PIERSON, N. S.; FARRAR, R. S.; RODNICK, K. J.; Idaho State University: Plasma calcium and cardiac responsiveness to extracellular calcium between male and female rainbow trout

Sex-related differences in cardiac size and function have been well documented in mammals. Male rainbow trout have larger hearts than similar-size female fish and sexual maturation selectively promotes cardiovascular performance in males. We examined whether there are sex-related differences in plasma concentrations of Ca and responsiveness to extracellular Ca. Venous blood was drawn from male and female rainbow trout and total plasma Ca was measured colorimetrically using Arsenazo III. Four uniform strips of ventricular tissue were tied to isometric force transducers, placed in individual 30 ml tissue baths containing oxygenated teleost Ringers and glucose, and stimulated maximally to contract at 0.5 Hz in the presence of a physiological (1.5 mM) concentration of Ca. After 1 h, Ca was added every 5 min to achieve 0.5 mM increments for 60 min. Total plasma Ca was higher in male vs. female fish (2.34 � 0.08 vs. 2.09 � 0.06 mM, P=0.018). Ventricle strips would not contract at extracellular Ca less than 1 mM and the inotropic effects of increasing Ca on contractile performance did not differ between the sexes. Both male and female fish showed a maximum responsiveness of 35 � 4% above baseline contractility. Despite comparable Ca effectiveness on promoting cardiac inotropism between sexes, elevated plasma Ca provides a possible mechanism for higher cardiovascular performance in males. Future studies will determine whether ionized Ca parallels total Ca levels in plasma and compare cardiac sensitivity and responsiveness to Ca using different energy substrates. Summary: Plasma Ca levels were higher in males than females; however, no differences were detected in responsiveness.