EFFECTS OF MODIFIED ST-THOMAS-HOSPITAL SOLUTION ON CORONARY-ARTERY ENDOTHELIUM-DEPENDENT RELAXATION IN THE ISOLATED RAT-HEART

Addition of magnesium to a preservation solution such as St Thomas's H ospital solution has been shown to improve myocardial preservation. Hi gh concentration of magnesium can affect coronary artery endothelial-d ependent relaxation. Isolated rat hearts were studied in the Langendor ff apparatus to investigate whether magnesium-enriched hyperkalemic ca rdioplegic solution (HCS) could alter coronary endothelial function. H earts in group 1 (n = 8) were perfused for 30 mins with a standard hyp erkalemic cardioplegic solution (potassium chloride 24 mmol/L). Hearts in group 2 (n = 8) were perfused with modified St Thomas' Hospital so lution (MST) containing 16 mmol/L of magnesium chloride and 24 mmol of potassium chloride. The endothelium dependent and endothelium indepen dent relaxation of the coronary arteries were respectively assessed by infusing 5-hydroxytryptamine (5-HT) (1x10(-6) mol/L) and sodium nitro prusside (SNP) (1x10(-5) mol/L) before and after perfusion of cardiopl egic solutions. Hearts in group 2 showed a reduction of the 5-HT-induc ed 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 dysfunct ion in the former. Coronary flow response to SNP was not affected in e ither group. Coronary vascular resistance during perfusion of both sol utions, as determined by the inverse of coronary flow (infusion durati on/infusion volume), decreased significantly during perfusion in group 1 (before, 5.39+/-0.21 s/mL; during, 3.98+/-0.33 s/mL, P<0.05) wherea s a significant increase was seen in group 2 (before, 5.79+/-0.41 s/mL ; during, 9.67+/-0.83 s/mL, P>0.01). These results suggest that perfus ion of magnesium-enriched cardioplegic solution increases coronary vas cular resistance and decreases endothelium dependent coronary relaxati on in the isolated rat heart.