Addition of magnesium to a preservation solution such as St Thomas's Hospital solution has been shown to improve myocardial preservation. High concentration of magnesium can affect coronary artery endothelial-dependent relaxation. Isolated rat hearts were studied in the Langendorff apparatus to investigate whether magnesium-enriched hyperkalemic cardioplegic solution (HCS) could alter coronary endothelial function. Hearts in group 1 (n = 8) were perfused for 30 mins with a standard hyperkalemic cardioplegic solution (potassium chloride 24 mmol/L). Hearts in group 2 (n = 8) were perfused with modified St Thomas' Hospital solution (MST) containing 16 mmol/L of magnesium chloride and 24 mmol of potassium chloride. The endothelium dependent and endothelium independent relaxation of the coronary arteries were respectively assessed by infusing 5-hydroxytryptamine (5-HT) (1 x 10(-6) mol/L) and sodium nitroprusside (SNP) (1 x 10(-5) mol/L) before and after perfusion of cardioplegic solutions. Hearts in group 2 showed a reduction of the 5-HT-induced coronary flow increase following the MST exposure (before, 8.66 +/- 0.86 mL/min; after, 5.66 +/- 0.97 mL/min, P < 0.01) whereas hearts in group 1 were not significantly affected (before, 8.00 +/- 0.68 mL/min; after, 6.99 +/- 1.02 mL/min, not significant), suggesting endothelial dysfunction in the former. Coronary flow response to SNP was not affected in either group.(ABSTRACT TRUNCATED AT 250 WORDS)