanced intracellular Ca2+ storage and release from the sarcoplasmic reticulum (SR). We also explored whether sex differences exist in extracellular Ca2+(Ca2+o) or cardiac sensitivity to Ca2+o. In the absence or at low concentrations (1 or 2 mmol l-1) of exogenous glucose, resting tension and relaxation time were increased selectively in cardiac tissue from females. Increasing glucose promoted twitch force in a bell-shaped manner, with 5 mmol l-1representing the optimal concentration for both sexes. The positive inotropic effects of physiological concentrations of testosterone (T) and 17β-estradiol (E2) in male and female trout ventricle strips,respectively, developed slowly (10-45 min) and were not apparent in glucose-free medium, in medium containing iodoacetate (IAA), an inhibitor of glycolysis, or medium containing 5 mmol l-1 lactate or pyruvate. Male ventricle strips had increased inotropic responses to glucose and T compared with female strips exposed to glucose and E2. Furthermore, sexually maturing males showed a greater inotropic response than immature males or females. Pretreatment with ryanodine (a specific blocker of SR Ca2+release) also eliminated the inotropic effects of sex steroids and exogenous glucose and reduced the post-rest potentiation of contractile force (a marker of SR Ca2+ storage). By contrast, the inotropic effects of epinephrine (Epi) or elevated Ca2+o were faster(developing within 1-3 min) and were not diminished by the presence or absence of glucose or by pretreatment with IAA or ryanodine. Sex differences were also found in responsiveness to caffeine (males > females) and the relationship between Ca2+ concentration and force development above baseline. The Ca2+50 was lower in female cardiac tissue than males, suggesting greater Ca2+ sensitivity, and although plasma albumin was higher in females, total and ionized plasma Ca2+ did not differ between the sexes. For the first time, our study highlights the importance of extracellular glucose, glycolytic activity and SR Ca2+ storage and release for sex steroid-induced inotropism in the trout ventricle. Conversely, the inotropes Epi and elevated[Ca2+o] do not require the presence or metabolism of exogenous glucose or the SR for signaling their positive effects on contractility. These results also demonstrate novel sex-related differences in cardiac reliance on exogenous glucose, Ca2+ sensitivity and SR function and thus should be considered in future studies.

Limited effects of exogenous glucose during severe hypoxia and a lack of hypoxia-stimulated glucose uptake in isolated rainbow trout cardiac muscle

“…Ryanodine reduced twitch force development at the first stimulus after a prolonged rest period [post-rest potentiation (Bers, 1985)] and the force developed at 1.1Hz in the presence of adrenaline at 15°C and in the absence of adrenaline at 23°C -a temperature close to the upper thermal limits of this species . Overall, the current results for Aarhus rainbow trout confirm the importance of the SR as a source of activator Ca 2+ and, in contrast to a previous study (Farrar et al, 2006), do not support an essential role of exogenous glucose for Ca 2+ loading/release in the SR of female rainbow trout. Even under aerobic conditions, glycolysis appears to be crucial for normal EC coupling in the heart of rainbow trout (Gesser, 2002) and mammals (Kockskämper et al, 2005).…”

“…However, previous (Gesser et al, 1982;Farrar et al, 2006;Battiprolu et al, 2007) and current data demonstrate that positive inotropic effects of a maximally effective dose of adrenaline occur during oxygen deprivation and recovery after oxygen deprivation, and these effects are independent of the presence of exogenous glucose. An additional study using iodoacetate to block glycolysis showed that inotropic responses to adrenaline under aerobic conditions do not require an active glycolytic pathway (Farrar et al, 2006). Together, these results suggest that aerobic and anaerobic metabolic pathways are capable of supporting adrenaline-induced changes in mechanical performance of the rainbow trout heart without exogenous glucose.…”

“…Under aerobic conditions, exogenous glucose accounts for a very minor component (<10%) of energy metabolism of intact (West et al, 1993;Blasco et al, 1996) or isolated hearts of trout (Bailey and Driedzic, 1993). Conversely, physiological levels of exogenous glucose (5mmoll −1 ) maintained resting tension and increased developed tension in isolated ventricle strips from immature female and maturing males, respectively (Farrar et al, 2006). Possibly related to this beneficial effect on myofilament function, glucose may influence the participation of the sarcoplasmic reticulum (SR) in cardiac excitation-contraction (EC) coupling.…”

“…Thus, evidence from fishes and mammals demonstrates that an active glycolytic pathway is required to maintain cardiac EC coupling for intracellular Ca 2+ homeostasis and mechanical performance under aerobic and oxygen-limiting conditions (Driedzic and Gesser, 1994;Gesser, 2002). Furthermore, under aerobic conditions, exogenous glucose appears to enhance SR Ca 2+ uptake, storage and release in ventricle strips from sexually immature rainbow trout (males and females), but only helps to maintain resting tension in females (Farrar et al, 2006). To what extent exogenous glucose or tissue glycogen fuels glycolytic activity and ATP production in teleost hearts is unknown, although there is evidence for functional compartmentation of glycolysis and glycogenolysis in the isolated working rat heart (Anousis et al, 2004).…”

Dichloroacetate selectively improves cardiac function and metabolism in female and male rainbow trout

“…Female ventricle strips do not maintain contractile force or resting tension in vitro under aerobic conditions as well as male preparations, whereas male preparations do not preserve contractile function as well as female preparations after hypoxia and reoxygenation (7). Inhibition of the glycolytic pathway during aerobic incubations of rainbow trout ventricle strips results in incomplete relaxation, increased resting force, and decreases in twitch force (22,35). However, compared with the mammalian heart, little is known about the relationship between anaerobic and aerobic energy metabolism in fish hearts.…”