Fig. 8. Effects of caffeine (8 mmol l^-1) on contractile performance of cardiac tissue in the presence or absence of elevated Ca²⁺ (5 mmol l^-1), epinephrine (Epi; 1 µmol l^-1), testosterone (T; males; 0.3 µmol l^-1) or 17ß-estradiol (E2; females; 1.0 nmol l^-1). All ventricle strips were exposed to 5 mmol l^-1 glucose for 1 h prior to addition of caffeine. After a 15 min exposure to caffeine, each strip received one of the inotropes. Within a sex there were no differences between treatments; however, the increase in twitch force was greater in immature males than females (^*P<0.05). Values are means ± s.e.m. (N=7 strips per group).