Fig. 2. The effects of hyperkalemia and acidosis (pH 7.5) on maximum cardiac output (A) and maximum cardiac power output (B) of perfused rainbow trout hearts at 10°C. Values are reported as mean ± s.e.m. Individual hearts (N=8) were exposed to the following sequence of perfusates: (1) control (normoxia), (2) 5.0 mmol l^–1 K⁺, pH 7.5, (3) control (recovery), (4) 5.0 mmol l^–1 K⁺ and pH 7.5 with 500 nmol l^–1 adrenaline (AD), and (5) 7.5 mmol l^–1 K⁺ and pH 7.5 with 500 nmol l^–1 AD. Values from series I are presented for comparison purposes (see Fig. 1). One-way ANOVA and a Holm–Sidak multiple comparisons test were used to compare treatment means. *Significant difference from control; a significant difference from pH 7.9 at that particular level of hyperkalemia, P=0.05. Three preliminary preparations were exposed to 7.5 mmol l^–1 K⁺ and 5 nmol l^–1 AD at a pH of 7.5 directly after the first normoxia step. However, this exposure resulted in an almost immediate decrease in cardiac output leading to a rapid (<5 min), irrecoverable cardiac collapse.