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.

The purpose of this study was to determine whether steroid hormones modulate cardiac function in rainbow trout (Oncorhynchus mykiss Walbaum). We assessed the effects of exogenously administered steroids on isolated ventricle strips and report that physiological concentrations of androgens, 17β-estradiol and cortisol rapidly (<10·min) enhance inotropism (30-40%) in a sexspecific manner. These effects were specific to the hormones studied, absent if animals were anesthetized chemically and dependent upon steroid concentration and contraction frequency. Based on the use of specific steroid receptor antagonists and key enzyme inhibitors, it appears that testosterone, 11-ketotestosterone and cortisol each act through specific intracellular receptors in males and that the positive inotropism requires the synthesis of polyamines and nitric oxide. Cortisol and 17β-estradiol, but not androgens, had similar effects in females and also involved similar signaling pathways. Androgen and cortisol effects were additive in males but cortisol and 17β-estradiol were not additive in females, suggesting sex differences in the pathways through which these hormones stimulate inotropism. In summary, gonadal steroids and cortisol promote ventricular contractility in a sexdependent manner through mechanisms that appear multifaceted. Ultimately, steroid-mediated improvements in cardiac performance might involve non-genomic pathways and be physiologically important during migration, spawning or stressful periods.